Determining Optimal Nutrient Management Strategies for Dairy Farms by oum18845


									                                      OUR INDUSTRY TODAY

Determining Optimal Nutrient Management
Strategies for Dairy Farms'
                                                      G. M. HENRY,2 M. A. DeLORENZ0,z D. K. BEEDE,?
                                                 H. H. VAN HORN,? C. B. MOSS? and W. G. BOGGESS3
                                                                                  University of Florida
                                                                                    Gainesville 32611

                     ABSTRACT                              Abbreviation key: MR = minimum require-
                                                           ment, N M R = no minimum requirement.
      A model for dairy nutrient manage-
   ment was constructed to evaluate strate-
   gies for nutrient recycling using farm-                                 INTRODUCTION
   specific information. The model used                        Large dairies with limited amounts of land,
   linear programming to determine optimal                 which import large quantities of nutrient-dense
   cropping systems that used nutrients                    feeds and possibly commercial fertilizers,
   from dairy manure applied to cropland.                  potentially develop an imbalance of N and P
   The optimal system minimized total feed                 for the total farm because N and P from pur-
   costs, including purchased commodities,                 chased feedstuffs can exceed the amounts leav-
   based on the ability of crops to take up                ing the farm in animal products, forages, and
   manure nutrients and supply forages for                 losses to the environment (volatilization, leach-
   cow rations. Nutrient uptake require-                   ing, and runoff). Recycling of manure nutrients
   ments to meet mandated regulations                      has been the most popular method to address
   were incorporated as constraints to the                 the problem of nutrient balance (12, 23). The
   linear programming model and were for                   recycling system has two nutrient balances, the
   either N or P.                                          balance between manure nutrient and crop up-
      Results from sensitivity analyses                    take (manure nutrient balance) and the balance
   showed that rations balanced for maxi-                  between crop nutrient and animal use (crop
   mum DMI versus solutions with less                      nutrient balance). Both of these balances are
   than maximum DMI affected the crops                     essential to determine strategies for manure
   used in the optimal cropping system and                 management.
   increased feed costs. The effect was                        Many aids have been developed to assist
   greater on feed costs at low production                 dairy producers in making decisions about nu-
   than at high production. The value of                   trient management. Programs have been devel-
   additional acreage was lower with no                    oped to evaluate problems associated with nu-
   minimum requirement on DMI. Winter                      trient pollution (20), to help determine the
   was the limiting season for N and P                     costs of constructing facilities for manure han-
   recycling because of less uptake by                     dling facilities (19), and to determine applica-
   crops. Feed costs were almost always                    tion rates and distribution patterns (9, 13, 15,
   lowered by increasing crop acreage be-                  22). The Manure Application Planner (13) used
   yond the minimum required for N recy-                   linear programming to minimize commercial
   cling.                                                  fertilizer costs by substituting manure
   (Key words: nutrient uptake and recy-                   nutrients. These aids, however, only consid-
   cling, manure management, linear                        ered specific areas of the nutrient recycling
   programming, cropping systems)                          system and not the overall effect of nutrient
                                                           management on the profitability of the dairy.
                                                               There have been many attempts to provide
    Received August 18, 1994.                              values of forages for cattle consumption. Rela-
    Accepted November 2, 1994.                             tive value methods, which evaluate the
    'Florida Agricultural Experiment Station Journal Se-   nutrients in many ingredients, including
ries Number R-04019.
    *Department of Diy and Poultry Sciences.
                      ar                                   forages, relative to value of equivalent
    3Department of Food and Resource Economics.            amounts of nutrients in reference ingredients

1995 J Dairy Sci 78:693-703                            693
694                                             HENRY ET AL.

(e.g., corn and soybean meal), are based on            uptake requirements. The model was con-
solutions of simultaneous equations similar to         strained by time, labor, land, governmental
methods described by Morrison (16). Altema-            regulations, feed and manure storage capacity.
tively, linear programming has been used to            forage supply, and nutrient requirements for
determine least cost rations and optimal strate-       cattle and crops.
gies for formulating rations (1, 21). These for-          The objective function was
mulation models determine rations based on
nutrient requirements of cattle and nutrient
availability of feedstuffs. Bath (1) used PC
DAIRY, a ration formulation model, to evalu-
                                                                         MIN          ct


