THE DIETARY-PROTEIN AND(OR) LYSINE REQUIREMENTS OF BARROWS AND GILTS

A cooperative research study involving three experiments and 2,318 pig s was conducted at 12 research stations to evaluate the protein (lysin e) requirements of barrows and gilts. The two sexes were penned separa tely and fed fortified corn-soybean meal diets containing protein leve ls ranging from 12.0 to 17.2%. Lysine levels in these diets ranged fro m .52 to .90%. Protein levels in Exp. 1 were 12, 14, and 16%; in Exp. 2, protein levels were 13, 14, 15, and 16%; and in Exp. 3, they were 1 3.2 15.2, and 17.2%. Fat (5 %) was added to one-half of the diets in E xp. 3. Each station that participated contributed a minimum of two rep licate pens of pigs per diet-sex combination in a given experiment. Av erage initial and final weights were 35 and 99 kg in Exp. 1 and 51 and 105 kg in Exp. 2 and 3, respectively. At the end of the test period, pigs were slaughtered and hot carcass weight, 10th rib fat depth, and longissimus muscle area were measured. Percentage of carcass muscle wa s estimated from these data. Overall, barrows gained weight faster tha n gilts (P < .01), but gilts required less feed per unit of gain (P < .05) and had less backfat, larger longissimus muscle areas, and a grea ter percentage of carcass muscle (P < .01) than did barrows. Lean grow th rate was similar for barrows and gilts (332 vs 329 g/d). Increasing the dietary protein or lysine level resulted in improved rate and eff iciency of gain and increased carcass leanness and lean growth rate in gilts, but the increase was less pronounced or did not occur in barro ws, resulting in protein level x sex interactions. Feeding low-protein (12 or 13%) diets decreased performance and carcass leanness to a gre ater extent in gilts than in barrows. The pooled data from the three e xperiments indicated that most traits tended to reach a plateau at 13% CP (.60% lysine) in barrows, whereas in gilts, weight gains, feed/gai n, carcass muscle, and lean growth rate continued to improve, but at a decreasing rate, with up to 17.2% CP (.90% lysine). The results indic ate that gilts require higher concentrations of dietary amino acids to maximize lean growth rate than do barrows.