The primary goal of gilt development programs is to prepare gilts to enter the breeding herd at a relatively young age, at a high level of productivity, and to maintain that level throughout a long, reproductive life.
Selecting the most effective gilt development program can be tough because there are many factors affecting reproductive longevity.
Nutrition plays an integral role in the development of gilts. But it can't overcome deficiencies in other parts of the gilt development program. Nutritional advice must be balanced against other management and economic challenges influencing gilt development.
Grow-Finish Period Replacement gilts in the grow-finish phase should receive specifically designed diets. With the widespread use of split-sex feeding, producers have the opportunity to feed these specially formulated diets.
In systems that generate their own replacement females, gilts are commonly selected off the finishing floor at market weight. One drawback to this approach is young gilts are not exposed to a boar to help trigger early puberty.
However, producers can identify and select the fastest-growing, leanest gilts. Those potential replacement gilts should be fed for rapid lean growth right up until they are moved to the gilt pool, much like their terminal littermates destined for market.
If gilts are raised at a site different from the breeding herd, an isolation period should be observed to protect the health of the existing sow herd and acclimate incoming gilts to endemic diseases in the existing herd. Gilts raised in a one-site production system can probably enter their gilt pool directly without an isolation period.
Many producers purchase maternal-line gilts from a seedstock supplier. These gilts may be purchased at a very young age and light weights (50 to 100 lb.) or they may arrive at the farm at or very near breeding age.These gilts were pre-screened before purchase so there is little need to evaluate their lean gain.
Purchased gilts should be moved to isolation facilities upon arrival at the farm. At a young age (140 to 150 days), boar exposure and other management practices should be implemented to encourage an early onset of estrus.
Ideally, specialized management and feeding of replacement gilts can be implemented when gilts enter the isolation facilities or the gilt pool (in one-site production systems) at 140 to 150 days of age. This approach may be ideal for reproductive management, but facility design and pig flow may delay implementation.
Regardless of pig flow or facilities, diets for gilts up to about 200 lb. should be formulated for maximal lean growth and skeletal integrity. Energy and amino acid density of diets for each phase of growth will depend on lean growth potential of the gilt and voluntary feed intake.
The calcium and phosphorus requirements for complete bone integrity are higher than the requirements for optimal growth rate and feed efficiency. Increasing bone mineralization has been shown to increase longevity of sows in some studies, but not all.
Diets for replacement females should be .1% higher in calcium and total phosphorus than diets fed to market gilts beginning at 100 lb. This assumes that diets for market gilts have not been over-fortified. Of course, the actual mineral concentration will depend on voluntary feed intake of the gilts.
Typically, these diets will contain .75% calcium and .65% total phosphorus. This mineral concentration will provide 17 grams of calcium and 14.5 grams of phosphorus intake daily for gilts consuming 5 lb. of feed.
Three Gilt Pool Feeding Strategies The primary goal in the gilt pool is to encourage early expression of pubertal estrus and successfully mate gilts while they continue to grow toward their mature body size. The nature of that body growth as it relates to sow longevity and overall reproductive performance has been the subject of much controversy. Three strategies for managing this body growth have emerged.
Strategy 1 entails full feeding gilts a moderate energy (about 1,500 kcal ME/lb), high protein (16%) diet. The goal is to ensure that the rapid growth of lean tissue, experienced at younger ages, continues right up until the gilt is mated. Increased body tissue reserves of fat and protein provide a nutrient reservoir for gilts to draw on, supporting their lactation and postweaning return to estrus. Some argue that these reserves will enhance longevity of the sow.
Strategy 2 employs restricted feeding (5 to 6 lb./day) of the diet described for Strategy 1 until 10 to 14 days before mating. This strategy allows growth to continue but at a slower rate than if the gilt was full-fed. Moderate restriction of growth rate keeps gilts from getting too fat. But this level of restriction may reduce ovulation rate and size of the first litter.
Therefore, gilts should be full-fed 10 to 14 days before expected mating to increase ovulation rate back to normal levels.
Discontinue this practice of flushing the day after mating because of the threat of increased embryo mortality linked to high feeding levels in the first 10 days after mating.
Strategy 3 involves full feeding a moderate-energy, low- protein (13%) diet to encourage body growth with a high proportion of fat tissue. The goal of this strategy is similar to Strategy 1, except that greater emphasis is placed on the value of fat tissue.
With this strategy, there is a real danger of getting gilts too fat. Excessive condition at mating increases the chance of females being over-conditioned at farrowing. Over-conditioned sows have more locomotor difficulties, farrowing problems and tend to crush more nursing pigs than leaner sows.