ate the relative feeding value of certain forages
based on their use in rations for cows at differ-      where cost, = total annual feed cost for year y
ent milk productions while holding constant            (r = 1.J).
the income over feed costs. Relative values               Within a year,
determined by this method thus were in-
fluenced by many of the constrants imposed                           n                      m   3
in the linear program, e.g., the potential for             cost =           xici       +              YfjGfj
greater DMI and much greater NEL intake with                        i= 1                    f=l J=1
higher quality forages than with poor quality
forages. Bath (1) concluded that this method
better estimated the value of high quality
forages in diets for very high producing cows
than did simpler estimates of relative value.
   The objective of this study was to develop a                     +            c
                                                                           p=l j=1
model that considered both the manure                                      n      7
nutrients and crop nutrients balanced to meet
mandated nutrient recycling requirements
while minimizing feed costs for a set milk
production. Optimal cropping systems were
chosen based on crop nutrient uptake and feed-
                                                                    -P         c
                                                                               j= 1

ing value of forages produced. This system
was to be similar to a least cost ration model,        where
but incorporated constraints for nutrient uptake
and crop utilization.                                       Xi = hectares grown of crop i (i =
                                                                   1, ...,n).
          MATERIALS AND METHODS                             Ci = variable cost per hectare of grow-
                                                                  ing crop i,
Model Description
                                                            YG = as-fed kilograms of commodity f
                                                                  used in season j (f = 1. ...,m;j = 1
   Linear programming was used to incor-                          to 31,
porate as many factors surrounding dairy nutri-             Gfi = price per as-fed kilograms of
ent management as possible. The General Al-                       commodity f in season j,
gebraic Modeling System (GAMS; The                        YFd = as-fed kilograms of forage on
Scientific Press, Redwood City, CA) was                           hand g (g = I , ...,r) used in season
selected as the linear programming language                       J?
and Minos 5.0 (Department of Operations Re-               GFa = cost per as-fed kilograms of using
search, Stanford University, Stanford, CA) as                     forage on hand g,
the solver. The objective was to minimize the            Pnutp, = kilograms of nutrient p [p = 1 to
cost of the dairy nutrient management system,                     2 (N or P)] purchased in season j,
including costs of feeding and nutrient recy-            Ncostp = cost per kilograms of nutrient p
cling. The model minimized feed costs by                          purchased,
selecting cropping systems based on their feed-           Tmij = tons of crop i transferred from
ing value and their ability to meet N and P                       period j to j+l,

Journal of Dairy Science Vol. 78, No. 3, 1995
                                           OUR INDUSTRY TODAY                                                  695

  Scostj = cost per ton of storing crop i per            HRVLi,? = percentage of harvest loss on as-
           season,                                                  is basis for crop i harvested in
      P = price per tonne of hay sold, and                          season j’.
    HS, = tonnes of hay sold in season j,                    Vci, = kilograms of forage i used by
                                                                    cow group c (c = 1,...,v) in season
  Annual nutrient constraints were                                  j,
                                                           STLij = percentage feed storage loss for
    n                                                               crop i in season j, and
  CXijApi       +   Tp, = Pnutpj     + Tnavp,      [3]      Tmi, = transfer of forage i to season j in
   1=1                                                              tonnes.
  CXij+lApi         +   Tpj+l = Pnutpj+l                   Seasonal dietary constraints for cow group
   i=l                                                   were
            + Tnavp,j+l + Tp,Dp,                   [41
                                                            n                             rn

   I=   1                                                               r

                                                                 +     CYF,~DC,,
                                                                             2 ~                     ~     ~       ~   &   ~   o
            +                                      VI                  g= 1                                     [91
                                                            n                             rn
    Api = kilograms per hectare of nutrient
          p taken up by crop i,
                                                                 +     CYFcgiDCwg I DMIcjUBwc
                                                                       g= 1                                    [ 101
    Tp, = kilograms of nutrient p transfer
          from season j to j + 1,
 Tnavp, = kilograms of nutrient p available              where
          from the dairy in season j, and
    Dp, = percentage of storage loss of nu-
          trient p in season j.                             DC,i             = amount of dietary characteristic w
                                                                               (w = 1,...k; e.g., DM basis), NEL
                                                                                 megacalories per kilogram as-
  Annual forage supply constraints were                                          sociated with crop i, commodity
                                                                                 f, or forage on hand g,
 XijrYieldi(1 - HRVLijp) = ~ V , i j / 2 0 0 0 + Tmjj           I,,          =   requirement per cow for dietary
                               c=l                                               characteristic w for cow group c
                                                   [61                           in season j,
                                                                 &,          =   number of cows in group c dur-
                                                                                 ing season j,
                                                             Oj =                number of days in season j,
                                                           DMIc, =               total DMI for group c in season j,
                                                            UB,,             =   maximum percentage of dietary
                                                                                 characteristic w allowed in cow
                                                                                 group c rations on a DM basis.

where                                                      Annual resource constraints were

      j’ = previous season Q - l),                                                   n
  Yield, = tonnes per hectare yielded by                                            CXiQli = M1                [Ill
           crop i,                                                                  1=1

                                                                      Journal of Dairy Science Vol. 78, No. 3, 1995
                                                 k l AL.

