The growth and development of gilts is a component of their lifetime potential for productivity. Growth and development affects not only their ability to reach puberty, it also likely affects their reproductive performance through later parities and their physical soundness as they age. Because of this, numerous studies have been undertaken to determine factors that matter in gilt development, using a variety of endpoints. Many studies have looked at the effects of nutrition and growth on attainment of puberty, fewer are available that focus on productivity and structural soundness to later parities. These studies provide useful information regarding the appropriate management of gilts, but leave many unanswered questions. In this article we summarize what we know, what we think, and what remains to be learned.
Clearly, getting gilts to reach puberty efficiently is an important component of their reproductive capacity. The key factor for a gilt reaching puberty is the maturity of the physiological components that cause her to begin to cycle. Unfortunately, the best measure of maturity of these components is that the gilt reaches puberty, and beyond this we do not have very good, reliable measures that are predictive of maturity. Nevertheless, relationships between onset of puberty (achievement of sexual maturity) and age, weight, growth rate, lean muscle (often measured as some measurement of loin muscle), and fat (often measured as back fat) exist and have been extensively studied. These relationships are made more complicated by the fact that in addition to being related to onset of puberty, they are also all related to each other, and trying to determine what is truly important with relation to puberty, independent of other factors, is complex. A second complication is that attainment of puberty in gilts is very much influenced by the timing of exposure to environmental stimuli that trigger the last phase of sexual maturation, such as transport and mixing (social) stress and boar exposure (“primer” pheromones produced in the saliva, auditory stimuli). Consequently gilts that are mature enough to reach puberty may not actually exhibit pubertal estrus and cycle unless exposed to these stimuli. When gilts are first mature enough to cycle, boar exposure still takes time to induce puberty, and this priming period needs to be accounted for in recommendations for manipulating age at puberty. The longer gilts continue to develop without having been stimulated to reach pubertal estrus, the lower the threshold becomes for environment stimuli to trigger a pubertal response. As discussed in the next paper in this series, one strategy may be to “store up” a pool of potential cyclic gilts beyond 140 days of age by deliberately not exposing them to transport/mixing stress or boar stimulation. This pool of gilts would then respond more synchronously to boar stimulation at a later age.
Thus, age at puberty is strongly influenced by the age when boar exposure begins. Although many experimental studies apply boar exposure early to capture all the variation in maturity that exists within a group of gilts, this is not practical on the farm and determination of an “appropriate age at puberty” is difficult to extrapolate to the farm situation. A better place to start is by asking the question how old, sexually mature (second estrus) and physically big (breeding target weight) does a gilt have to be when she is mated to perform efficiently and then, with that information, calculate when to begin boar exposure to reach that point, taking into account how long it takes for a boar to induce puberty in a mature gilt.
Optimal gilt weight
Several studies indicate that an optimal lower limit for breeding of gilts is unrelated to age if weight is at least 300 pounds. Most production systems do not weigh gilts, but correlated measures such as heart girth or flank to flank measurements can be used to approximate their weight. Most studies indicate that a 300 pound weight at breeding optimizes the chance of the gilt remaining productive into adulthood. Studies have also shown that breeding on the second estrus provides a greater litter size in first parity. Thus if we start with the age at which a majority of gilts reach 300 pounds, then subtract one estrous cycle (21 days), we arrive at the oldest age at which gilts should reach puberty, taking into account their specific growth rates. Then we need to add the time it takes for boar exposure to induce puberty in a group of gilts. This time varies, but early studies suggest that the time is relatively uniform when boar exposure is begun after 150 days of age. Figure 1 indicates age at puberty in a group of about 1,200 gilts from a recent National Pork Board-funded study that received boar exposure beginning at 160 days of age. This figure suggests that on average it takes 24 days for gilts to express estrus and most gilts within the early puberty group reached puberty by 40 days.