Gilt Development Strategies The National Pork Producers Council in cooperation with the University of Nebraska and the University of Tennessee studied development strategies similar to the three feeding regimens described above, looking at gilts from five different genetic lines.
In short, the three groups reported that a higher proportion of gilts assigned to Strategy 2 farrowed their first litter and stayed in the herd long enough to farrow a fourth litter (Table 1).
Significantly fewer females fed for rapid lean gain through 180 days of age (Strategy 1) farrowed their fourth litter, indicating a higher culling rate compared with the other two strategies.
In this experiment, it appeared that feeding to increase fat stores at first mating was neither harmful nor beneficial to sow longevity.
Underlying these three strategies is the central question: What is the most appropriate age, weight and body composition of gilts at first mating?
Work conducted at the University of Minnesota suggested that age, body weight, and body composition at first mating were no different between females that conceived their fourth litter and those females that were culled from the experiment (Table 2).
Similarly, number of pigs weaned over three lactations was not influenced by age or body composition at first mating.
Evidently, age, body weight, and body composition of gilts at first mating are not the most important factors that control sow longevity and lifetime productivity.
Don't ignore feeding and management practices employed during gestation and lactation. Limit feeding during gestation to control weight and fat gain, and maximizing feed intake during lactation to minimize weight loss are key components determining long-term success of the sow herd.
Feed Restriction Advised A moderate restriction in feed intake to slow growth rate of gilts in the late finishing and pre-breeding phases seems to be the most prudent strategy. A target weight of 240-260 lb. at mating on the second estrus is a reasonable target.
Ovulation rate and size of the first litter increases when mating is delayed until the second or third estrus (Figure 1). However, lifetime productivity of third or fourth parity sows was not influenced by delaying mating past the second estrus.
Workers at Michigan State University projected the economic impact of delaying first mating until the second, third or fourth estrus. They found no economic advantage for delaying mating past the second estrus. Delaying mating past the third estrus dramatically decreased returns per hundredweight of pork produced. Facilities, production flow and effectiveness of the gilt pool will dictate the appropriate estrus period for first mating. In most systems, gilts will be mated at second or third estrus. Mating at the first estrus or after the third estrus is not recommended.
In many instances, with fast-growing pigs, moderate feed restriction in late finishing and pre-breeding will be necessary to prevent gilts from getting too heavy and fat.
However, the specific approach may vary from farm to farm depending on genetics and management practices. Restricting feed intake to 5.0-5.5 lb./gilt/day of a diet that contains about 1465 kcal metabolizable energy (ME)/lb. and .8% total lysine will provide 7.3-8.0 Mcal of ME and 18-20 grams of lysine daily. This level of restriction should allow continued growth of body tissues without excessive fattening while having little or no negative effects on age at puberty.
In some production systems, restricting feed intake is not feasible. Use of low- energy feed ingredients that dilute the energy content of the diet may be a reasonable alternative but can be difficult to implement effectively in some systems.
Breeder Vitamins, Minerals Vitamin needs of the reproducing female are greater than those of the terminal animal. For gilts entering the breeding herd, the typical vitamin premix in the finishing diet should be replaced with a "breeder" vitamin premix. This premix should contain elevated levels of the fat-soluble vitamins A, D, E and K, and the water-soluble vitamins with special attention to choline, biotin and folic acid that are relatively low or absent in typical finishing diets.
Likewise, mineral requirements of breeding swine are elevated relative to market animals. Calcium and total phosphorus concentrations of diets should be .9% and .8%, respectively. Concentration of other minerals are also increased in breeder premixes.
Switch to higher levels of vitamins and minerals typical of a sow diet when gilts are moved into the isolation facilities.
The following recommendations can be used as guidelines for the development of gilts from 50 lb. through first mating.
1. House gilts to allow feeding of specially formulated gilt development diets. 2. Feed for rapid lean gain until gilts weigh about 200 lb. 3. Beginning at 100 lb., increase dietary calcium and phosphorus concentration .1% above typical grow-finish diets for improved skeletal development. 4. Initiate boar exposure at about 150 days of age (200 lb.) if possible, to encourage early expression of puberty. 5. Feed diets in the isolation facilities and gilt pool that contain the same vitamin and mineral fortifications as diets for the sow herd. 6. Restrict feed intake in the gilt pool to 5.0-5.5 lb./gilt/day of a corn-soybean meal diet that contains .8% lysine. The primary goal is to slow growth by restricting energy intake. 7. Full-feed gilts 10-14 days before mating on second estrus to achieve increased ovulation rates associated with flushing. 8. Reduce feed intake back to pre-flushing levels immediately after mating to reduce embryo mortality.