where                                             Default Inputs

                                                     The model used specific information to de-
      Qli =  amount of resource associated        termine optimal nutrient management strate-
             with the production of an acre of    gies for dairies. General farm information in-
             crop Xi (= 1 to 2, land and
                         l                        cluded location of the dairy, percentage of herd
             labor), and                          in cattle groups @gh, medium, low, dry, heif-
        MI = maximum or minimum quantity          ers, and yearlings), crop acreage availability,
             of resource 1 available,             labor availability, harvest and storage losses,
                                                  cost of purchased N, storage losses of N, stor-
and where                                         age capacity for manure and feed, and the
                                                  concentration of N and P in manure
          Costy, Xi, Yfi, YFgi, Pnutpj,           wastewater. Percentage of millung cows varied
          Tmi,, HSj,   Yieldi, Vcij 2 0.          by season to reflect changes in milk production
                                                  and cow management practices.
   The objective function [ l ] minimized feed       Default values for available excreted N and
cost over the planning horizon t, which was the   P were 34 kg/yr per cow of N (33.6%of total
sum of the yearly feed costs defined by [2].      N excreted; 101 kg/yr per cow total N) and 21
Annual feed costs were composed of crop           kg/yr per cow of P (23). These values were
production costs, purchased feed costs, costs of  slightly higher than some other excretion esti-
feed on hand, purchased fertilizer costs, and     mates, e.g., 92 kg/yr of N (8) and 17 kg/yr of P
sale of bermudagrass hay. Costs for feed on       (14). Nutrient excretion is affected by milk
hand were only included for yr 1, the transi-     production and ration nutrient concentration
tional year.                                       (W.
   Two constraints were included to control           The amounts of N and P available for crop
the balances for manure and crop nutrients uptake are variable. Inorganic forms of plant
within the nutrient recycling system. The ma- nutrients are readily available for crop uptake,
nure nutrient constraints ([3], [4], and [5]) en- but organic forms must be broken down by
sured that the manure nutrient balance was microorganisms before they are available (11).
met. These constraints allowed for transfers of Holloway et al. (8) estimated that between 23
N from winter to spring and from spring to and 45 kg/yr per cow of N were available for
fall, but forced the annual manure nutrient crop uptake during the initial year of applica-
balance at the end of the fall growing season. tion. The percentage of P from manure that is
   Forage supply constraints ([6], [71, and [81) available for crop uptake during the initial year
ensured that the crop nutrient balance was met. was estimated to be 70% (1 1). Nutrients may
These constraints allowed transfers of forage be lost during the recycling process by volatili-
between the first and second and between sec- zation, leaching, and runoff or introduced by
ond and third growing season following har- rainfall (23). Manure handling facilities have
vest, but forced an annual crop nutrient bal- the greatest effect on nutrient losses, which can
ance at the end of the third growing season range from 10 to 80% of excreted nutrients
following harvest.
   Constraints for dietary characteristics were (14). Application method is an additional factor
[9] and [lo]. Equation [9] forced the amount of for crop N availability. Losses of N associated
a dietary characteristic to be greater than the with irrigation of dilute wastewater have been
amount required by the cow. The second con- estimated to be between 30 and 40% of total N
straint [lo] forced the percentage of a dietary (14). After application, nutrient loss is depen-
characteristic in a diet to be less than a certain dent on soil pH, buffering capacity, cation
percentage of DMI. Two constraints for each exchange capacity, and urease activity, as well
characteristic (greater than and less than) were as temperature, soil water content, and soil air
included to allow a range for the model to exchange capacity ( )       4.
select an economical level.                           Crop nutrient uptake has been determined
   Resource constraints limited the use of land by crop yield and concentration of N or P in
and labor to be less than the amounts available. DM (3). Johnson et al. (10) determined N and P
Other special constraints were included to en- uptake for a rye, corn, and bermudagrass crop-
sure realistic cropping systems and rations.       ping system at different manure applications.
Journal of Diy Science Vol. 78. No. 3. 1995
                                           OUR INDUSTRY TODAY                                                     697
TABLE 1. Nutrient uptake and yields for crops grown in central Florida.]
Crop                                               DM                     N                  P
descnption          Season    Type       DM        Yield     CP           Uptake2   P        Uptake3     Cost
                                         (9%)      (tonnes/ha)(%)         (kplha)   (%)      (kg/ha)     ($/ha)
Temperate corn      Spring    Silage     35        18.8        8.1        244       .22      41          7 54
Temperate corn      Fall      Silage     35        15.6        8.1        203       .22      35          744
Tropical corn       Spnng     Silage     35        17.2        8.4        232       .I9      32          731
Tropical corn       Fall      Silage     35        18.2        8.4        244       .20      36          766
Sorghum             Spring    Silage     35        16.3        7.5        195       .21      35          529
Sorghum             Fall      Silage     35        15.0        6.0        143       .26      39          534
Sorghum4            Fall      Silage     35        15.0        6.0        143       .26      39          343
Bermuda             Spring    Silage     35          7.8      11.5        146       .28      22          259
Bermuda             Fall      Silage     35        11.8       11.5        216       .28      32          314
Bermuda             Fall      Hay        93          9.8      12.0        188       .20      20          376
Bermuda afters      Fall      Silage     35        11.8       13.3        250       .20      24          395
Bermuda after       Fall      Hay        93          9.8      12.0        188       .20      20          457
Oats                Winter    Silage     35          7.8      13.0        164       .21      17          34 1
Rye                 Winter    Silage     35          5.6      12.8        112       .32      18          306
   'Source: Gallaher et al. (3). Henry and Gallaher (6). Overman et al. (18). and Johnson et al. (10).
   2Uptake of N was calculated by (DM yield x 1000) x (%CP/100)/6.25.
   3Uptake of P was calculated by (DM yield x 2000) x (96 P/100).
   5After the spring crop

