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:
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).
|Breeding Age (days)||< 200||200-210||210-220||220-230||230-240||240-250||250-260||> 260|
|* 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.
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;
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.
Britt JH. New technologies to increase litter size. Proceedings of The Swine Reproduction Symposium. Sponsored by the American Association of Swine Practitioners and Society for Theriogenology. Kansas City, MO, 1996.
Jindahl R, Cosgrove JR, Aherne FX, Foxcroft GR. Effect of nutrition on embryonal mortality in gilts: Association with progesterone. J. Anim. Sci. 1996;74:620.