This experiment was undertaken to develop diets that could be fed ad libitum to developing gilts that would result in differences in growth rate or body composition during development. A further component of the experiment was to determine whether diets affected age at puberty. This study is a prelude to a larger study to measure dietary affects on sow productivity to third parity. Diets were developed that had normal levels of metabolizable energy (~3200 kcal/kg) but differed in SID lysine levels, with a control designed to be adequate, and two further diets with progressively less SID lysine (medium and low lysine, respectively). Diets were fed in two phases, a grower phase that was applied for 6 weeks, and a finisher phase that was fed from then until gilts left the experiment at 220 days of age. Diets were begun at 100 days of age. Gilts were weighed and measured for backfat and loin depth by ultrasound when diets began and at 28 day intervals until 212 days of age. Blood samples were collected from gilts that had not expressed estrus by 210 and 220 days of age for progesterone analysis to assess whether they were prepubertal or behaviorally anestrus. Finally, at 220 days of age all gilts not experiencing a standing estrus were injected with PG600 and observed for 1 week to determine their estrus response.

Gilts on this trial experienced a porcine epidemic diarrhea virus outbreak at approximately the time estrus detection and boar exposure was begun. It is likely that this outbreak temporarily impaired the growth of gilts, but there was no uninfected control to allow conclusions to be drawn on the PED effects, if any. On the other hand, all treatments were similarly afflicted, thus conclusions from the trial regarding comparisons among diets are likely to be valid.

Compared to the Control diet, the medium and low SID lysine diets progressively reduced body weight, loin depth and backfat gain in gilts, with no changes in relationships (body composition) between loin depth and backfat. Only approximately 30% of gilts reached puberty by 220 days in this experiment, likely due to the effects of PED. The number of gilts experiencing puberty per the number of gilts exposed to the boar was numerically less for the medium and low lysine diets, but these differences were not statistically significant. However, mean age at puberty was significantly greater in gilts fed the low lysine diet. Most noncyclic gilts experienced puberty in response to PG600, suggesting that they were prepubertal, rather than behaviorally anestrus. However, about half the gilts that were confirmed to be behaviorally anestrus using progesterone concentrations responded to PG600 by exhibiting standing estrus within one week.

These results indicate that we were successful in developing ad libitum fed diets that resulted in reduced growth of developing gilts by reducing the lysine level of the diet. Although the number of gilts reaching puberty did not differ, age at puberty in the low lysine diet was 7 days older than the control and medium lysine diets. These diets can now be used to determine effects of reduced growth rates on retention of gilts in the breeding herd.