Nitrogen uptake varied from 422 to 588 kgiha, (17). The nutrient requirements for cow and
and estimated P uptake varied from 62 to 67 heifer groups were calculated using the Spartan
kgha when manure N application increased Dairy Ration EvaluatorBalancer (21). Dietary
from 381 to 740 kgha. Chase et al. (2) deter- limitations of forages and commodities were
mined that N recovery efficiency of forage and obtained from Harris and Staples (5) and West
grain sorghum decreased from 104 to 22% (25).
when N fertilization rate increased from 45 to
269 kg/ha on sandy soils. The N uptake by Sensitivity Analyses
bermudagrass in response to N fertilization, N
and P uptake for double cropped corn and           Sensitivity analyses were performed to de-
                                                termine model behavioral response to changes
sorghum, and N and P uptake for oat and rye
                                                in input variables. Variables analyzed included
on sandy soils have been determined (3, 6, 18). DMI,milk production, and crop acreage. The
Crops grown in Florida for nutrient recycling default dairy was a 1000-cow dairy averaging
include corn, forage sorghum, bermudagrass, 8165 kg of milklyr per cow.
wheat, oat, rye, and ryegrass. These crops vary    The gross economic benefit of changing
in quality and yield, depending on the area and from one strategy to another was calculated by
season in which they are grown. Crop costs for
dairies in central and northern Florida were
                                                  GEB% = [(Ti - T2)/(1 + i)']
estimated by Hewitt (7). A summary of nutri-
ent uptake, yields, and costs based on cited                    r n                    1
literature (3, 6, 10, 18) are shown in Table 1.
    Nutrient concentrations for forages were
taken from NRC (17) and from the studies just
discussed. The commodities available for use        GEB = gross economic benefit between
in rations were those typically available in                  system 1 and system 2, consider-
Florida (3, their seasonal costs were aver-
             and                                              ing only variable costs,
ages for 1988 to 1992. Nutrient concentrations        T1 = transitional year feed costs for
for these commodities were taken from NRC                     system 1,
                                                                     Journal of Dairy Science Vol. 78, No. 3, 1995
698                                             n t N K 1 c1 AL.

