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                              GILT DEVELOPMENT AND MANAGEMENT

                                                Glen Almond and John Deen
                                               College of Veterinary Medicine
                                               North Carolina State University
                                                     Raleigh, NC 27606


Introduction

Gilt development and management represents one of the most critical factors affecting the reproductive
performance of a sow herd. With replacement rates of 50%, gilts are integral to production. A gilt replacement
program optimizes the number of gilts maintained, has the appropriate number of cycling gilts available when
needed and minimizes the cost of feed, labor and facilities for open and bred gilts. Problems in gilt management
result in the following repercussions:

      Inadequate number of available, cycling gilts interferes with culling programs. Problem sows are kept to
       maintain breeding group size.
      Gilts with delayed puberty are often kept for prolonged periods and become a heavy pig (300 lb.), which
       is difficult to market. The extra feed, facility and labor costs are not recovered.
      Gilts bred at a late stage have increased costs of rearing.

This paper is not intended to review the numerous studies regarding specific factors involved in gilt
development and management. Hopefully, we will raise challenging questions regarding a few, important
factors that emphasize the critical significance of gilt pool management.

Age at First Mating

The influence of gilt breeding age on lifetime performance was the focus of countless studies. The results are
conflicting, particularly, when one considers the different genetic lines, rearing conditions, breeding schemes
and facilities used in the investigations. One study, conducted by the Pig Improvement Company, assessed
breeding age on lifetime performance of Camborough 15 gilts (Table 1).

 Table 1. Influence of gilt breeding age on lifetime performance (Pigtopics 1995; 14(4):1-4) Age (days) at
                                                   Breeding

                                                   200-    210-      220-      230-      240-     250-
            Breeding Age (days)         < 200                                                               > 260
                                                   210     220       230       240       250      260
            No. Gilts                   1105       1176    1266      1459      1129      925      607       1022
            Parity/Lifetime             5.10       5.05    5.18      5.10      5.14      5.27     5.16      5.07
            Born Alive/Lifetime         48.9       48.8    49.9      49.0      50.1      51.8*    49.8      49.1
            * Born alive/lifetime for 240-250 day gilts differed from gilts bred at < 200 days, between 220-230 days
            and > 260 days.
This study indicated that age at first breeding resulted in no significant differences in regard to parity/lifetime
(i.e. longevity) and few differences in productivity (i.e. pigs/lifetime). One needs to be critical when interpreting
the meaning of these results. For example, what is the additional cost of keeping a gilt until she is 240-250 days
of age to gain 1-2 pigs/lifetime? How many producers intentionally wait until a gilt is 260 days of age before
she is mated? Another way to interpret the information is that a producer can mate gilts on first estrus (< 200
days) with minimal loss (just 2 pigs!) of lifetime performance. Because so many other farm-specific factors
influence the reproductive performance of sows, it is almost impossible to make generalizations about long-
term influence of age at first mating.

Feeding the Gilt to Improve Ovulation Rate and Embryo Survival

Restricted feeding versus ad libitum feeding? An excellent topic for debate! Generally, energy intake is
considered critical for about 2 weeks before estrus (flushing), immediately after breeding and during the rearing
stage for gilts that are not intentionally flushed (Britt, 1996). Results from a recent study indicated that feeding
level after mating plays an important role in embryo survival (Jindal et al., 1996). Gilts were fed ad libitum until
1st estrus and then approximately 2.5 kg/day/gilt (7 Mcal DE/day) until one day after the 2nd estrus. During the
2nd estrus, gilts were bred twice by AI. One group (Group N1) of gilts were fed the NRC recommended feed
allowance (1.5 x maintenance; 1.9 kg/day or 5.7 Mcal DE/day) for pregnant gilts on day 1 until day 15 post
mating. The second group (Group N3) were fed the 1.5 x maintenance quantity starting from day 3 to day 15
post mating. The last group (Group H1) were fed an allowance of 2 x maintenance (7.8 Mcal DE/day) from day
1 to day 15. From day 15 onward, all gilts were fed a standard allowance of 1.8 kg of feed/day until they were
slaughtered between days 25-31 of pregnancy. The effects of the feeding schemes on ovulation and embryo
survival are given in Table 2.

