SELECTING GILTS FOR LIFETIME PRODUCTIVITY

Simultaneous advances have been made in the number of pigs a sow can produce each year, predominantly due to management, and in lean (growth rate and percentage), predominantly due to genetics. Today’s gilts grow faster, reach puberty at heavier weights, and are mated both younger and leaner. Therefore, we are ultimately managing a more prolific mature sow that may be both leaner and larger in mature body size. For gilts to be successful in the breeding herd, young breeding females must be carefully selected and developed.

Genetics
The most important genetic considerations are performance testing methods, selection objectives, and a crossbreeding program (See , 1998). Research has documented that selection for aspects of efficient lean growth can adversely affect reproductive performance. Selection for reduced backfat results in reduced daily feed intake, increased age at puberty, fewer pigs born alive, greater pre-weaning mortality and an increase in nonproductive days. Litter size at birth (and weaning) and piglet weight are also reduced with selection for low daily feed intake. Producing today’s genetically leaner animal can result in reduced reproductive performance and, most importantly, reduced appetite.

Perhaps the most common management mistake related to genetics is assuming that all females, regardless of genetics, have similar reproductive behaviors. Genetics will influence feed intake, growth, health, longevity (culling rates), and all aspects of fertility. Remember that farm-to-farm differences will exist. Therefore, validation of characteristics that indirectly affect reproductive performance will improve the management level of breeding females and the consistency of reproductive performance.

Most sow productivity traits have low heritability values, while production and carcass traits have higher values. Selection for strong signs of estrus is also possible; the heritability is .31 for the ability to return to estrus (standing reflex and ovulation) within 10 days after weaning. The duration of the vulvar signs of estrus has the highest heritability (.38) of all estrus signs at puberty, and the duration of standing estrus when checked twice daily is lower (.16) than for any other estrus sign.

Crossbreeding is also an important part of commercial swine production systems because of the improvement in efficiency from heterosis and the potential to exploit differences between breeds. A terminal, static cross in which all offspring are market animals takes the greatest advantage of differences in strengths of lines or breeds. Lines that have superior genetic merit for reproduction provide the females, and lines that are superior for production traits provide the males. The pigs marketed then have high genetic potential for production, and the sow herd has high merit for reproductive traits. Heterosis has the most significant benefit in maternal performance, especially litter size and farrowing rate. Ultimately in commercial pork production, selection and crossbreeding are combined to achieve the highest level of performance.

Gilts Selection
Gilts of the appropriate genotype that have 12 to 14 teats should be selected from the sows with the best reproductive performance. In addition, research has demonstrated that gilts reared in smaller litters (= 7) have greater reproductive performance than gilts reared in large litters (= 10). At weaning, approximately 2.5 times the number of needed replacement females should be retained. Pre-selection can be completed in the nursery where gilts that have structural defects, hernias or ruptures, and inadequate growth rate can be eliminated from consideration. Final selection at approximately 140 days of age should include a visual evaluation of structure, underline, and external genitalia. Gilts with a very slow growth rate (< 1.3 lb day) or that are extreme in either fat or leanness should not be selected. The number of gilts selected should be about 1.25 times the number of replacements required.

Visually evaluate replacements
Females that are purchased or retained as replacements should be genetically superior, reproductively sound, and structurally correct. Replacement females need to have a thorough visual examination to determine their structural and reproductive fitness.

Feet and legs are important because sows are expected to farrow more than two litters per year, nurse a large litter of pigs for two to three weeks, breed back in seven days or less, and live their entire life on solid concrete or wire floors. An ideal foot on a hog is comparatively large, with both toes the same size. The pastern should be relatively soft (not rigid or erect), and the rear hocks and front knee should be angled so as to not put extreme pressure on the leg joints when the animal is walking. The foot should be perfectly flat against the floor and not rotate or turn when the hog takes a step. When the foot rotates or turns on the floor as the animal walks it sets up a possible sore on the bottom of the foot that can become infected and cause the animal to be in pain and unable to perform. Hogs that have sore inside toes have a tendency to become unsound at an early age. This is due to the uneven balance of weight on the feet and the fact that the foot does not set down on the floor surface evenly.

The overall structure of an animal is the sum total of bone muscle, fat, and skin and how it is assembled to make an animal functional for a specific purpose. The structure of the skeleton is very important because it affects longevity and function. A correct skeleton is one that is shaped in such a way that the hog has ample interior body space for essential organs to function. In the case of females, a long, wide, deep skeleton allows for more space for reproduction. Correct structure allows a hog to move around on most any surface without difficulty. An incorrect structure will cause a sow to have difficulty getting up and down while she is in the farrowing crate.

The underline of a gilt or sow is extremely important. Replacement gilts must have at least six functional nipples per side, and they should be evenly spaced and prominent. The nipples should start far forward on the underline, and the underline should be free of pin nipples and inverted nipples. A pin nipple is any underdeveloped nipple that replaces a functional one. An inverted nipple is one where the end of the nipple is held up in the body of the mammary gland and therefore is “inverted.” Pin nipples never become functional. Inverted nipples will sometimes pop out when the sow farrows, but gilts with inverted nipples should generally be eliminated before they are put into the gilt pool because a large majority of them will not be functional.