       T2 =    transitional year feed costs for          recommended DMI. This study evaluated the
               system 2,                                 economic implications of requiring a minimum
         t =   year being discounted,                    DMI constraint to be used in balancing rations
        n =    number of years discounted (n =           and its impact on cropping systems.
               2 to 15).
      M1 =     optimal year feed costs for sys-          Milk Productlon
               tem 1,
      M2 =     optimal year feed costs for sys-             Milk production was varied from 6350 to
               tem 2, and                                9978 kg of milldyr per cow in 907-kg incre-
         i =   interest rate.                            ments to determine the effects of milk produc-
                                                         tion on optimal manure management strategies.
Types of Producers                                       Excretion of N was adjusted for milk produc-
                                                         tion and associated diets.
   At least three situations must be considered
for dairy nutrient management. The first re-             Crop Acreage
quires meeting the nutrient uptake requirement
on the least amount of acreage possible. For                Crop acreages considered were 80.9, 101.2,
the second, acreage is fixed, and the producer           115.7, 134.8, and 202.3 ha to determine the
wants to utilize available acreage to maximize           effects of land availability on optimal manure
profit. The third situation is a new producer            management strategies.
designing a new dairy and needing to know the
optimal acreage needed to grow crops. This                         RESULTS AND DISCUSSION
study applies to the first two situations. The
model could be used to address the new dairy             Variability in DMI
producer if land were included as a resource
rather than a constraint in the model.                      Results from variable DMI showed a sig-
                                                         nificant change in the optimal cropping system
                                                         when a minimum requirement (MR) constraint
Acreage Assumption
                                                         was placed on DMI compared with only an
   Dairy producers were considered risk-                 upper bound of 105% (Table 2).
averse about meeting N uptake requirements                  Corn was the preferred spring and fall crop
for governmental regulations and would not               when no minimum requirement (NMR) was
apply more manure N or P to sprayfields dur-             placed on DMI. When the MR on DMI was
ing a season than crops could use. As a result,          increased, bermudagrass and sorghum acreage
the winter growing season became the limiting            increased to meet the DMI requirements at
season for land availability. In the studies in          lower costs.
which the amount of N varied, acreage was                   As the DMI requirements were varied from
adjusted so that enough was available for N              NMR to a MR of 100%, the annual total feed
uptake during winter. Furthermore, the dairy             costs increased from $889,000 to $927,000, a
producer was assumed not to have any more                difference of $38,000 (Table 2). The greatest
acreage than the minimum required during the             difference in feed cost occurred when DMI
winter. As a result, no N was purchased for              was increased from 90 to 95% ($15,000) and
winter, and the crop acreage was likely less             from 95 to 100% ($19.000). There was little
than optimal in the long run.                            difference when MR was 90% ($3600).
                                                            Rations for the NMR and the MR of 100%
DMI                                                      DMI scenarios are shown in Table 3. There
                                                         was little difference in the feeds used in the
   The DMI was evaluated using upper and                 high cow rations for winter, and DMI was only
lower bounds. The first scenario placed an               lowered to 97.3% of the recommended DMI
upper bound of 105% of NRC recommenda-                   when unconstrained. The DMI was lowest in
tions (17) and no minimum, which allowed the             the cow r o u p s with medium and low DMI for
model to determine the optimal DMI for each              the NMR scenario, 90 and 86% of recom-
cattle group. The final three scenarios had              mended DMI, respectively. The medium ration
minimum bounds set at 90. 95, and 100% of                for the 100% MR scenario added sorghum to
Journal of Dairy Science Vol. 78. No. 3, 1995
                                       OUR INDUSTRY TODAY                                                 699
TABLE 2 Cropping systems for varying DMI constraints.
                                            ~             ~~

                                                                            DMI Requirement
system                                    Season               NMR1       90%2         95902       100%
             ~          ~                                 ~~                     ~        ~          ~

                                                               ala)       (ha)         (ha)        (ha)
Temperate corn                            SPWz                  EO 1       80 1         76 5        42 1
Forage sorghum                            Spring                 36         36           73         34 4
Temperate corn                            Fall                  38 9       30 4         15 0
Tropical corn                             Fall                  41 3       49 8         61 5         69
Ratooned forage sorghum                   Fall                   36         36           73         34 4
Bermudagrass hay after corn               Fall                                                      10 9
Oats                                      Winter                83 8       83 8         83 8        83 8
Total feed costs, $loo0                                        889        893          908         927
   INMR = No nunimum requirement (no lower bound consmnt); upper bound of 105% of recommended DMI
   *Minimum requirement (lower bound) of 90. 95. or 100% of recommended DMI

meet DMI. The low ration for the 100% MR                the optimal cropping systems changed, and
scenario added both sorghum and ber-                    feed costs, land shadow prices, and feed stor-
mudagrass silage to meet the minimum DMI                age capacity were lower at all production lev-
requirements. Sorghum was used in rations of            els when NMR was placed on DMI. Only
high producing cows to meet DMI require-                results with NMR on DMI are reported.
ments because bermudagrass was excluded in                 The optimal cropping system used corn and
high cow rations because of its negative effect         oats to meet nutrient disposal and cow nutrient
on milk production. The NMR rations used                requirements (Table 4). Sorghum entered the
corn silage and whole cottonseed to meet cow            optimal cropping system in the scenarios with
nutrient requirements. D y cow rations were
                          r                             8165 and the 9978 kg of milk, but comprised
the same for both studies. All NMR rations              -3% the annual crop acreage. Two types of
met nutrient intake requirements and other die-         corn were used in the fall growing season. As
tary requirements that ensure normal ruminal            milk production per cow increased, the per-
function.                                               centage of tropical corn increased from 10% at
    Three conclusions can be drawn. First, corn         7258 kg to a high of 19% at 9978 kg of milk
silage was the preferred forage for both nutri-         production per cow.
ent uptake and cow nutrient intake when NMR                As milk production increased, feed costs
was placed on DMI. Second, enforcement of               increased, but not at a constant rate. Feed costs
DMI recommendations was more expensive                  increased as production was increased from
                                                        7258 to 8165 kg and from 9072 to 9978 kg of
than more nutrient-dense rations under condi-
                                                        milk, by $65,000 and $52,000, respectively.
tions of the default dairy. And finally, if DMI
                                                           Forage storage capacity requirements in-
requirements were enforced, growing sorghum
                                                        creased as milk production increased because
or bermudagrass was more economical than
                                                        of the increased crop acreage required by the
purchased cottonseed hulls to meet DMI
                                                        greater amounts of manure nutrients that
recommendations at the prices and costs used            needed to be recycled as milk production in-
in this study.                                          creased.
                                                           The value of an additional hectare also in-
Milk Production                                         creased as milk production increased. An addi-
                                                        tional hectare was $507 greater in value for the
   The effects of milk production on the op-            producer at 9978 than at 6350 kg of milk/yr.
timal cropping system were analyzed with and               Reduction of milk production was not eco-
without a minimum constraint being placed on            nomically feasible because the estimated loss
DMI. Results from these studies showed that             of milk production ($300,000/yr) was greater

