Improving management, nutrition, and health will result in short-term improvement in reproductive efficiency. To assure continuous improvement, genetic improvement is needed. The only way to produce genetically superior females is with a good selection and culling program. Gilt selection and sow culling should be based on mothering ability soundness, and performance.
The only information we have on a gilt's ability to perform in the farrowing barn is the performance of her dam. However, this is the most difficult characteristic to select for. Heritability estimates for mothering ability range from 10 to 15 percent, meaning the environment is responsible for 85 to 90 percent of the variation in these traits. Even with the low genetic effect, we cannot afford to ignore mothering ability.
At birth, all gilts in litters from superior sows should be identified (ear-notched) for future selection. After receiving colostrum, even up litters by moving the largest male pigs from large to small litters. The records on each sow should contain the number of pigs born alive, number of pigs weaned, and 21-day litter weight. Notes on farrowing ease, temperament, and pig abnormalities (cryptorchid, spraddle legged, etc.) should also be included.
The accuracy of records is improved with each litter a sow produces. Therefore, lifetime records should be used in gilt selection. Breed associations participating in the STAGES program rank sows based on maternal traits. The Expected Progeny Difference (EPD) is calculated for every animal registered and tested. This information is available for commercial producers to use in selecting replacement gilts or boars.
For operations without adequate sow records, selection of a litter must be based on the sow's current performance. Gilts from large litters farrowed without difficulty and abnormalities should be identified by ear notches. Only these gilts should be used for further selection.
For more information on selection, see "Selection Guidelines For Commercial Pork Production," Extension Circular ANR-822.
Underlines. Unless a gilt has an adequate number of evenly spaced functional teats, she cannot be expected to raise a large litter. Blind nipples are often caused by contact with a rough floor during the first few days of life. Pin nipples are not functional and should be selected against. Inverted nipples may not function; they should also be avoided.
Underlines can be evaluated at weaning and before gilts are moved to the finishing area. Keep records on gilt underlines. Aim to select gilts that have at least twelve evenly spaced, functional teats. Whether you select a gilt with less than twelve functional teats should depend upon performance in your herd. If you wean an average of eight pigs per sow, it might be a mistake to cull a high-indexing gilt with eleven sound teats.
Reproductive soundness. The only portion of the reproductive tract visible on a gilt is the vulva. Abnormal vulvas can be an indication of abnormal reproductive tracts or can cause breeding problems. Gilts that have up-turned vulvas are often difficult for the boar to mate. Infantile or small vulvas can be an indication of a small reproductive tract. Select only those gilts with properly shaped vulvas of adequate size.
Structural soundness. Gilts selected from the best sows, even with perfect underlines and vulvas, are worthless unless they are sound in their feet and leg structure. Only those gilts with adequate slope in the shoulder, cushion in the pasterns, a level top, and cushion in the rear leg should be selected.
Gilts with straight shoulders and straight or "bucked over" front legs should not be kept. Many of these gilts cannot stand up under the weigh a boar or under the rigors of confinement. Gilts that are too straight in their back legs or have a high arch down the top have trouble standing during breeding and getting up in a farrowing crate. Selecting structurally sound gilts will reduce the number of breeding problems and crippled sows.
It might seem odd to mention performance when discussing ways to improve reproductive efficiency in gilts. However, backfat and growth rate are also economically important traits. Putting some emphasis on performance traits in boar and gilt selection will result in leaner, more efficient market hogs. Consider performance in the selection decision after you have identified the gilts with the greatest reproductive potential. Better yet, rank your gilts on a maternal index that includes backfat, growth rate, and litter traits.
Commercial producers use crossbreeding to get a large boost in reproduction. There are several factors to consider in your crossbreeding program.
Breeds. Research has compared various breeds and breed crosses. For litter traits, Landrace cross sows ranked first, Chester cross sows ranked second, and Yorkshire cross sows ranked third. Thus, crosses among the white breeds generally produce the most prolific gilts. One exception is the Hampshire-Landrace F1 gilt. Because of variation within breeds, you should select a prolific line in the breed(s) you decide to use.
Hybrid vigor. While the heritability estimates for mothering ability
are low compared to other traits, the expected improvement due to crossbreeding
(hybrid vigor) is moderate to large. Gilts with a large amount of hybrid
vigor should then be more productive (5 to 15 percent). Maximum hybrid vigor
is found when F1 gilts are used. Gilts from a 3-breed rotational cross show
86 percent as much hybrid vigor as F1's, while gilts from a two-breed rotational
cross show 67 percent as much.