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Nutrition and Postpartum Breeding in Cattle

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					                     Nutrition and postpartum rebreeding in cattle
                                     R. D. Randel


                               J ANIM SCI 1990, 68:853-862.




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                                                  R. D. Randel
                                 Texas A&M University Agricultural
                            Research & Extension Center, Overton 75684

                                                     ABSTRACT
       Body weight and condition score, although perhaps imprecise or subjective, are
   functional indicators of energy status and rebreeding performance after calving. Inadequate
   precalving and(or) postcalving energy or protein nutrition lowers pregnancy rates as well as
   first-service conception rates and extends postpartum intervals in suckled postpartum beef
   females. Normal numtional regimens for dawy cows that are fed for maximal lactation do
   not exhibit long postpartum intervals or reduced fertility. Yet excessive protein intake may
   depress postpartum rebreeding performance, especially in older dairy cows. Feeding of
   ionophores, with increased ruminal propionate levels in the rumen, results in an earlier
   return to estrus postpartum. Underfeeding of the postprnum cow extends the period of
   ovarian inactivity. The underfed postpartum cow’s lack of ovarian activity appears to be
   due to a suppression of the pulsatile release of LH from the anterior pituitary gland, which
   in turn is controlled by release of GnRH from the hypothalamus. Some metabolic
   compound(s) presumably act on the hypothalamic-pituitary-ovarianaxis as the nutritional
   state of the animal is altered.
   (Key Words: Bovines, Nutrition, Postpartum Interval, Conception.)
                                                                               I. h i m . Sci. 1990. 68:853-862

                      Introduction                                Body WeightlFat and Fertility. Dietary
                                                              restriction during the late prepartum period
   The deleterious effect of undernutrition on                results in weight loss and decreased body fat at
postpartum rebreeding performance of cattle                   calving, which lowers the number of cows and
has been recognized for many years. A review
by Guilben (1942) pointed out that the                        first-calf heifers that return to estrus early in a
principal deficiency influencing reproduction                 defined breeding season (Whitman, 1975;
is general undernutrition due to feed shortage                Wettemann et al., 1982; Dziuk and Bellows,
or poorquality feed. Other reviews have                       1983). Similar results have been reported when
described nutritional influences on reproduc-                 dietary restriction occurs during the postpar-
tive performance of canle (Reid, 1960 Wilt-                   tum period (Rutter and Randel, 1984; Rake-
bank et al., 1965; Baker, 1%9; Lamond, 1970                   straw et al., 1986; Richards et al., 1986).
T o w , 1977; Bowden et al., 1979; Dunn and                   Regression equations relating energy status as
Kaltenbach, 1980 Dziuk and Bellows, 1983;                     expressed by BW change against reproductive
Entwistle, 1983; Hanzen. 1986). A typical                     performance were developed by Dunn and
finding of these reviews is that undernutrition               Kaltenbach (1980) from data published by a
extends the period from calving to the first                  number of researchers. They also developed
postpartum estrus. The purpose of this review                 regression equations by regressing the mean
is to discuss the interrelationship between                   length of the postpartum interval against mean
nutrition and postpartum rebreeding in cattle.                prepartum weight changes. When no pre-
                                                              partum weight loss occurred, 91% of multipa-,
                                                              rous and 64% of primiparous cows would be
                                                              predicted to be in estrus by 60 d after
    ’Journal Paper TA 23828, Texas Agric. Exp. Sta Re-        parturition. Postpartum intervals of 47 d were
sented at a symposium titled “Postpartum Rebreeding in Cat-
tle” at the ASAS 801h Annu. Mtg., New Brunswick, NJ.          predicted for multiparous and 54 d for
    Received January 3.1989.                                  primiparous cows. Wettemann et al. (1982)
    Accepted April 7, 1989.                                   using both weight change and body condition
                                                          853




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854                                                   RANDEL

      TABLE 1. E F m OF DIETARY ENERGY ON PRUjNANCY RATES IN SUCKLED COWS AND H E W E R S
                FE

              b e r m status
Adequate             hadequare                               P                  Source
                                                      ProCalVitlg
68'                  60                                    c.05                 Dunn et al., 1969
78                   60                                     .10                 Bellows and Short. 1978

