The goal of the breeding herd is to consistently meet pig production targets by ensuring farrowing facilities are fully utilized. To achieve predictable pig flow, groups of mature females are bred within a defined timeframe. This allows for accurate scheduling for facilities, labor, supplies and production, and facilitates use of all-in and all-out disease control methods. Meeting production targets requires the numbers of pregnant females match the number of farrowing crates. Determination of the number of females needed in a breeding group must account for expected fertility successes, failures, and incidence of culling. In an established breeding herd, the proportion of females from each parity should represent the structure of the breeding groups. The herd parity distribution results from the frequency of retention and removals following entry with an ideal and actual parity structure that differ in the sows retained into the later parities (Figure 1). Risk factors for removal are led by reproductive failure, structural problems and removal for old age and genetic improvement.Replacement gilt pool
Most established sow herds have breeding groups made up of 80% sows and 20% gilts. Replacing sows as a result of culling and for fertility failures is corrected by entry of replacement gilts. Proper planning avoids problems with too few gilts to breed or too many gilts that are pregnant. In either case, lost profit, increased costs and problems in space and animal flow can occur. The gilt pool must be managed continuously for induction and maturation to ensure mature gilt numbers required are available for each breeding group. It is also important to cull gilts that have failed to meet established selection criteria in a timely manner. Gilt pool management can be influenced by environmental variables that can affect the fertility and development and effective management must anticipate variation in the expected responses over seasons. Since final selection as “breeding eligible” is below 100%, the pool size must be increased to account for fertility and structural failures.
Expectations for gilt fertility
Gilt maturity is recognized when targeted age, weight and cycle maturity are reached. Reproductive performance and longevity are optimal when gilt breeding occurs near 230 days of age, 300 pounds (136 kilograms), with adequate backfat and at second or third estrus. Problems in fertility and longevity result when breeding occurs far outside the targets. Breeding under and overweight gilts appears most related to problems. These problems in body weight are related to issues with attainment of puberty and lifetime growth performance. Pubertal estrus is induced in batches of gilts over a 60 day period. Earlier age of at start of boar exposure results in earlier age at puberty but reduces synchrony of estrus. Delaying boar exposure increases age of first estrus but improves overall synchrony (Figure 2). Expectations for gilts can be charted (Table 1) to include age, weight and backfat at puberty or first service. It is expected that select gilts will show a “normal” progression of vulval swelling, associated with changes in proceptive to receptive behavior (full standing estrus). Only gilts with a recorded standing heat over a period of two days, that cycled in a regular 19 to 22 day interval, should be considered “breeding eligible” and moved to the breeding line or sow farm. Although a recorded standing heat is the only acceptable measure of sexual maturity (eligibility to be bred), daily records of behavioral and vulval development help guarantee that effective induction and selection protocols are in place (Figure 3).
Physiology of puberty
Puberty is a heritable trait with as much as 40% of the variation attributed to genetics. Yet emphasis is often placed on traits with greater economic importance such as growth and carcass measures. As a result, average age of puberty typically ranges between 190 and 230 days of age. The variation in puberty can be large among batches of gilts, herds and different genetics. Puberty induction patterns can are influenced by the interaction of the genes and environment. Large seasonal, health and housing factors may explain much of the variation but not necessarily allow prediction of the response. Puberty is defined when the behavioral expression of estrus coincides with the occurrence of ovulation. However, puberty is not the same as sexual maturity and can often as there are instances where gilts appear to express estrus but fail to ovulate and cases where gilts ovulate but do not express estrus. For breeding, maturation should include evidence normal estrous intervals, standing for two days at estrus period, ovulation of 14 or more eggs, and a uterine size to accommodate a targeted number of pigs. A two-day standing estrus helps to remove errors in pubertal estrus identification and reduces failures in estrus intervals.
The collective conditions for maturity are not met until the gilt has reached mature body size, and has cycled so that the uterus and neuroendocrine tissues have been exposed to progesterone during the normal cycle. When conditions for maturity are sensed and external environmental signals are stimulatory, pathways to initiate reproduction begin. In the gilt, the brain, endocrine and reproductive systems are capable of hormone production and signal reception, but do not reach functional maturity at the same time and can be demonstrated by the proportions of gilts that express estrus and the magnitude of hormone release as gilts age. These responses influence estrus and ovulation, numbers of eggs produced, uterine growth, normal cycles, pregnancy establishment, and litter size. Failure in any of these maturational steps during development can result in problems in delayed puberty, anestrus, irregular cycles, regular and irregular returns, and small litter size. Since determination of internal measures of maturity cannot be assessed, gilt selection is based on the symptoms of estrus and measures of weight, age and body condition, linked to a requirement to breed gilts at second estrus.