 Table 2. Influence of feeding pattern after mating at 2nd estrus on ovulation rate and embryo survival
                                    (Adapted from Jindal et al., 1996).

                                                                Total Embryo Survival
                                  Group        Ovulation Rate
                                                                        (%)
                                    N1           14.50 + .38          86.0 + 4.5
                                    N3           14.95 + .42          77.0 + 6.0
                                    H1           14.95 + .38          67.0 + 8.0


N1, N3 are gilts with feed intake reduced to 1.5 x maintenance from d 1 and 3 of pregnancy, respectively. H1
gilts received 2 x maintenance from day 1 of pregnancy.

The obvious implication is that if flush feeding is used to increase ovulation rates in gilts, this management
practice must be discontinued immediately after mating to avoid negative effects of increased feed intake on
embryonal survival.

Influence of the Gilt Pool on Productivity

The number of pigs weaned/week is considered an important economic parameter for sow farms. Several
factors influence pigs weaned/week; however, recent studies at Minnesota have attempted to determine the
relative importance of each factor. Pigs weaned/litter was more important in weaned pig output on 1000-sow
farms than on 250-sow farms. One major conclusion was that the number of females served/week had a
tremendous influence on pigs weaned/week. The number of services/week depended on the number of gilt and
weaned sow services, with repeat breeders and delayed returns representing minor factors. It should be no
surprise that if a producer fails to reach breeding targets, then the pigs weaned/week will suffer 19-20 weeks
later.
The aforementioned observations stress the importance of gilt pool management. The demand for gilts is
determined by the breeding target and the supply of sows to be rebred. Thus, the demand for gilts varies greatly
from week to week, especially, due to changes in the characteristics of each weaned group. The total sows
weaned, minus the involuntary and voluntary culls represents the sow availability. For each breeding week, the
difference between the breeding target and the available sows is the weekly demand for gilts. Gilt demand
varies considerably from week to week on most farms. This variation causes the problem of hitting the "moving
target", i.e., how many gilts to breed each week. Another problem is not having enough gilts to be bred, when
required to meet the breeding target. Therefore, producers must consider the basic economic rules of breeding
gilts;

      breed any gilt to meet the breeding target
      the cost of breeding a gilt early must be balanced against the cost of retaining a voluntary cull sow
       (breed a gilt early or keep a sow of questionable productivity)
      the ability to retain and acclimatize gilts is a function of the number of gilts delivered and frequency of
       delivery.

Summary

Management of gilts is often neglected. Prior to entry into the breeding herd, a gilt typically is treated the same
as a finishing pig. A scarcity of scientific information is available to support the concept that rearing
environment affects the subsequent performance of a gilt. However, recent studies at NCSU (Jeff Armstrong
and K. Swanchara) indicated that reduced insulin-like growth factor-I (IGF-1) concentrations during the grow-
finish stages, impaired ovulation rates in post-pubertal gilts. The IGF are important factors associated with feed
intake and growth in pigs. In view of these recent findings, it is evident that early development, perhaps as early
as nursery age, does influence subsequent reproductive potential.

There is a strong association between culling rates and gilt replacements or gilt demand. High culling rates
increase gilt demand and conversely, lack of gilt availability hinders management decisions to cull sows with
questionable productivity. Gilt pool management depends on many farm-specific factors and a general
recommendation for all farms would not be appropriate.

Take-Home Message

      Gilts are fundamental to sow-farm productivity
      Pay careful attention to feed intake of gilts after mating.
      Ensure that inadequate gilt availability does not interfere with meeting your breeding targets.

								
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