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Manitoba Agriculture, Food and Rural Initiatives

Managing Efficiency of the Breeding Gilt


Roy N. Kirkwood and Frank X. Aherne, Alberta Pork Research Centre, Alberta Agriculture, Food & Rural Development, 905 O.S. Longman Building, 6909-116 Street, Edmonton, Alberta T6H 4P2 

Introduction

The foundation for cash flow and long-term profitability in pork production resides with the breeding herd and, within the breeding herd, the ability to meet breeding targets will have the greatest impact on the numbers and variance of pigs produced. Since efficiency of the breeding herd is in large part driven by gilt management, the areas we will address are:

Index:

Management of the Gilt
Nutrition and Development
Stimulation of Early Estrus
Breeding Management
Conclusion


 Management of the Gilt

The difference between new and established operations is a function of numbers of gilts. The management requirements of the gilts, per se, are not different. However, before addressing specifics of gilt management, we need to appreciate what we want to achieve. In our opinion, the only purpose of a gilt is to generate revenue. Therefore, the only objective of gilt management is to cost effectively produce prolific and long-lived breeding females, ie. maximize lifetime productivity and minimize non-productive days.

A gilt management strategy has the following aims:

• After entry into the breeding herd, minimizing entry-to-service intervals. This ensures that non-productive days are minimized.
• To have a supply of service-ready gilts available for breeding as required to meet breeding targets. This ensures that the supply of breeding animals will not form a constraint in the pork production process.
• To maximize subsequent reproductive performance and longevity. This ensures that production efficiency (eg. pigs/crate/year) does not negatively impact on pork production.

Nutrition and Development

Currently, there is much interest in the nutrition of gilts during rearing, ie. nutritional conditioning prior to breeding. However, most producers purchase gilts near market weight which leaves little opportunity for nutritional intervention and the major decision is whether to breed sooner or later. For those with the capability for in-house replacement or that purchase very young replacements, an often asked question is how do you feed the gilts? Different advisers and breeding companies have different opinions and these opinions likely are not appropriate for each genotype. This lack of consensus likely reflects the probability that there is not one correct answer and that the best gilt nutrition program is genotype/farm-specific. In other words, our interpretation of the available literature is that (excepting extremes) age and/or weight at first breeding will have little or no impact on the lifetime performance of an incoming gilt. Similarly, unless extreme, backfat depth and estrus of mating will have little impact on lifetime performance. It is the management of the gilt through the first pregnancy and lactation that will have by far the greatest impact on lifetime productivity. Therefore, the question to be answered is; how do the gilts of your genotype perform (in terms of body condition maintenance and lifetime productivity) under the conditions of your farm environment and management? Only when this information is known can you attempt to decide the gilt nutritional regimen required to achieve your farm-specific target age, weight, backfat depth, or estrus number to breed at.

Stimulation of Early Estrus

To minimize entry-to-service intervals, it is necessary to have gilts reach puberty as soon as possible after arrival on farm. This is independent of whether the gilt enters an isolation facility or directly enters the breeding barn since the decision on when to breed gilts (in terms of age, weight, backfat depth, estrus number) is a separate issue. The ability to meet breeding targets requires a predictable supply of service ready gilts which is best realized by having gilts achieve an early puberty, regardless of when they are to be bred. Two methods to stimulate an earlier onset of estrous cycles are appropriate boar exposure and the injection of exogenous gonadotrophins.

Boar exposure is the most common practice for inducing early puberty. However, it is important to understand the difference between estrus stimulation and estrus detection. Adequate stimulation requires direct physical contact while detection may only need fenceline contact (although direct physical contact is better). If the efficacy of boar exposure is questioned, it is important to evaluate whether the rules of boar contact are being followed.

The first four of these rules must be strictly adhered to. The latter two, while desirable, are frequently difficult to follow in commercial practice. Other variations, such as using different boars on different days or housing gilts with a boar continuously, will probably not improve the response over that obtained by daily contact with a single boar of at least 10 months of age. Although not absolutely necessary, the stimulus boar should be vasectomised or otherwise rendered sterile. This allows for gilts to be bred and so removes the requirement to separate estrus gilts from their pen mates before boar exposure. Also, if using artificial insemination, a prior sterile mating will enhance the reproductive performance of gilts after a fertile breeding.

As stated earlier, occasionally boar exposure appears not to be effective. If, as far as possible, the rules of boar exposure are being followed, then an intervention strategy must be considered. Gonadotrophin treatment (eg. PG600) is effective for the induction of estrus and ovulation. However, research and clinical experience demonstrates that up to 30% of treated gilts may not show behavioral estrus (but will probably ovulate) and about 30% of those having a behavioral estrus may fail to cycle normally. Failure of normal cycles can be minimized by continuing boar exposure during the induced cycle. Due to potentially unpredictable responses we recommend that, if inducing with PG600, gilts should be bred at the induced estrus but expect some possible depression in farrowing rate and/or litter size (Table 1).

Table 1. Performance of gilts bred at a PG600-induced or natural first estrus (lsm ? se).
Control PG600 P
Service ready gilts, % 37.5 78.0 0.0001
Farrowing rate, % 88.6 74.4 0.01
Litter size (total) 9.7?0.3 9.4?0.3 NS
Pigs/entered gilt 3.22 5.46

Kirkwood (1999)

If gilts are known to be post-pubertal, the options for control of estrus onset are limited to breed-and-abort and the feeding of the orally active progestagen, allyl trenbolone (Regumate). Note that, unlike cattle, a single injection of PGF2a will not induce luteolysis before day 12 of the estrous cycle so is of limited value. However, if required, estrous cycle length can be reduced by 2 to 5-days by injecting prostaglandin every 24 or 36-hours between days 6 an 10 of the cycle.