95                   50                                    <.01                 Wiltbank et al.. 1962
92                   72                                    >.05                 Wiltbank et al., 1964
87                   64                                     c.01                Dunn et al., 1%9
92                   76                                    c.05                 Richards et al., 1986
      'Percent pregnant.



score (1 = emaciated; 9 = obese) developed                   nument intake; when cows were classified as
regression equations relating either BW change               to whether they maintained or lost body
or body condition against reproductive perfor-               condition during the first 20 d after calving,
mance and found that both equations were of                  even more of the differences in postpartum
similar predictive value.                                    interval could be accounted for. Somerville et
    The concept of a target BW and(or) body                  al. (1979) suggest that BW loss during the
condition score at calving was first proposed                postpartum period is of greater importance
by b o n d (1970). Whitman (1975) found that                 than absolute BW in determining postpartum
cows calving at body condition scores of 7 to 9              rebreeding performance. Rakestraw et al.
(range of 1 to 9) were capable of returning to               (1986) reported that rebreeding performance
estrus within 60 d after calving regardless of               was suppressed in cows that calved in good
pre- and post-partum change in BW. Review-                   body condition and were subjected to nutrient
ers have concluded that prepartum numtion is                 restriction during the postpartum period. They
more important than postpartum nutrition in                  concluded that despite a good body condition
determining the length of the postpartum                     ( . )at calving and adequate energy reserves at
                                                               25
interval @unn and Kaltenbach, 1980; Dziuk                    parturition, optimal postpartum rebreeding effi-
and Bellows, 1983). Dziuk and Bellows (1983)                 ciency is uncertain unless postpartum changes
suggest a minimum body condition score of 25                 in BWlcondition are considered.
at calving. Richards et al. (1986) also support                  Body weight and condition score are useful
the concept of a target body condition score 25              indicators of energy status and rebreeding
at calving. These authors indicate that this                 performance after calving. Although both these
minimum body condition score will ensure that                measurements may be imprecise or subjective,
body stores of numents are adequate for                      these simple measurements remain as func-
postpartum reproductive performance.                         tional indicators for both producers and re-
    Other recommendations for predicting re-                 searchers. The mechanism(s) by which these
productive performance relative to BW or                     simple indicators affect rebreeding perfor-
condition are based on the time of mating                    mance remain unclear.
rather than on calving (Topps, 1977; Kdkenny,                    Dietary Energy and Pregnancy Rale. Preg-
 1978; Entwistle, 1983). Some controversy                    nancy rates of lactating beef cows and heifers
exists as to the ability of a target BWkondition             are affected by precalving or postcalving
to ensure adequate postpartum rebreeding                     energy intake (Table 1). Inadequate energy
performance in all production situations. Rich-              intake during late pregnancy lowers pregnancy
 ardson et al. (1976a) suggest that a curvilinear            rates even when energy intake is adequate
relationship exists between postpartum weight                during the postcalving period. Further declines
 change and rebreeding performance, which                    in pregnancy rate occur when lactating beef
 suggests that rebreeding performance is related             cows and heifers receive inadequate energy
 to actual BW at breeding rather than rate of                during the postcalving period. Pregnancy rates
 change in BW from calving to breeding. Rutter               in animals with restricted energy intake during
 and Randel (1984) reported that postpartum                   the postpartum period ranged from 50 to 76%
 interval decreased with increasing levels of                 compared with 87 to 95% in adequately fed




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                             NUTRITION AND POSTPARTUM REBREEDING                                     855
               TABLE 2. EFFECT OF PRECALVlNG DIETARY PROTEIN SUPPLEMENTATION
                             ON PREGNANCY RATE OF SUCKLED COWS
                                                                              ~




                                                        P                  Source
85”              71                                    <.I1                Wenemann et al., 1980
91               80                                                        Clanton, 1982
93               86                                                        Clanton, 1982
92               76                                    >.IO                Rasby et al., 1982
58               21                                    <.02                Mobley et al., 1983
84               12                                    <.o1                Garmendia et al.. 1984
39               12                                    <.07                Selk et al., 1985
92               80                                    >.lo                Selk et al., 1985
88               56                                    <.11                Heck and Lusbv. 1986
  “Percent pregnant.