Figure 1 confirms many earlier studies in establishing a normal distribution in the pubertal response to boar stimulation starting at 160 days, but also illustrates the presence of separate populations of gilts that take longer to respond. Results suggest that these later responding gilts likely have a reduced rate of remaining in the breeding herd to later parities, so strategies that incorporate them into the breeding herd should be avoided. From this figure, combining 21 days plus 40 days results in boar exposure beginning at 61 days from the age at which the majority of gilts reach 300 pounds.
Figure 1 was generated from gilts that were ad libitum fed. Although most gilts in production today are fed ad libitum, a couple of recent studies have suggested that restricted feeding might provide greater herd longevity, and one study suggested that restricted feeding provided about 10% increase in productivity, but this was not statistically significant. To explore this important question further, a series of studies was funded by the NPB to develop diets that could be fed ad libitum to developing gilts and would result in different growth rates and/or body composition, and then determine the effects of the diets on puberty and productivity to third parity. Two trials developing gilts to puberty have been completed, Figure 1 is from the first gilt trial. The trial used six diets, the first diet was a control diet developed from an informal survey of swine producers to obtain the levels of metabolizable energy (ME) and standard ileal digestible lysine content in their gilt diets. In order to use diets that were practical in current production, ME values in test diets were either 10% below or 10% above the average obtained from the survey. Lysine levels were either control or 15% less than control, giving six treatments: 90% ME-85% Lysine, 100% ME-85% Lysine, 110% ME-85% Lysine, 90% ME-100% Lysine, 100% ME-100% Lysine, 110% ME-100% Lysine. The diets were fed in two phases, a grower and a finisher phase, and gilts received the diets from 100 days of age until 260 days of age when they were slaughtered (the switch from grower to finisher diets occurred at 200 pounds). As indicated in Figure 1, boar exposure began at 160 days of age and gilts were heat checked daily until slaughter.
This trial provided several interesting results. Gilts grew identically on all six diets, because they adjusted their feed intakes according to the ME in the diet and the low-lysine diets also did not affect growth. In addition, the diets did not affect puberty, except for a very minor increase in the number of gilts that failed to reach puberty in the low-Lysine diets. From these results we can conclude that the ME and Lysine levels in the control diet (which were based on the industry) were above the requirement for growth of the gilts, taking into consideration their ability for compensatory feed intake. They were also mostly greater than the needs to reach reproductive maturity. To the extent that ME and Lysine levels in diets are economically important, this result suggests that the industry could lower these by 10 and 15% respectively.
However, the initial trial was designed to develop diets that limited growth and/or body composition of gilts. For that objective it failed, necessitating a second trial. In a second trial, new diets were developed for which ME in the diet was held relatively constant, and Lysine levels were reduced in the control diet, and further progressively reduced in two additional test diets. This trial was just recently completed. The new diets differing in Lysine resulted in progressive reductions in growth rate, without altering body composition. However, there was also an increase in age at puberty in the low lysine diet, suggesting that the low diet slowed the maturity of gilts. These results provided diets that can be fed ad libitum that will reduce growth rates of gilts, and we are now poised to begin a third trial to determine the effects of gilt development diets on productivity to three parities.
Returning to the timing of boar exposure, there are some additional considerations, which are:
- The lack of any relationship between average to good growth rates and age at first estrus
- The high and variable growth performance of replacement gilts
- The variability in “stimulation to pubertal estrus” interval
The first two issues are illustrated in Figure 2. In the recent NPB gilt study, there was no relationship between growth rates and age at puberty. There were essentially no gilts with a lifetime growth rate at 160 days below 1.3 pounds per day (0.6 kg per day) and the lack of a relationship indicates that growth rate in these gilts is not limiting for onset of puberty. However, the variable growth rates observed, combined with the variable number of days to first estrus after starting boar stimulation at 160 days, results in nearly half the gilts studied exceeding 300 pounds at pubertal estrus. If these gilts were then bred as proposed at second estrus, most would exceed an upper target breeding weight of 350 pounds. Data supporting a lower weight limit are more abundant than those supporting an upper weight limit at first breeding, in terms of lifetime retention and productivity. However, reported detrimental effects of overweight gilts include poor return to estrus after first parity and increased losses due to lameness and other structural problems. Effects of weight on return to estrus after weaning may be confounded with age at first estrus, because there is a relationship between late puberty and postweaning return to estrus. Because of these reports, an upper weight limit of 350 pounds has been suggested, but information confirming that an upper weight limit exists and what that limit should be, are needed. The upcoming NPB-funded trials should provide further data allowing an evaluation of this important question.