                                                               Journal of Dairy Science Vol. 78. No. 3, 1995
700                                                   HENRY ET AL

TABLE 3. Winter rations for two DMI scenarios at various milk production levels.
                                                                       DMI and m l oroduction status
Feed                                      NMR1                           MR                               NMR                     MR
                                    0              (5%)               0%)                 (%
                                                                                           9)          (kg)      (S)           (kg)            (To)
                                                               High                                                  Medium
Temperate corn, spring                                                  1.58                7.9          1.41      7.9
Temperate corn. fall                                                                                     1.92     10.7
Tropical corn, fall                     8.34        42.8               6.65                33.2          4.31     24.5      6.07                30.7
Ratooned sorghum, fall                   .77         4.0               2.01                10.0                             3.81                19.3
Whole cottonseed                        2.92        15.0               1.37                 6.9           2.68    15.0
Corn, ground                                                                                                                2.08                10.5
Soybean meal. 48%                       3.74        19.2               4.27                21.4           2.77    15.5      3.42                17.3
Dicalcium phosphate                      .I1          .6                .I2                  .6            .OB      .4       .09                   4
Citrus pulp                             3.59        18.4               4.00                20.0           3.75    21.0      3.31                16.8
Purchased grass hay                                                                                        .89     5.0       .99                 50
 Total DMI                          19.47          100.0               20.00              100.0          17.87   100.0     19.77               100.0
 % of Required DMI                  97.3                              100.0                              90.4             100.0
                                                               Low                                                       Dry
Temperate corn. spring                  3.84        24.7
Ratooned sorghum, fall                                                 4.24                23.7
Bermudagrass silage                                                    1.14                 6.4
Whole cottonseed                        1.56        10.0                                                   .87     7.8                .87        7.8
Corn, ground                            1.56        10.0                5.40               30.2            .51     4.6                .5 I       4.6
Soybean meal. 48%                       2.16        13.9                1.46                8.1            .42     3.8                 42        3.8
Corn distillers grams                                                   1.46                8.1
Dicalcium phosphate                      .06              .4             .01                 .1
Calcium carbonate                                                        .11                 .6
Citrus pulp                          3.27           21.0                1.62                9.1           .65      5.8            .65            5.8
Purchased grass hay                  3.1 1          20.0                2.46               13.7          8.65     78.0           8.65           78.0
 Total DMI                          15.56          100.0               17.90              100.0         11.10    100.0          11.10          100.0
 % of Required DMI                  86.9                              100.0                            105.0                   105.0
   *NMR = No minimum requirement; MR = minimum requirement.

TABLE 4. Effects of annual milk production per cow on the optimal cropping system when no minimum constraint is
placed on DMI.
                                ~   ~      ~   ~      ~         ~~           ~~~                   ~

                                                                                    Annual milk production per cow
Cropping system                                     6350 kg                 7258 kg               8165 kg        9072 kg          9978 kg
                                                    ala)                                          ala)           ala)             (ha)
Temperate corn, spring                                    71.2                     76.9             80.1           82.6                 91.1
Forage sorghum, spring                                                                               3.6            1.2
Temperate corn, fall                                      71.2                     53.0             38.9           33.6                 47.3
Tropical corn. fall                                                                23.9             41.3           49.0                 43.7
Ratooned forage sorghum, fall                                                                        3.6            1.2
Oats, winter                                           71.2                   76.9                  83.8           83.8             91.1
Maximum land, ha per season                            71.2                   76.9                  83.8           83.8             91.1
Feed costs, $ x 103                                  775                     825                   890            933              985
Forage storage, tonnes                              3 142                   3575                  4103           4028             4514
Economic benefit,' $/yr
 additional hectare                                 7074                    6514                  675 1          7119             7581
   'Feed cost savings.