herdmates. Hence, energy nutrition has a                1989). Inadequate protein intake during both
strong influence on pregnancy rates in lactat-          the prepartum and postpartum periods resulted
ing beef cows and heifers, and body condition           in a pregnancy rate of 32% in cows with low
(body nutrient stores) at calving interacts with        protein intake compared with 74% in cows
nutrient availability in the diet to influence          with higher protein intake receiving isocaloric
postpartum rebreeding performance.                      diets (Sasser et al., 1989). These data indicate
   Dietary Protein and Pregnancy Rate. Preg-            that protein nutrition influences pregnancy rate
nancy rates of lactating beef cows and heifers          in lactating beef cows and heifers. Because
may be affected by precalving and postcalving           pregnancy rates in most experiments with beef
protein intake (Tables 2 and 3). Data from              cattle are determined after a limited breeding
lactating beef cows and heifers that received           season, these fertility results are confounded
inadequate protein With various energy intakes          with the effects of nutrition on postpartum
during gestation have uniformly lower preg-             interval. Pregnancy rates therefore are a
nancy rates compared with herdmates receiv-             function of both conception rate and postpar-
ing adequate amounts of protein. Lower                  tum interval.
pregnancy rates were found in lactating cows               Nutrition and First-Service Conception
and heifers that received inadequate amounts            Rate. First-service conception rates are af-
of protein with various energy intakes during           fected by energy and(or) protein intake of
the postpartum period. Although the above               postpartum beef cows and heifers (Table 4).
diets were not isocaloric between groups, and           First-service conception rates were lower in
the data are confounded by differences in               p s p r u cows and heifers receiving inade-
                                                         otatm
energy intake, the conclusions agree with               quate energy than first-service conception rates
studies with isocaloric diets (Sasser et al.,           in adequately fed herdmates. A similar sup-


              TABLE 3. EFFECr OF POSTCALVING DIETARY PROTEIN SUPPLEMENTATION
                            ON PREGNANCY RATE OF SUCKLED COWS

           Protein status
Adeauate         Inadmuate                               P                  Source
94*              44                                    <.05                 Forero et al.. 1980
96               82                                    <.03                 Canuell et al., 1982
91               71                                    <.01                 Kropp et al., 1983
92               77                                    <.01                 Hanccck et al., 1984
95               80                                    <.01                 Hanccck et al., 1985
79               50                                    <.01                 Rakestraw et al., 1986
87               65                                    >.IO                 Rakesvaw et al., 1986
89               85                                    >.IO                 Rakesuaw et al., 1986




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856                                                      RANDEL

                  TABLE 4. EFFECT OF POSTPARTUM D[ETARY ENERGY AND PROTEIN STATUS
                   ON FIRST-SERVICECONCEPTION RATES IN SUCKLED COWS AND HEIFERS

Adequate              Inadequate                              P                   Source
                                                        ---- w   I---




Ma                    38                                    >.05                  Wiltbank et al.. 1%2
83                    54                                    <.05                  Wiltbank et al.. 1964
68                    56                                    C.05                  Dunn et al., 196'3
84                    66                                    >.05                  Somnville et al., 1979
66                    62                                    >.05                  Richards et al.. 1986
                                                        RDtein status
71                    25                                    <.05                  Sasser et al., 1989
      "Percent conceiving t first service.
                          o


