Inducing and Synchronizing Estrus in Pre-Pubertal Gilts

Swine Webinar Series Inducing and Synchronizing Estrus in Pre-Pubertal Gilts Rob Knox, Ph.D. Swine Reproductive Specialist Department of Animal Sciences University of Illinois Urbana IL USA rknox@illinois.edu Introduction Expression of pubertal estrus soon after selection of gilts based off of their age and weight and the application of puberty induction methods would facilitate breeding herd management. Further, it would be of great value if estrus lasted two days and if the cycles would repeat at 21 day intervals until breeding was desired. However, despite this wish list, the reality is that gilt puberty is often spread out over 60 days or more, with some gilts never expressing puberty before 300 days of age. In addition, the symptoms of heat are highly variable for duration of heat, reaction to human application of stimuli, and vulval swelling or color. Puberty in Gilts Puberty is a complex process. It is based on the maturation of the neural pathways associated with the hypothalamic-pituitary axis (HPX). This axis is responsible for release of hormones that must pulse, that must respond to positive hormonal feedback and also to external stimuli. The positive feedback stimulus originates from the estrogen from the immature ovarian follicles. Puberty can also be uncertain as to true status since true fertility would be classified by estrus, ovulation and normal cycling activity or the ability to induce these states. However, estrus and ovulation in pubertal gilts are not always associated, and cyclic activity does not always follow induction. Studies have shown that in L X LW gilts, in tropical conditions, estrus and ovulation are linked in 34% of gilts, ovulation without estrus in 21%, and estrus without ovulation in 45% of gilts [1]. Natural puberty results from a process of physiological maturation. Age of maturation is responsive to genetics, environmental cues, and internal physiologic signals. The presence of medium- sized estrogenic follicles is evident after 100 days of age. But pulsatile LH release is not evident in most commercial line gilts until after 150 days of age. The proportions of gilts with larger follicles and pulsatile LH capability increases with chronological age and physical body weight. Induction of Puberty in Gilts Induction of puberty can ensure breeding of a mature gilt that would have the potential to farrow, and produce a larger litter. This would result from a heat no service event and the cycle in which exposure to normal patterns and levels of HPX hormones, would result in production of high levels of ovarian estrogen and progesterone and other hormones that would feedback to the HPX to finally mature the reproductive center, and to allow maturation of the ovary and uterus to increase ovulation rate and growth of the uterus. There are limited methods for inducing puberty in pigs but most are additive. These include boar exposure, which can induce 20-40% of gilts in a 10 day period, relocation or regrouping stress, each which can induce 5-10% of females, and gonadotropin hormone treatment, such as with PG600, which 1

can induce 50-90% within a 10 day period. Of course, all responses depend upon the age and physiological stage of maturity for the gilts. Some of the factors that influence puberty in pigs include genetics. Approximately, 40% of the variation in age at puberty is attributable to genetics, and direct selection has been used to reduce pubertal age by 20 days or more. Breed influences puberty, with the greatest difference noted between the Meishan pig which averages 100 days to maturity while occidental breeds show 200 day averages until puberty. A review of records suggests that in the last 50 years, most common pigs breeds used in commercial production have average ages of puberty between 145 to 228 days [2] with an average of ~205 days of age. The variation ranges by and within breed the average can vary by 25-50 days. Heterosis can also be used to reduce pubertal age by 10-20 days depending upon genetics utilized. A study comparing several lines of maternal crossbred females [3] in 2004 showed age at pubertal estrus ranged from 209 to 225 with percent of gilts pubertal by 300 d of age ranging from 97% to 87%. Factors such as gilt nutrition, diet, growth, and body composition can all impact pubertal age and data from Canadian researchers suggests that gilt body growth rate can be important to puberty and gilts should be selected for growth >600g/day [4-7] to ensure age at puberty less than 200 days of age and a high proportion of gilts cycle before this age [8]. Boar exposure, above all others appears to have the greatest impact on puberty induction and increases LH in gilts for ~10 days following exposure [9]. Boar exposure is most commonly delivered to gilts in pens but stalls have been used as well. Research suggests that moving females to the boar housing area improves induction and detection rates but moving boars to the gilt housing areas can work well also. Data suggests physical boar exposure is a stronger stimulus for puberty [10, 11] compared to fenceline, while other research shows little effect of method [12]. However, the results could depend upon many factors in the method of delivery such as duration of exposure, age at start of exposure, and frequency of exposure. Age at start of boar exposure can influence age at puberty and synchrony of estrus. The earlier boar stimulation begins, the earlier puberty will be induced in the population. However, pubertal onset will be spread over a greater period of time. As the age of puberty is delayed, greater synchrony is achieved (figure below). This has been tested and shown that induction starting at 180-200 d of age, allows an average age to estrus of ~10 days in 68% of gilts [13]. Patterns for estrus induction in gilts have been reported when boar exposure is initiated at 160 days of age. In this data set, 67% of gilts display estrus in 20 days, 84% in 30 days, and 93% in 40 days from start of exposure [5] but this may vary from herd to herd and batch 2