In addition, to underlines, other external signs can be evaluated for possible reproductive problems. Two problems can be detected in the vulva: (1) An infantile or extremely small vulva is a possible sign of an immature internal reproductive tract. (2) A vulva that is tipped up on the end indicates a gilt that may be difficult to breed. Gilts with infantile or tipped-up vulvas should not be selected as replacements. A comprehensive guide for the visual evaluation of replacement gilts (Stalder et al., 2005) is available free from the National Pork Board.

Gilt Development
Achieving the desired combination of age, weight, and fat depth in lean genotypes may necessitate a gilt development program that is very different from conventional finishing. Replacement gilts raised as finishing pigs will lead to increases in mature body weight and maintenance requirement of the breeding herd. Breeding females should most likely be reared on a conservative diet of adequate protein and relatively low energy. Some restriction of feeding may also be required to achieve the target fat, weight, and age endpoint. Evidence (Foxcroft, et al.; 1996) supports the concept of short-term “flushing” before first mating. Flush feeding by allowing gilts to eat to appetite is simply a corrective process to ensure the potential ovulation rate of the female. Restricted feeding of gilts nearing puberty will inhibit the secretion of luteinizing hormone (LH) which regulates ovulation. When the animals are put back on ad libitum feeding, LH releases immediately begin again.

The flush feeding should end immediately after mating in the gilt. Research has demonstrated (Foxcroft et al., 1996) that high feed intakes immediately after mating in the gilt result in a significant decrease in embryonic survival. Therefore, flush feeding should only continue until mating and then feed intake of gilts and sows should be restricted to normal gestation levels of 1.5 times maintenance. For group-housed females this poses an additional management problem because unmated gilts should be fed ad libitum and gilts that are mated should be restricted in their intake. Forming new groups is also undesirable during the first three weeks of pregnancy as fighting during the establishment of the social hierarchy may lead to embryo losses and a subsequent reduction in litter size.

When is the gilt ready for breeding?
Appropriate age and weight at first mating is very dependent on genotype. In many cases, a mating weight of approximately 275 lb. live weight and an age of 210 days appears most appropriate for optimum subsequent fertility and longevity. It is also important at first mating that the gilt have adequate fat stores for good lactation and a short weaning to estrus interval. This adequate body composition may be represented by a fat measure in excess of .7 in. (18 mm). However, age, fat depth, and live weight are not themselves the targets for the right time for first mating. They are indicators of age pattern of puberty, the weight pattern of fatty tissue growth, and the relationship between ultimate mature size and the proportion of mature size that is required before reproduction should be initiated.

Sizing the Gilt Pool
Season of the year, disease, environment, age, and genetic makeup influence the number of females showing estrus and conceiving at a particular time. The number of replacement gilts needed to complete a farrowing group must be determined in advance. When determining the number of replacement gilts needed, selection of as many as three replacement gilts for each farrowing crate to be filled may be necessary. During hot weather, the number of gilts needed to insure one pregnant gilt at the desired time doubles or even triples. The more gilts in the pool at any one time, the greater the chance of obtaining more than enough pregnant females for a predetermined schedule. However, space allowances for the gilt pool are often allotted based on the average annual need. Increasing the number of available females without simultaneous increases in space allowance will most likely result in additional stress conditions on the gilts through crowding, which may ultimately increase the incidence of anestrous.

Induction of Estrus and First Breeding
Transportation of replacement gilts to the sow farm will generally result in the onset of first estrus. If this is true in your management system, you may find a high percentage of the gilts in heat after a three-week acclimatization period. Providing for quality boar exposure will induce puberty in the gilts. Studies have shown that gilts that respond to boar exposure by the onset of puberty at a younger age tend to have a greater retention in the herd than those gilts that respond at a later age. PG600 is also commonly used to stimulate puberty in 145- to 160-day-old gilts. This is generally very effective and may also be useful during periods of high ambient temperatures to stimulate a first estrus in incoming gilts where cyclicity is suppressed.

It is also important to consider that a gilt’s estrus behavior pattern will differ from a sow’s. Estrus lengths are generally shorter and often less pronounced in gilts. Therefore, the first insemination should occur immediately following detection of estrus. A follow up insemination 24 hours later should follow only if the gilt is still in standing heat.

Summary
A comprehensive gilt selection and development program is critical to the long-term success and productivity of the sow herd. Genetic and management programs will dictate the future reproductive performance and longevity of the females. It is nearly impossible, however, to overcome poor gilt selection and development with later management actions in the breeding herd.

References

• Foxcroft, G. R., J. R. Cosgrove, and F. X. Aherne. 1996. Relationship between metabolism and reproduction. Proceedings of the 14th IPVS Congress, Bologna, Italy, 7-10 July 1996. pp 6-9.
• See, M. T. 1998. Exploiting genetics within your herd. Professional swine Managers Training, Oct. 6-7 Guymon, OK. National Pork Producers Council. pp 27-36.
• Stalder, K. J., C. Johnson, D. P. Miller, T. J. Baas, N. Berry, A. E. Christian, T. V. Serenius. 2005. Pocket guide for the evaluation of structural, feet, leg, and reproductive soundness in replacement gilts. Des Moines, Iowa: National Pork Board. Pub. no. 04764.

  - M. Todd See

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  Last modified January 4, 2008.

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