pression in first-service conception rate (25 vs                  compared with moderate levels of dietary
71%) was found in beef cows receiving                             protein. The effect of excessive dietary protein
inadequate protein diets compared with ade-                       on serviceslconception in dairy cows is not
quately fed herdmates (Sasser et al., 1989).                      consistent (Table 5). Reports by Jordan and
Because first-service conception rates are                        Swanson (1979) and Folman et al. (1981)
suppressed similarly in both beef cows and                        indicate a tendency for excessive protein to
heifers, it appears that underfeeding energy                      increase the number of serviceslconception,
and(or) protein lowers fertility in cattle.                       but reports by Howard et al. (1985) and
Whether these effects are due to poor fertiliza-                  Aalseth et al. (1986) indicate that there is no
tion rates or to a lack of ovulation and(or)                      influence of protein excess on serviceslconcep-
subsequent luteal function is not known. When                     tion. Similarly, the data regarding the effect of
the combination of suppressed fertility and                       excessive dietary protein on days open in dairy
extended postpartum interval are combined,                        cows is not uniform (Table 6). Reports by
final pregnancy rates are lowered.                                some researchers would seem to indicate that
                               f
   Nutrition and Fertility o Dairy Cows.                          excessive dietary protein results in an increase
Dairy cows normally are maintained on a                           in days open and other do not. The reason for
plane of nutrition high enough to maximize                        the inconsistency of results of excessive
milk production. Problems with undernutrition                     dietary protein may be due to differences in
during the post partum period are less preva-                     the age of the experimental herds. Kaim et al.
lent for dairy cows than for beef cows (Dunn                      (1983) reported that fertility is impaired more
and Kaltenbach, 1980). Yet, reduced fertility                     by feeding excessive protein for older cows, in
has been reported by a number of researchers                      their fourth or later lactation than for younger
(Girou and Brochart, 1970; Treacher et al.,                       cows.
 1976; Sonderegger and Schurch, 1977; Jordan                         Although the mechanism by which high
and Swanson, 1979 Folman et a . 1981; Kaim
                                 l,                               levels of protein adversely affects reproduction
et al., 1983) in dairy cows that were fed                         in dairy cows is unknown, current efforts are
excessive (above 19% crude protein) amounts                       being made to quantitate the changes in the
of protein. Other reports indicate that high                      uterine environment induced by feeding exces-
levels of protein do not impair fertility                         sive amounts of protein. Carroll et al. (1987)


       TABLE 5. EmcT OF EXCESSIVE DIETARY PROTEIN ON SERVICES/CONCEPTION IN DAIRY COWS

               Rolein s u
                       ms
Normal                Excessive                                   P                Source
                                             ~~                                                         ~~   ~




1.9                   2.5                                        <.05              Jordan and Swanson, 1979
1.8                   2.2                                        >.05              Folman et al., 1981
2.0                   1.7                                        >.10              Howard et al., 1985
1.5                   1.7                                        >.lo              Aalseth et al.. 1986




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                                       NUTRITION AND POSTPARTUM REBREEDING                                      857
                TABLE 6. EFFECT OF EXCESSIVE DIETARY PROTEIN ON DAYS OPEN I DAIRY COWS
                                                                           N

~               ~~
                     Rotein status
                       ~




    Normal                 Excessive                              P                  Source
    140                    192                                   >.05                Treacher et al., 1976
    %                      106                                   >.05                Jordan and Swanson, 1979
    98                     102                                   >.05                Folman et al., 1981
    85                      81                                   >.10                Howard et al., 1985
    82                      Bo                                   >.lo                Aalsctb et al., 1986




reported that vaginal fluid was higher in urea                    and Randel, 1983). Cows managed in a system
nitrogen (20.9 vs 8.2 mg/100 ml) in dairy cows                    that resulted in short (30 to 50 d) postpartum
fed 20% CP than in those fed 13% CP. Cows                         intervals did not show much improvement in
conceiving at f i s t service did not have                        postpartum interval due to ionophore feeding
elevated urea nitrogen in vaginal fluid. When                     (Turner et al., 1977, 1980). Cows with
vaginal fluid urea nitrogen was 140 mg/100                        moderate (60 to 85 d) postpartum intervals
ml,no cow conceived. Perhaps high concentra-                      showed moderate improvement (Belcher et al.,
tions of certain metabolites or catabolites                       1980; Hardin and Randel, 1983), but cows
inhibit postpartum rebreeding performance in                      with extended (290 d) postpartum intervals
 cattle.                                                            Msn
                                                                  ( a o and Randel, 1983) showed the greatest
     Ionophore Antibiotics and Rebreeding Per-                    improvement due to feeding of the ionophore
fonnance. The ionophore antibiotics, monensin                     (Table 7). The effect of ionophore feeding was
 and lasalocid, improve feed efficiency in cattle                 not uniform with respect to pregnancy rates
by increasing ruminal concentrations of propi-                    (Table 8). Some experiments reported small
onic acid and reducing acetic and butyric acids                   increases and one reported a small decrease,
 without changing concentrations of t t l vola-
                                         oa                       none of which was significant. It would appear
 tile fatty acids (Raun et al., 1974; Dinius et al.,              from these data that the increased ruminal
 1976; Richardson et al., 1976b; Davis, 1978;                     propionate production found in cows fed
 Bartley et al., 1979; Thonney et al., 1981). The                 ionophore results in an earlier return to estrus
 increased ruminal concentration of propionate                    but does not consistently increase pregnancy
 is responsible partly for increased feed effi-                   rate.
 ciency observed with ionophore feeding, al-                         Nutrition and Ovarian Function. Underfed
 though some evidence points to improved                          lactating cows have extended periods of
 nitrogen utilization as well (Poos et al., 1979;                 ovarian inactivity. Postpartum rebreeding of
 Horn et al., 1980; Muntifering et al., 1980).                    the cow depends on recovery from the
 Feed efficiency is increased in beef cows and                    pregnant state, escape from suckling-induced
 heifers as well as in growing animals (Turner                    inhibition of gonadotropin secretion, initiation
 et al., 1977, 1980; Clanton et al., 1981; Mason                  of follicular development, occurrence of estrus