to batch as other data sets reveal that boar exposure to gilts at 180 days of age is effective for induction but >25% of gilts still may fail o express estrus within 60 days [14]. A clear example of batch variation in gilt induction responses was shown [15] where gilt regrouping, relocation, and boar exposure revealed induction responses within 25 days averaging 20%, 50% or 70%. The reason for the variation is uncertain but it others factors that could contribute to variation in response could be related to health, season, space allowance, environmental variation, and boar exposure quality. Boar exposure variables and effects on puberty have been reviewed [16]. Boars with lower libido are shown to have less of an effect on puberty advancement within 40 days [11]. Duration of boar exposure on age at puberty and percentage of gilts expressing estrus within 60-80 days shows mixed results with no clear advantage to extended times greater than 5-10 minutes [16] to an advantage of 10 to 30 minutes of daily exposure and continuous exposure with additional exposure [17]. However, the number of gilts and the space allocation could impact the responses. There is data to support the beneficial effect of twice or three times daily exposure compared to only once daily exposure on age at puberty and in some cases percent of gilt pubertal within 60 days [18]. Studies by Hughes et al. [19] suggests some interaction of boar exposure frequency and transportation when gilts are exposed to boars once or three times daily following transportation of gilts for 20 minutes for 10 days. There was a reduced age at puberty when using once or three times daily boar exposure and a reduced age at puberty when transport was applied to 3X daily contact. There was little effect in this study of once daily exposure over three times daily exposure alone though. Gilt group size was also reported to impact the proportion of gilts that were cyclic by 270 days of age. There was little effect in group sizes from 9-27 gilts but with only 3 gilts per group, estrus was lower. Large groups >40 may be a concern due to the inability to effectively deliver boar exposure to all females in a reasonable period of time [11]. Floor space for gilts has also been shown to have an effect. When floor space/gilt was < 1.13 m 2 (12 sq. ft), age at puberty and gilts pubertal by 200 days was reduced [20]. Sterle and Lamberson [21] investigated the impact of an estrogenized female continuously housed with gilts when compared to boar daily exposure. The estrogenized did not effectively induce puberty but did have an effect on a small population of gilts. This could suggest some value of maintaining estrual gilts in with non-pubertal pen mates to aid in induction responses. Hormonal Induction of Puberty in Gilts Induction of puberty using hormones has been reviewed [22, 23] and is most commonly performed using PG600 which contains a combination of PMSG and hcg although in some countries PMSG alone is approved. However, in most cases hcg or some other hormone such as plh or GnRH is used to induce ovulation. PG600 is used and has been well characterized and studied for use as an induction agent. A summary of numerous studies indicates that 60-70% of treated gilts express estrus within 5 days of treatment, with estrus lasting 1.5 days, 75% of gilts ovulating 13 eggs. The treatment is typically injected intramuscularly, where the hormones diffuse into the blood and act at the ovary where they bind to medium sized follicles to stimulate growth and development of a cohort of follicles that produce estrogen and result in the symptoms of pubertal estrus. PG600 comes in single doses or 5 dose packages. It is in lyophilized form and is reconstituted with supplied diluent. The drug should be stored undiluted until use but if diluted, should be used within days. When reconstituting the hormone, transfer losses of hormone between bottles should be avoided as loss of hormone will reduce the effective dose provided. Injection of PG600 should be placed i.m. or s.c. and not in the fat or in the skin. To avoid leakage of dose, use 18 g needles that are 38 mm in length (1.5 inches). Needles that are too thin, may bend and break if the gilt moves, while needles that are too large may allow leakage. Injection failures may also occur from multi-dose syringes that do not allow for 3