Breed-and-abort is effective but of questionable value economically, since there is no return to the costs associated with breeding. The feeding of allyl trenbolone is an effective means of controlling estrus. While being fed, allyl trenbolone does not prevent normal luteolysis but will maintain the block on estrus onset after luteolysis occurs. Ideally, gilts should be individually fed so that they consume at least 15 mg/d. While there is likely no problem with overdosing (except economic), underdosing allyl trenbolone (<13 mg/d) may cause cystic follicles. Since allyl trenbolone needs to be fed only from the time of luteolysis, if cycle dates are known you can minimize its feeding by only providing it from day 14 of the estrous cycle until 5 days before gilts are scheduled to be bred. Expect 90 to 95% of gilts to achieve estrus on days 4 to 8 after last feeding.

In practice, an often encountered problem is that some proportion of incoming gilts (5 to 15%) fail to show estrus within a reasonable time period after entry (eg. 28-days). It has been suggested that these gilts are likely either having a silent estrus or are prepubertal but relatively infertile. In either case, an argument can be made that they should be culled. However, where pressure exists to keep them but the use of Regumate as outlined in the previous paragraph is not possible, then they can receive a "last chance" injection of PG600 and be bred at the induced estrus. Thereafter, our advise is that any gilt failing to exhibit estrus by 7-days after PG600 should be culled. If a well developed gilt fails to show a natural estrus and than subsequently fails to respond to gonadotropic stimulation should not be considered to be a reasonable candidate for becoming a productive and profitable sow. Also, arguably, any bred gilt failing to conceive to the PG600-induced service should be culled as infertile.

Breeding Management

In grossly simple terms, successful breeding management requires the deposition of an appropriate number of fertile sperm into the reproductive tract of a fertile sow at the appropriate time during the estrous period. This paper will not address sow, boar or extended semen fertility but will focus on the timing of breeding since this is most frequent breeding problem encountered in practice.

Based on studies employing artificial insemination, to maximize both fertility and fecundity sperm deposition should occur about 10 to 12 hours before ovulation, with good performance observed if deposition occurs during the 24 hour period before ovulation. However, at best, the time of ovulation can only be determined retrospectively. Recent studies using gilts have indicated that, based on four times per day estrus detection, ovulation occurred about 80% through the estrous period. However, as the number of daily estrus checks decreases (four, twice or once daily) the mean and variance of time of ovulation relative to onset of estrus decreases and increases, respectively. With four daily estrus checks, 85% of gilts ovulated between 40 and 50 hours; with twice daily estrus checks, 83% ovulated between 35 and 45 hours; with once daily estrus checks, time of ovulation was evenly distributed between 25 and 45 hours. The implication for gilt breeding management is that with once daily estrus detection, gilts should be bred not later than the afternoon of the day of detection and again the following morning and afternoon (assuming a good standing response is elicited at each breeding). With more frequent boar exposures, the timing and number of breedings can be refined.

In many instances, the reproductive performance of females bred by artificial insemination is poorer than that achievable with natural breeding. The reason why some females perform relatively poorly following artificial insemination is not fully understood. However, it is reasonable to suggest that the chemical and hormonal content of natural seminal plasma has a functional role and the absence of these chemicals and hormones from artificially extended semen, at least in part, results in the performance depression.

In support of the above suggestion, previous work has shown that reproductive performance may be improved with the addition of hormones to extended semen (eg. estrogen or oxytocin). The possible mechanism involved may be that the seminal hormones induced a uterine release of PGF2a which in turn induced a uterine oxytocin release. These two hormones will act to stimulate uterine contraction which may enhance sperm transport. Additionally, the PGF2a can be transported to the ovarian follicle and act directly to facilitate the process of ovulation. Indeed, it has been shown that natural breeding may advance the onset of ovulation. What is likely occurring is that ovulation is advanced only in those females that would otherwise be late ovulators (eg. estrus to ovulation intervals >48 hours). The consequence of this will be improved synchronization (ie. reduced variance) between sperm deposition and ovulation and, therefore, a potential improvement in fertilization and conception rates.

It has also been shown that an injection of PGF or oxytocin at the time of AI will enhance reproductive performance following artificial insemination. It is possible that this effect is, again, mediated by improved synchronization between insemination and ovulation as a result of the exogenous hormone substituting for the effect of hormones missing in extended semen. Clinical experience with two 1,200 sow start-up herds supports the beneficial effect of oxytocin. Both reported a 5% increase in farrowing rate. Although oxytocin is very inexpensive, and therefore cost-effective, its use has been associated with adverse behavioral sow reactions (mild to extreme agitation) in 10 to 15% of sows, reflecting the potency of this hormone and the likelihood of pain associated with its use. If adverse sow reactions are encountered, a low dose (25 to 50%) of PGF intravulvally should be substituted for the oxytocin which should provide the same benefit.
 

Table 2. Effect of inseminator experience and oxytocin on sow performance
  Farrowing rate (%) Litter size (alive)
Inexperienced 77 9.4
Inexperienced + oxytocin 89 10.2
Experienced 86 10.1
Experienced + oxytocin 91 10.5

Flowers (1996)

Conclusion

An important component of breeding herd efficiency is appropriate gilt management. This requires an early onset of puberty. When to breed a gilt for optimum lifetime performance is farm specific and can only be determined after analysis of farm specific records. Breeding management can impact farrowing rate and first litter size. The timing of breeding during estrus will depend on the intensity of estrus detection. The choice of natural or artificial breeding will depend on available on-farm expertise. Hormones can be used to partially offset the relatively poor performance occasionally observed when first implementing artificial insemination. However, this should be viewed as a short-term strategy. If gilts continue to demonstrate poor performance to artificial insemination, breed gilts to boars and reserve artificial insemination for sows.