     TABLE 7. EFFECT OF IONOPHORE FEEDING ON POSTPARTUM INTERVAL IN BEEF COWS AND HEIFERS

             Postpamrm interval,d
    IonopbDrc              concsol                                 P                  Source
    30                      42                                   c.05                 Turner et al., 1977
    49                      48                                   2.10                 Turner et al., 1977
    59                      69                                   >.05                 Belcher et al., 1980
    67                      72                                   >.05                 Belcher et al., 1980
    41                      44                                   >.IO                 Turner et al., 1980
    65                      86                                   c.01                  adn
                                                                                      H r i and Randel, 1983
    92                     138                                   <.03                 Mason and Randel, 1983




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858                                                    RANDEL

 TABLE 8. EFFEn OF IONOPHOREFEEDING ON FINAL PREGNANCY RATE IN BEEF COWS AND HEIFERS

            Rerrnancy rate, 46
Ionophore            Control                                 P                  Source
 82                   78                                    >.IO                Belcher et al., 1980
I00                   84                                    >.05                Turner et al.. 1980
 %                   100                                    >.lo                Clanton et al., 1981
 %                    93                                    >.lo                Clanton et al.. 1981
 88                   82                                    >.lo                Clanton et al., 1981




with ovulation and adequate luteal lifespan for               GnRH (Rasby et al., 1986). Conflicting results
maternal recognition of pregnancy (Malven,                    were found with cows losing body condition
1984). Although underfed cows may not                         having a lower response to GnRH (Rutter and
exhibit ovarian activity, the numtional mecha-                Randel, 1984). Conflicting data may reflect
nism controlling ovarian activity may be                      differences in pituitary stores of LH and ability
having its effect on the hypothalamus, pituitary              to respond to GnRH.
gland or ovary (Entwistie, 1983). Thin Here-                      Diets that are low either in energy or in
ford cows (body condition 14) had lower                       protein may suppress pulsatile release of LH
ovarian, corpus luteum and follicular fluid                   and increase the releasable pool of pituitary
weights than did cows that were in moderate                   LH when given a pharmacological GnRH
(body condition 25) or better body condition                  challenge. Replenishment of pituitary stores of
(Rasby et al., 1986). Because ovarian function                LH may be a limiting factor in return to
is controlled by gonadotropin secretion from                  reproductive cyclicity after calving (Moss et
the pituitary gland, the site of numtional                    al., 1985). Postpartum interval and nutritional
influence on the ovary probably is located at                 status affect the pituimy content and concen-
the hypothalamic-pituitary axis.                              tration of LH and FSH, but do not affect the
    Nutrition and Pituitary Function. Low-                    number of GnRH receptors (Moss et al., 1985;
energy diets decrease mean serum concentra-                   Nolan et al., 1989). Compared with cows fed
tions of LH in postpartum cows (Terqui et al.,                adequate diets, cows fed diets deficient in CP
1980; Echtemkamp et al., 1982; Whisnant et                    had elevated pituitary LH and FSH concentra-
al., 1985). Mean serum concentrations of LH                   tions early in the postpartum period.
are lower in postpartum cows losing body                          Diets that are lower in energy, protein or
condition that in cows maintaining body                       both energy and protein lead to a lower
condition (Rutter and Randel, 1984). Cows fed                 pulsatile release of LH, which indicates that an
low-energy diets had suppressed pulsatile LH                  increased concentration of gonadotropin is
release on the day of calf removal as well as                 stored in the pituitary gland and can be
on d 1 of calf removal, but responses were                    released by GnRH challenge. These data
equal to those of cows fed a high-energy diet                 indicate that hypothalamic release of GnRH is
on d 2 of calf removal (Whisnant et al., 1985).               being suppressed.
Cows fed a diet deficient in protein failed to                    Nutrition and Hypothalamic Function. Cir-
have increased LH pulse frequency as postpar-                 cumstantial evidence that numtional status
tum intervals increased (Nolan et al., 1989).                 affects hypothalamic release of GnRH has
    The numtional status of the postpartum cow                been obtained by the use of estrogens to
alters pituitary release of LH following a                    mediate LH release (Echtemkamp et al., 1982;
GnRH challenge (Mason and Randel, 1983;                       Hardin and Randel, 1983; Nolan et al., 1989).
Rutter and Randel, 1984; Whisnant et al.,                     Although response to an estrogen stimulus
 1985; Rasby et al., 1986; Nolan et al., 1989).               varied and doses of estrogen varied between
Cows receiving low-energy diets had an                        experiments, all authors found that animals on
increased response to GnRH (Whisnant et al.,                  a lower plane of numtion had lower profiles of
 1985) that was similar to that of cows                       LH release than did animals on higher planes
receiving diets with inadequate CP (Nolan et                  of numtion. At early stages postpartum and
al., 1989). Cows receiving diets low in both                  with more dramatic dietary resmction, com-
energy and CP also had a greater response to                  plete failure to respond to a challenge of 1 mg