precision in dosing volume. Use a 5-10 cc syringe for greatest accuracy. When giving injections, to avoid drug loss, pull back slightly on syringe to test for blood, and if no blood is pulled, then administer injection dose. In cases where excessive injection losses occur, try gilt restraint to aid in administration. To improve responses to PG600, differences in age between 160-180 day old gilts had little effect, but boar exposure for 30 minutes/day for four before PG600 increased the estrus induction response by 10-20% [12]. PG600 is approved for i.m. use in the USA but i.m. or s.c. use elsewhere. Comparison of route has been shown in some instances to improve the estrus response by 10-20% [24]. A report on the use of PG600 in a large commercial system showed that 170 day old gilts exposed to boars for 23 days, and which failed to express estrus during that period, were injected with PG600 at ~193 days. The response showed the majority of gilts expressing heat on day 4 with 75% in heat on d 3 to 5 [15]. Gilt growth rate and body weight may play a role in PG600 response. A study feeding three different amino acid to energy levels to gilts from day 100 to day 175 reveled slow (< 600 g/days) and adequate growth rates (>600 g/day). PG600 given at day 175 showed estrus induction rates >90% in the high growth rate and diet gilts, while estrus rates were < 70% in slow growth gilts on a low amino acid:energy diet (Knox, unpublished). Estrus induction with PG600 results in induction of estrus within 3 days but synchrony changes in subsequent cycles. The synchrony of estrus is spread by the second cycle over 7 days 25-30 days following the initial injection, and is spread by 10 days or more by the time of the third cycle. However, estrus expression rates may be higher in subsequent cycles as maturity of gilts allows full expression of both estrus and ovulation in more gilts [25]. The issue of induction and subsequent cycles has been a dilemma for some time. There has been speculation and reports that some induced gilts fail to cycle a second time. Studies have tested the impact of PG600 with no boar exposure or daily boar contact on subsequent estrus responses. Daily boar exposure following initial treatment and response indicated estrus expression was improved by 25% or more [26, 27]. Dealing with gilts of unknown reproductive status has been a difficult problem as there is no information on whether the animals are still prepubertal, are cycling but were missed or did not express an standing heat, or may have a permanent or temporary fertility defect. If the gilts are prepubertal, they have failed to respond to weeks or months of stimuli and continue to accrue open day costs. If they are cyclic, they have shown a silent heat or were missed by the farm staff with no indication they will be in heat 20 days later. If there is a defect, the outcomes are uncertain as to heat and diagnosis is unlikely. PG600 was tested for its effects on gilts of mature size and age but with presumed unknown reproductive status. In reality, real time ultrasound assessment and estrus measures were recorded. This study injected PG600 into cycling gilts on d 10 of the luteal phase (10 days after previous estrus), into a silent estrus gilt, and into two gilts that were still prepubertal. The results indicated that PG600 induced an immediate estrus response in both prepubertal gilts, and all other gilts returned to estrus and regular cyclic activity within the next 8-16 days. There were no abnormalities in any measures. Synchronization of Estrus in Gilts Synchronization of estrus in mature cycling gilts can be an effective method for meeting breeding targets within specified windows. The synthetic progestagen, Matrix (altrenogest, Regumate), is a top dress feed additive that is fed for 14 days to cycling pigs. In the USA it is approved at 15 mg/head/day but elsewhere it may be labeled for 20 mg/head/day for up to 18 days. Regardless, randomly cycling pigs after entry and initiation will show estrus at intervals over at 40 day period. This product and approach has shown excellent synchrony. Other methods for synchronizing estrus in pigs through use of estrogens and prostaglandins are far less effective and limited in their use. 4

Use of Matrix has been shown to synchronize estrus in >80% of females within 4 days from last feeding of the hormone. In fact, most females show estrus on days 5-8 with ~5% on days 4, 9 and 10. A new approach to synchronize induced gilts with Matrix following pubertal induction with PG600 without regard to initial estrus response was reported (ref). In this study, PG600 was also used in half of the replicates following last feeding of matrix. In the study, 170 day old gilts were all treated with PG600. Approximately 70% of gilts expressed estrus but all were fed Matrix for 14 d starting on day 10 after PG600. Following last matrix feeding, some gilts expressed poor rates of estrus (<40%) so PG600 was given at 24 hours following last Matrix feeding. The results suggested that regardless of initial induction response, Matrix with or without PG600 following matrix, effectively synchronized estrus in > 85% of gilts. While the need for PG600 following Matrix was unclear, and unrelated to initial estrus response, use of PG600 did induce >85% of gilts to show estrus ~ 5 days following synchronization. In summary, boar exposure can be modified for effect. Maximize induction effects from use of the boar through high libido males, proper duration of boar contact, frequency of contact, and optimizing method of boar contact delivery. For gilts, control group size, provide adequate floor space, consider maintaining estrus gilts in pens, mixing and relocation, and select fast growing gilts. PG600 is effective at estrus induction in pubertal aged gilts. PG600 can also be used to determine the status of females of mature age and weight but without reproductive information in less than 3 weeks. The response of PG600 can be improved by modifications for administration. Estrus can be synchronized using Matrix. This hormone treatment is highly effective for synchronizing estrus in cycling gilts to within four days of each other. Lastly, combination methods for induction of gilts with PG600 with Matrix synchronization and with or without PG600 following Matrix is effective at estrus control and synchronization in > 85% of treated gilts. Citations 1. Tummaruk, P., et al., Age, body weight and backfat thickness at first observed oestrus in crossbred Landrace Yorkshire gilts, seasonal variations and their influence on subsequence reproductive performance. Animal Reproduction Science, 2007. 99(1-2): p. 167-181. 2. Dyck, G.W., Factors influencing sexual maturation, puberty, and reproductive efficiency in the gilt Can. J. Anim. Sci., 1988. 68: p. 1-13. 3. Moeller, S.J., et al., The National Pork Producers Council maternal line national genetic evaluation program: a comparison of six maternal genetic lines for female productivity measures over four parities. J. Anim. Sci., 2004. 82: p. 41-53. 4. Beltranena, E., et al. Designing effective boar stimulation systems as a critical feature of the gilt development unit. in Allen D. Leman Swine Conference Pre-conference Workshop. 2005. St. Paul, MN: College of Veterinary Medicine, University of Minnesota 5. Foxcroft, G.R. Gilt management for the new millennium-research to reality. in Manitoba Swine Seminar 2001. Manitoba, Canada. 5

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