Puberty induction procedures
Induction of puberty is managed for groups of gilts housed in pens under scenarios typical of the grow-finish period with ad libitum access to food and water. Induction is often initiated at 24 weeks (160 to 180 days of age) when non-select gilts can still be marketed. At induction, gilts are moved from finishing to induction pens which have reduced numbers of gilts and more floor space per female. Ideally, even during earlier development in the finisher stage, gilts will have been provided more space than is typical of a commercial finishing facility. Induction is performed using exposure to a mature boar that may benefit with both relocation and regrouping. Selection of gilts from dams that expressed earlier ages at puberty can improve responses. Selection for growth rate is a key factor and slow growing gilts (less than 1.3 pounds per day or less than 600 grams per day from birth or weaning) are more likely to show delayed puberty than their faster growing counterparts. Age is also related to response since the proportion of gilts that respond to induction is low and increases with age (Figure 4) and (Figure 5).Qualities of the boar identified as important for induction include: greater than 9 months of age (seven months or more for Meishan boars), high libido (active champing and salivation for pheromone production), vocal and with a calm temperament. In addition, daily is more effective than intermittent exposure (occurring every other day or for weekdays but not weekends). Duration of exposure shows that a minimum of 10 minutes of exposure per female is needed for effect, with little benefit beyond 30 minutes per day. Continuous housing of gilts with or next to boars, while stimulatory, reduces accuracy for detection of estrus due to the refractory behavior effects, and also tends to lead to boar complacency and over conditioning. Direct physical and fenceline boar exposure have been utilized for successful induction but physical exposure is clearly a more potent stimulator for induction. The cumulative non-productive days associated with less effective induction protocols, in combination with the likelihood of delayed gilts being overweight at breeding, are risk factors that must be considered when deciding on gilt management programs.
Floor space is important for gilts during development and at induction and the negative effects of limited floor space result from stress associated with aggression in competition for living space and feed. With space limitations in pens, reduced feed intake and access to the boar can occur. Similarly, the number of gilts per pen can affect puberty with delayed puberty occurring with fewer than four gilts and more than 50 gilts in each pen. The problems in the small pen likely occur due to the stress resulting from intense social dominance while the problems in the larger pen likely result from inability to effectively provide adequate boar exposure to all gilts. It is essential that energy and protein for gilts prior to breeding are not restricted as each will have negative consequences on growth, weight and body composition as well as fertility. Other housing conditions that involve poor air quality, thermoregulation and factors that affect the ability of animals to respond to stimuli from the boar involving visualization, perception of pheromones and reception of boar vocalization could be expected to reduce fertility. Very low lighting levels are considered less than optimal for pig and human visual acuity. Poor air quality resulting from manure gases or dust and including poor air exchange, could affect respiratory health and nasal perception of pheromones and smell. To avoid problems, gilt housing areas should be evaluated for auditory, air, and lighting conditions. Although limited data are available, it is generally assumed that problems in health, will be linked to reduced growth performance, delayed gilt puberty and perhaps problems in fertility. Delayed puberty could result from the disease itself or the side effects associated with lethargy, reduced appetite, and reallocation of nutrients to maintain health. Diseases such as PEDV, PRRS, circovirus and others would affect growth and health. Acclimation, vaccination, and other health programs are used for proactive control.
Variation within a group of gilts in age, weight and body measures are anticipated, and the extent of acceptable variation in any of these measures for the pool would be expected to increase variation in the induction response. The success of the gilt induction program can be evaluated by the gilts that are identified in estrus on each week, the synchrony of the response, and the cumulative expression of estrus. The expectation is that 15 to 20% respond each week with 45 to 65% cycling by week 3 and 90% by week 6 (Table 1). However, the data suggest variation in the response by week and the proportion culled after week 10. Allowing for a 10 week induction response to occur from 170 days of age would set 240 days as the final cut. However, most gilt development units have limited capacity, in terms of space, labor availability and boar power. Programs that set an earlier limit for a “selection” decision (30 days after the start of induction) may function more efficiently (Table 2). Based on available data, there is an expectation that 10 to 30% of gilts could be culled from each gilt batch.PG600
PG600 (Merck Animal Health) is an important product that can induce follicle development and estrus in prepubertal gilts that are near the targeted age and body weight for natural attainment of puberty. The product has generally been effective at inducing 60 to 70% of the females into estrus and 70 to 80% to ovulate within six days of treatment. It is recommended that breeding be delayed until the next estrus after induction, which is consistent with the goal of achieving greater uterine maturity and increased ovulation rates in gilts that have completed a full estrous cycle before breeding. PG600 has mostly been used to treat gilts that have not been detected in estrus after a 30- to 60-day induction period. This treatment has had mixed success, since some gilts may be prepubertal, while others may be cyclic but not have a recorded estrus. One approach that has been used is to pre-schedule PG600 use for all gilts not showing estrus following a period of boar exposure of 23 days. This has worked with some success and has been effective at inducing gilts that did not respond after 23 days of boar exposure (Table 2). There are also data to suggest that treatment of gilts with PG600 without any information on cyclic status, can allow the status of the animals to be resolved within a 30-day period following injection. For gilts that are prepubertal, most appear to respond, and for those that are already cycling, the treatment may delay estrus expression by 10 to 20 days following injection.