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                           NUTRITION AND POSTPARTUM REBREEDING                                      859
estradiol-17P was noted (Nolan et al., 1989).           estrus and formation of corpora lutea (McClure
The reduced ability of cows receiving diets             et al., 1978). Postpartum cows that were
with lower energy, protein or energy and                induced to exhibit glucosuria and to become
protein to respond to estradiol suggests that the       hypoglycemic by infusion of phlorizin had a
hypothalamic response to estradiol is altered           depression in amplitude of LH pulses (Rutter
by nutrition. Perhaps this decrease in hypotha-         and Manns, 1987). Experiments using glucose
lamic responsiveness is due to a decrease in            infusions (McCaughey et al., 1985; Garmendia
estradiol receptors as well as faulty synthesis,        et al., 1986) suggest that in beef cows insulin
storage and secretion of hypothalamic GnRH.             increases and lipolysis decreases in response to
The site of nutritional control over postpartum         glucose infusion in an attempt to maintain
rebreeding in cattle could be at the hypothala-         relatively constant glucose concentrations. This
mus.                                                    tendency of the animal to maintain relatively
   Blood Metabolites and Metubolic Hor-                 constant glucose concentrations in the blood
MOWS. Measurement of blood metabolites may              may explain partially the lack of a relationship
be useful as an indicator of nutritional status         between circulating glucose concentrations and
and subsequent rebreeding performance. Con-             reproductive performance. Further, circulating
centrations of blood glucose during the post-           glucose concentrations may modulate repro-
partum interval were correlated (r = .51) with          duction at a threshold level with no advantage
conception rate during the 1st yr, but not              above the threshold but deleterious results
during the 2nd yr (Selk et al., 1985). Russel           below this threshold concentration. Changes in
and Wright (1 983) measured plasma glucose,             gluconeogenesis may be more important than
3-hydroxybutyrate and nonesterified fatty acids         circulating glucose concentrations alone in
at various stages of the reproductive cycles of         explaining the mechanism by which nutrition
cows with different nutrient intakes. Glucose           alters reproductive performance in cattle.
was not useful in evaluating energy status,                 Gluconeogenesis. Ruminants absorb only
3-hydroxybutyrate was useful in pregnant                limited quantities of glucose from the gastroin-
cows, and nonesterified fatty acids were useful         testinal tract. The primary energy substrates
both in pregnant and nonpregnant cows. When             absorbed are volatile fatty acids, and propio-
sampling schedules were intensified to produce          nate is the primary gluconeogenic volatile fatty
24-h profiles, greater concentrations of nones-         acid absorbed (Bergman, 1973). Abomasal
terified fatty acids, P-hydroxybutyrate and             infusion of propionate enhanced blood glucose
growth hormone and lower insulin were found             concentrations and release of LH following a
in high-producing dairy cows (Hart et al.,              GnRH challenge in prepuberal heifers (Rutter
1979). The relationship between increased               et al., 1983). In a previous study, prepuberal
growth hormone and decreased insulin during             heifers that were fed protein-protected lipids to
early lactation suggest a role for the metabolic        increase ruminal escape of energy reached
hormones to promote mobilization of adipose             puberty at an older age and heavier weight
tissue stores to fulfill energy needs (Hart et al.,     than did heifers receiving a normal diet
 1978; Vasilatos and Wangness, 1981; Kunz et            (Rhodes et al., 1978). Feeding of ionophores
al., 1985). Postpartum beef cows have negative          (Moseley et al., 1977; McCartor et al., 1979)
correlations between GnRH-induced release of            or altering the concentrate to roughage ratio
LH and plasma urea nitrogen, plasma albumin             (McCartor et al., 1979) to favor propionate
and serum glutamic oxaloacetic transaminase             production decreased age and weight at pu-
(Nolan et al., 1989). Measurement of metabo-            berty in heifers. Similar results have been
lites and metabolic hormones is one method to           noted when ionophores were fed to bulls
detect mobilization of adipose tissue and               (Neuendorff et al., 1985). Thus, the form of
protein catabolism during nutritional stress.           energy supplied and the amount of absorbable
Intravenous infusion of glucose in postpartum           metabolites are critical. Ionophore feeding to
beef cows has not established a direct role for         increase ruminal concentrations of propionate
increased glucose in enhancement of postpar-            increases the ovarian response to endogenous
tum rebreeding (McCaughey et al., 1985;                 and exogenous gonadotropins (Bushmich et
Garmendia et al., 1986). Yet, glucose appears           al., 1980) and enhances release of LH follow-
to be necessary for reproductive function in            ing a challenge with either GnRH (Randel and
cattle because the metabolic inhibitor,                 Rhodes, 1980) or estrogen (Randel et al.,
2-deoxy-D-glucose, prevents occurrence of                1982) in prepuberal heifers. Postpartum beef




