Tri-State Swine Nutrition Guide

Bulletin 869-98

Acclimation

Replacement gilts are either commonly reared in a grower-finisher facility along with market hogs, or purchased at market weight from a seedstock supplier. It is becoming more common to purchase replacement gilts at 40 to 60 lbs. and then to isolate and allow them to acclimate to herd conditions for several months. By five to six months of age, gilts should be introduced into a gilt replacement pool and fed a diet with a higher nutrient content (Table 6). During the following two- to three-month period, they should be acclimated to the different housing conditions, exposed to sow herd diseases, have fence-line or direct contact with boars, and be monitored daily for estrus activity. Gilts should not be bred until their second or third estrus.

During the period prior to breeding, two feeding strategies have emerged in the United States for replacement gilts.

The first strategy, used largely with maternal lines that are genetically lean, is to increase the gilt's body fat content during the prebreeding period. By feeding a lower protein diet, the rate of muscle growth will be slightly reduced, but there will be an increasing body fat content. Producers with "high-lean" gilts use this method because of the importance of body fat on later lactation and rebreeding performance.

The second strategy, used largely with gilts with less lean potential, invokes the feeding of a lower quantity of a diet during the pre-breeding period. This method results in a lower body weight at breeding. Moderate increases in body fatness occur prior to the gilt's initial breeding while maintaining maximum lean. This strategy is used for gilts that have a lower mature body weight than the higher-producing genotypes. If feed is not restricted, many of these gilt lines often get too fat and heavy, which later results in poorer lactation feed intake, less milk production, and lower litter weaning weights. Vitamins and minerals should be formulated at a higher concentration in order to meet the daily quantitative needs of the replacement gilt (Table 8).

When gilts of either strategy are thin at breeding, the provision of a high quantity of feed for 11 to 14 days prebreeding (i.e., flushing) is recommended. Flushing will result in an increased ovulation rate and litter size.

The decision of when to breed gilts is unfortunately often based on the need to fill farrowing groups. Gilt age, weight, and backfat thickness should, however, be the criteria for selecting animals for breeding. Target ages, weights, and backfat thicknesses are presented in Table 7. Research has shown that genotypes with a high-lean and/or high-producing capacity should be bred at heavier body weights than industry average gilts. Breeding gilts after they attain physical and body compositional maturity will help to ensure that they will have sufficient body nutrient stores to meet the metabolic challenges of reproduction. Backfat thickness at the 10th rib may be slightly less than that collected at the last rib.

The influence of gilt age and body condition on their subsequent productivity and lon-gevity depends on several factors including nutrition, feeding management, lactation length, culling criteria, breeding practices, herd health, and housing condition. Allowing gilts to remain in the replacement pool for an extended time can increase production costs and is discouraged. Although the onset of ovulation is largely under genetic control, feeding and management practices can result in gilts being mated by seven months of age without compromising subsequent lifetime performance. The number of estrus cycles should be monitored and daily records kept so that gilts can be bred by their third standing heat or third ovulation. Gilts that do not demonstrate strong estrus behavior should not be bred.

The effective environmental temperatures that gilts are exposed to during the acclimation period is important. The effective environmental temperature is different than air temperature. It is described in practical terms as the temperature that the "animal feels" or its comfort zone. Behavioral characteristics of gilts will generally be a good indicator of environmental conditions that are satisfactory. Drafts of 4 to 10 mph on wet concrete floors can produce an effective environmental temperature that is 7 to 13 degrees colder than air temperature. Conversely, straw bedding and group housing makes pigs "feel" warmer. Increased feed intakes with subsequent decreases in nutrient densities in the diet can be used for colder housing situations (see Table 8).