Several procedures can be used to increase the weight gain of the smaller pigs in an attempt to reduce variation in a group. Split suckling, use of supplemental milk and shifting the smallest pigs to better milking sows are examples. These management techniques have been proven to slightly increase the weight gain of the small pigs and, thus, reduce variation at market weight. However, the impacts are all relatively small, so the economic payback must be carefully calculated before implementation.

Split-nursing — allowing the smallest half of the litter access to the sow for two hours within a 24-hour period after farrowing — has been shown to reduce variation in average daily gain and, thus, numerically reduce the variation in weaning weight. But, again, the impact is relatively small (about 2% lower coefficient of variation [CV]).

Research has also shown that offering milk replacer to nursing litters can effectively increase weaning weight and that weight is not lost during the nursery stage. Thus, the increased weight at weaning remains at market time, but it must pay for the entire cost of the milk replacer.

Similarly, trials with complex nursery diets have demonstrated increased weight gain in the nursery. Although data has not been entirely consistent, the advantage often does not become larger during the finishing stage.

These changes in pig weight are relatively small compared to the differences caused by weaning age, sex or health status.

Since barrows grow faster than gilts, raising pigs within a common group based on gender will reduce the variation within that group simply because the variation in growth rate compared to mixed-gender feeding is reduced. Because barrow and gilt growth rates differ, diets are often formulated for specific genders at given weights.

Another approach is to feed barrows and gilts the same diets, but alter the feed budget to match their nutrient needs at different stages of growth.

Although split-gender feeding is simple in concept, the production system must be large enough to fill a barn or site with one gender within a reasonable amount of time.

If split-gender feeding is practiced, but it takes several weeks to fill the barn, the variation in pig weights within that barn will be greater than if the pigs were housed as mixed-gender groups from a common weaning group.

Likewise, if a common barn is filled by gender in a short period of time, but the pigs come from multiple sow production sites, the health status of the pigs may be jeopardized, which may reduce growth rates of the entire barn.

The bottom line is this — filling a site or barn over multiple weeks or from multiple sources produces other large sources of variation (weaning age, health status) that may overwhelm any advantage of single-gender housing. As a rule of thumb, if a barn cannot be filled with a single sex within seven days, pigs should be housed as mixed sex to improve facility utilization.

Grouping pigs by gender in order to feed higher amino acid levels to the gilts has been practiced in the industry for some time. However, simply dividing groups based on weight has not been commonly practiced.