Under commercial conditions it is more typical to start boar exposure at 170 days of age and observe gilts for 30 days or more as is indicated in Figure 2. If it is valuable to limit the upper weight of gilts at breeding to 350 pounds, then feeding the fastest growing gilts with diets that limit growth performance, or physical feed restriction, would be possible management approaches to reduce the number of gilts that are too heavy at breeding. Thus, a second reason for the development of ad libitum diets for gilts that limit growth rate might be to use them to limit the growth of the fastest growing gilts so that an upper weight limit is not exceeded. Alternatively, a second management approach for limiting weight and age at pubertal estrus might be to stimulate pubertal estrus with exogenous hormone treatment at the time that non-pubertal gilts reached some predetermined weight threshold (say 300 pounds). This would also have the advantage of limiting the number of gilt non-productive days between entry and breeding. The possible negative impacts of exogenous hormone treatment on sow lifetime productivity is discussed below and further discussed in the next paper in this series.In the absence of the results of the future trial, we can return to the two trials for information on gilt age at 300 pounds under commercial conditions. In the first trial, gilts reached a mean of 300 pounds at around 180 days of age. In the second trial, in the control diet, they reached a mean of 300 pounds at around 200 days of age. The sites and other conditions differed between the two trials, but the difference between them is a useful illustration of the dependence of decision making on the capability of gilts in a particular production environment. To get the age when most gilts reach 300 pounds in the two experiments we need to add about 20 days to when the average reaches 300 pounds (200 and 220 pounds, respectively). If we are to use data from the two trials to calculate when to begin boar exposure, we get initiation of boar exposure at 140 and 160 days, respectively (age where the majority is 300 pounds minus 60 days). As indicated previously, waiting to initiate boar exposure can reduce the time for gilts to respond to the boar by reaching puberty. However, the third issue, variability in “stimulation to puberty interval”, makes this decision somewhat challenging. There are not extensive published data that address this question well for modern gilts. Nevertheless, the cost of waiting to initiate boar exposure, in terms of non-productive days, must be balanced by the savings in the reduction in labor involved in estrous detection for a shorter period that would result by later initiation of boar exposure.
The discussion to this point relies on the natural ability of gilts to reach puberty, but hormonal tools (e.g., PG600) are available, such that the natural ability of the gilt to start cycling is not necessarily a limiting factor when establishing the most efficient puberty induction programs. The use of PG600 to limit age at puberty and breeding, and to reduce gilt non-productive days has been discussed above. The cost of inducing pubertal estrus is partly justified by the savings in non-productive days. In this context, the data in Figure 1 raise some interesting questions. Although the main group of gilts reached puberty within 40 days of boar exposure, a second population (distribution) was identified that reached puberty within 60 days of boar exposure, and a third population that reached puberty within 100 days of boar exposure. As studies suggest that gilts experiencing late puberty are generally less fertile when they enter the breeding herd the use of PG600 to advance puberty of these second and third population gilts seems unwise. Upcoming sow trials funded by the NPB incorporate the use of PG600, and will generate valuable data that will help address the optimal use of this product.
In conclusion, numerous studies have investigated various growth related factors on puberty attainment and sow longevity. The best evidence suggests an optimum weight of 300 pounds at breeding, regardless of age, for optimizing productivity. Boar exposure at the appropriate time in relation to the 300-pound milestone for breeding is essential and should be adjusted according to the growth trajectories of the gilts. In contemporary gilt populations there is little evidence that growth rate, lean tissue mass or fatness of the gilt also limits age at puberty or productivity through later parities. Further evidence is needed to determine the impact of high growth rates on SLP and this evidence will become available as two large NPB-funded studies on these topics are completed.
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