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860                                                     RANDEL

cows fed an ionophore had decreased release                         rumen fermentation and feed efficiency. J. Anim. Sci.
of LH following GnRH challenge (Mason and                           491066.
                                                               Belcher, D. R., R.D. Wyatt, S . W. Coleman, R. L. Hinlz. G.
Randel, 1983) and enhanced release of LH                            L. Crosthwait and S. L. Armbruster. 1980. Effect of
following an estradiol challenge (Hardin and                        monensin on reproductive performance and winter
Randel, 1983) and returned to estrus earlier in                     weight change of fall-calving, first-calf heifers.
both experiments. Body condition scores of                          Oklahoma Agric. Exp. Sta. MP-107:68.
                                                               Be.Uows, R.A. and R. E. Short. 1918. Effects of precalving
ionophore-fed cows and control-fed cows did                         feed level on birth weight, calving difficulty and
not appear to differ, and ionophore-fed bulls                       subsequent fertility. J. Anim. Sci. 46:1522.
and heifers reach puberty at an earlier age and                Bergman, E. N. 1973. Glucose metabolism in ruminants as
at a decreased weight; perhaps the increased                        related to hypoglycemia and ketosis. Cornell Vet. 63:
ruminal propionate had favorable metabolic                          341.
                                                              Bowden, D. M.. R. Hironaka, P.J. Martin and B. A. Young.
and endocrine effects.                                             1979. Feeding beef cows and heifers. Agnc. Can. Publ.
    From these data it could be postulated that                    1910.
an increase in gluconeogenesis from propio-                   Bushmich, S.L.. R. D. Randel, M.M. McCartor and L. H.
nate, which would result in decreased glucone-                     Carroll. 1980.Effect of dietary monensin upon ovarian
ogenesis f o amino acids, is detected at the
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