Journal of Dairy Science Vol. 78. No. 3, 1995
                                          OUR INDUSTRY TODAY                                                         70 1
than the decreases in feed costs at all milk             by $12,000 and $6000, respectively. However,
productions.                                             feed costs were not lowered significantly as
                                                         land was increased over 115.7 ha.
Crop Acreage                                                Shadow prices for an additional hectare of
                                                         land showed an economic benefit for an addi-
   The effect of crop acreage was evaluated              tional hectare to each acreage level until 202.3
with and without N M R on DMI. Only results              ha, at which point the economic benefit
with N M R are discussed.                                reached zero. Because no value was assigned
    All N from dairy manure was used with
                                                         to land in production costs, the estimated
83.8 ha of forages, so all additional acreage
                                                         benefits could be used to evaluate adding acre-
over 83.8 ha could be viewed as imgated crop
acreage and not as required nutrient spray-              age to the cropping system. If land were val-
fields. Therefore, this study addressed the abil-        ued at $2471/ha and fixed costs of irrigation
ity of homegrown forages to decrease feed                and other improvements were $988/ha, the de-
costs when grown in addition to crops on                 fault dairy could afford to add acreage until
nutrient sprayfields. Total annual feed costs,           about 101 ha, at which point the benefit
land shadow prices, and the economic benefit             ($5683) was greater than the costs ($3470) of
of changing available crop acreage were com-             adding a marginal hectare. Marginal returns
pared.                                                   equaled marginal costs between 101.2 and
    Corn was the preferred crop when N M R               115.7 acres.
was placed on DMI (Table 5). 0 ats were used                Feed storage costs increased as land avail-
exclusively for the winter crop, but were not            ability increased (Table 3, and these costs
grown on all of the available acreage in the             should be considered when the purchase of
134.8 and 202.3 ha scenarios. Oats were grown            land is evaluated. In addition, purchased N
only to meet the N uptake requirements in the            increased as more land became available.
winter. The model preferred to grow more corn
acreage in the spring and fall with purchased N                                        CONCLUSIONS
rather than to grow more oats in winter. In
addition, bermudagrass was used increasingly                  Evaluation of the effects of recycling
in the optimal cropping system as more land               nutrients on the profitability of the whole farm
was made available.                                       is important for dairy producers who are facing
    Feed costs decreased as hectares changed              environmental regulations. The model
from 80.9 to 101.2 and from 101.2 to 115.7.               presented is a framework that can be used to

TABLE 5. Effects of land availability on the optimal cropping system with no lower bound on DMI.
Cropping system                            80.9 ha              101.2 ha        115.7 ha      134.8 ha        202.3 ha
                       ~        ~     ~           ~      ~~~~                                            ~~

                                           (ha)                 (ha)            (ha)          rn)             ala)
Temperate corn, spring                            77.3                 97.5        108.9         120.2           144.5
Forage sorghum, spring                             3.6                  3.6          4.8           4.8             4.5
Bermudagrass. spring                                                                 2.4          10.1            19.8
Temperate corn, fall                           34.8                    56.3        100.0         108.1            80.1
Tropical corn, fall                            42.5                    41.3          7.7           2.4             2.0
Ratooned forage sorghum, fall                 3.6                       3.6          4.8           4.8             4.5
Bermuda silage. fall                                                                 2.4           7.3             1.6
Bermuda hay, fall                                                                                  2.8            18.2
Bermudagrass and corn, fall                     1.2                  1.2             1.2           1.2             1.2
Oats, winter                                   80.9                101.2           115.7          83.8            83.8
Maximum land, ha per season                    80.9                101.2           115.7         134.8           202.3
Feed costs, 5 x 103                           892                  880             874           873             874
Forage storage, tonnes                      3917                 5375            6153           6888           7098
Manure storage, kg of N                       458                    0               0             0               0
Purchased N, kg/yr                         14,963               26,963          34.286        34,885          38,326
Economic benefits, $/15 yr
 Additional hectare                         7922                  5683            1534          912                  0

                                                                         Journal of Dairy Science Vol. 78, No. 3. 1995
702                                             HENRY ET AL

