Phenylalanine Requirement of the Lactating Sow

Six mature Yorkshire × Landrace sows were randomly assigned to a 6 × 6 Latin-square experiment on d 3 of lactation to determine the phenylalanine (Phe) requirement in the presence of excess tyrosine (Tyr). A control diet of corn sugar, cornstarch, whey, L-glutamic acid, solka floc, soybean oil, amino acids, minerals and vitamins was supplemented with L-Phe to provide .175, .250, .325, .400, .475 and .550% Phe and .47% Tyr. Each diet was fed to a maximum of 5.5 kg/d within each of six 7-d periods. Sow milk yield and average pig weight gain decreased (cubic, P<.01) with increasing time period. Feed intake and average pig weight gain were depressed (quadratic, P<.05) on the lowest Phe diet, but milk yield was not significantly affected. Daily nitrogen (N) intake was depressed (quadratic, P<.01) when sows consumed the .175% Phe diet, but percent fecal N was not affected. Percent urine N decreased (quadratic, P<.001) and percent N retained (without milk) increased with increasing Phe to .288 and .296%, respectively. Daily secretion of milk N as a percentage of N intake was not affected by diet. Percent overall N balance (with milk) increased with increasing Phe to .307%. Urea N decreased with increasing dietary Phe in both plasma (linear, P<.01) and urine (quadratic, P<.001) to a breakpoint at .285% Phe. A sharp rise (quadratic, P< .01) in plasma Phe occurred at .299% dietary Phe. Plasma Tyr increased (quadratic, P<.05) as dietary Phe increased, but no clear inflection point was obtained. Plasma lysine and leucine decreased, and plasma isoleucine and valine increased with increasing Phe to plateaus at .317, .303, .367 and .359% Phe, respectively. If equal weight is given to the N balance and plasma amino acid data, 30% Phe seems to meet the dietary requirement for the lactating sow. In a diet containing natural ingredients, the requirement would increase to .34%, or 18.7 g/d for sows fed 5.5 kg/d. It was suggested that Tyr could supply 48 to 54% of the total aromatic amino acid requirement of the lactating sow.