Managing the cyclic gilt
Following the identification of a pubertal estrus without breeding (known as a heat no serve, or HNS), the females enter the breeding pool. Following the HNS, gilts may remain in their pen, be relocated to another pen or be moved into stalls. Moving gilts in weeks 1, 2 or 3 following the HNS to allow time for acclimation tot eh stall, has no effect on gilts showing normal 21-day estrous cycles (84%) but can affect the proportions showing abnormally short or long cycles. A greater proportion of gilts moved in the first week after the HNS show short cycles (less than 17 days) while those moved in week 3 after estrus, show delayed returns to heat (>24 days) but with no effect on fertility (Table 3).However, gilts displaying short cycles were more likely to have reduced litter size while those showing delayed cycles were more likely to show reduced farrowing rates. The effect of week of movement following HNS could be related to disruption of hormonal control of the estrous cycle. The cycle of the gilt is regulated by hormones that act to control the follicular (days 14 to 21 of the cycle) and luteal (days 1 to 13 of the cycle) phases of the cycle. Follicles develop quickly in the last week before estrus and produce estrogen which is critical for events associated with expression of estrus, sperm transport and ovulation. Following ovulation, corpora lutea form to produce progesterone to allow for embryo development. Once corpora lutea form, the length of the cycle is regulated by the timing of corpora lutea destruction and decline of progesterone. In non-mated females, the uterus releases prostaglandin on day 14, to end progesterone production and start a new follicle phase. While the mechanism for cycle disruption is not known, it may occur through stresses that disturb hormone control of follicles or corpora lutea in a small proportion of gilts.
Fertility in gilts at breeding depends on inseminations occurring at the optimal time. Two inseminations usually occur at 12 to 24 hours apart during estrus to help ensure one AI occurs close to the time of ovulation. Proper insemination with adequate numbers of fertile sperm allows fertilization rates to remain very high. Following fertilization, embryos develop and pass into the uterus on day 4 and distribute themselves throughout the uterus. In most pigs, 25% of the embryos may be lost by day 10, with another 10% degenerating by day 25. In bred gilts with healthy embryos, pregnancy is recognized beginning on day 13 following AI and allows progesterone to remain elevated in blood for establishment and continuation of pregnancy until time of farrowing.
Following breeding, factors that increase embryo loss and pregnancy failure are major concerns. As a result, attention has focused on seasonal and environmental stressors involving housing conditions for the gestating gilt. Gilts are expected to show 90% conception rates 30 days after breeding, and exhibit farrowing rates near 85%, with total born litter size of 13-14 pigs. For housing gestating pigs, many farms restrict movement of bred females to specific periods of time to prevent embryo and pregnancy loss. When comparing mixing gestating females in the first, second or fifth week after breeding, from a welfare and reproduction standpoint, mixing in the fifth week was best and closest to the performance of the pregnant females housed in stalls (Table 4). Other reports that suggest mixing at other times may not be detrimental to reproduction, or in some cases the effects are only marginal. Regardless, it will be important to evaluate both short- and long-term measures for reproduction and welfare. These measures may include farrowing rate, litter size, sows rebred after weaning and welfare measures that can include lesions, lameness and body condition. Proper feeding of the gestating gilt should be a priority as each female is expected to gain approximately 100 pounds prior to farrowing. The general recommendation is to have gestating gilts in proper body condition in the first third of gestation. Managers must prevent under or over conditioning gilts for farrowing as this can have detrimental impacts on live-born pigs and feed intake in lactation. In the end, for bred gilts, the largest risk factors for removal from the herd and reduced longevity will include failure to establish pregnancy, failure to produce a large litter and failure to rebreed after weaning.
Parts of this report were made possible by research funding from National Pork Board projects 09235 and 12-111.