determine the economic impact of the nutrient                Conf., Coop. Ext. Serv., New Mexico State Univ., Las
recycling system on farm profitability. Results              Cruces.
                                                           2 Chase, C. R., G. M. Henry. and R. N. Gallaher. 1994.
from sensitivity analyses showed that many                   Yield and nutrient concentration and content in re-
variables affect the optimal cropping system                 sponse to nitrogen fertilization in forage and grain
and indicate the complexity of the dairy nutri-              sorghum. Page 27 i Proc. Southern Conservation
ent recycling system.                                        Tillage Conf., Dep. Agron. Soils, Clemson Univ.
   The DMI study showed that placing a MR                    Coop. Ext., Clemson, SC.
                                                           3 Gallaher, R. N., S. A. Ford, R. McSorley, and J. M.
constraint on DMI changed the optimal crop-                  Bennett. 1991. Corn forage and forage sorghum dou-
ping system and increased feed costs compared                ble cropping yield. economics, crop nutrient removal
with allowing DMI to be selected by the                      and quality. Agron. Res. Rep. AY-91-05, Dep.
model. Corn was preferred to other forages                   Agron.. Univ. Florida. Gamesville.
because of its higher nutrient density and its             4Hargrove. W. L. 1988. Evaluation of ammonia
                                                             volatilization in the field. J. Prod. Agnc. 1:104.
ability to be used with purchased commodities              5 Harris. B.. Jr.. and C. R. Staples. 1989. Feeding by-
in rations when no minimum requirement was                   product feedstuffs to dairy canle. Pub]. No. DS 20,
placed on DMI. Sorghum and bermudagrass                      Dep. Dary S a . , Florida Coop. Ext. Ser.. Univ.
were grown when DMI requirements were                        Florida, Gainesville.
fixed at maximum amounts.                                  6Henry. G . M.. and R. N. Gallaher. 1993. Forage
                                                             quality and nutrient contents for oat and rye grown in
    At the level of production and estimated                 central Flonda. Agron. Res. Rep. AY-93-02, Dep.
commodity prices of the default dairy,                       Agron., Univ. Flonda, Gainesville.
homegrown forages decreased feed costs com-                7 Hewin, T. 1988. Production and harvesting COSIS for
pared with purchase of high fiber feeds such as              forages for silage. Paper presented to an Escambia
                                                             County dairy meeting, Dep. Food and Resource Econ..
cottonseed hulls. Although a feeding require-                Flonda Coop. Ext. Ser., Univ. Florida Gamesville.
ment for DMI (17) may avoid some animal                    8 Holloway, M. P.. A. B. Bottcher. R. A. Nordstedt, and
health problems, our study showed definite                   K. L. Campbell. 1991. BMPs for mitigating nitrate
economic costs from forced consumption of                    Contamination of the groundwater under dairies. Paper
required DM in all cases. Thus, best solutions               No. 91-2134 in Summer Mtg. Am. Soc. Agric. Eng.
                                                             Am. Soc. Agric. Eng., St. Joseph, MI.
occur when DMI is permitted to range between               9 Jacobsen. K. W., and G. D. Bubenzer. 1992. A deci-
the minimum and the maximum limits estab-                    sion support system for manure management. Page
lished for production characteristics of the par-            572 in Proc. 4th Int. Conf. on Computers in Agric.
ticular dairy being studied.                                 Ext. Prog., Florida Coop. Ext., Univ. Florida. Gaines-
    The optimal cropping system changed as                   ville.
                                                          IOJohnson, J. C., G. L. Newton. and J. L. Butler. 1991.
milk production increased when a minimum                     Recycling liquid dairy waste to sustain annual triple
constraint was placed on DMI. As milk                        crop production of forages. Page 41 in Proc. Florida
production increased, more corn was grown to                 Dairy Prod. Conf.. Dairy Sci. Dep., Univ. Florida
meet the nutrient requirements and the in-                   Coop. Ext., Gainesville.
creased N uptake requirements caused by the               11 Klausner, S. D. 1989. Managing the land application
                                                             of animal manures: agronomic considerations. Page 79
greater N excretion by the high producing                    in Proc. Dairy Manure Manage. Symp., Northeastern
cows. The annual feed costs were lower at low                Reg. Agric. Eng. Serv.. Cornell Univ., Ithaca. NY.
production when DMI was determined by the                 12Lanyon. L. E.. and D. B. Beegle. 1993. A nutrient
model than for the scenario in which a fixed                 management approach for Pennsylvania: nutrient
constraint was placed on DMI.                                management decision-making, Agron. Facts 38-C,
    Alteration of crop acreage indicated an eco-             Coll. Agric. Sci.. Pennsylvania State Univ.. University
nomic benefit to increasing the amount of crop            13 Levins. R. A., and M. A. Schmin. 1992. Manure
acreage above the acreage needed to recycle                  Application Planner, Users Manual. Ctr. Farm Finan-
manure nutrients, which reflected the eco-                   cial Management, Minnesota Ext. Sew., Univ. Min-
nomic advantage of growing forages. There-                   nesota, St. Paul.
fore, limiting acreage to the amount needed for           14 Midwest Plan Service: Structures and Environment
                                                             Handbook. 1987. 11th rev. ed. Iowa State Univ.,
N uptake for the winter season complicated the               Ames.
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                                                                  Journal of Dairy Science Vol. 78, No. 3, 1995

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