Utilization of dietary protein or non-protein nitrogen by lactating mares fed soybean meal or urea

Abstract

Two trials using 27 (trial I) and 28 (trial II) mares of predominantly Quarter Horse breeding were conducted to determine the quantitative and qualitative protein requirement of the lactating mare. Mares were blocked by expected foaling date and mating selection and assigned to dietary treatments containing different levels and sources of dietary nitrogen. Treatments were evaluated by foal growth, nitrogen balance, milk yield and milk composition. Rations in trial I were formulated and fed to contain a slight nitrogen deficiency in the control and a significant excess in the supplemented diets. Concentrate diets were formulated to contain different levels and sources of nitrogen as follows: (1) control (corn and oats) 10% C.P.; (2) control + soybean meal (SBM), 15% C.P.; (3) control + urea, 15% C.P. Average quality (9.96% C.P.) coastal bermudagrass hay was fed with each diet at 4.5 kg per mare daily. Trial II rations were formulated and fed to create a severe nitrogen deficiency in the control while the supplemented diets met (N.R.C., 1973) protein requirements of the lactating mare. Dietary treatments for trial II were: (1) control (corn and oats) 10% C.P.; (2) control + SBM, 17% C.P.; (3) control + SBM/urea, 175 C.P.; (4) control + urea, 17% C.P. A low quality (4.01% C.P.) coastal bermudagrass hay was fed at the rate of 4.5 kg per mare. Concentrate diets were fed to meet energy requirements of lactating mares. Concentrate consumption in both trials was approximately 6.75 kg per day. Mares were placed on treatment immediately following parturition, were fed individually (separated from foal) twice daily and maintained (together with foal) at all other times in dry lots with free access to water. All mares were weighed weekly and fed to maintain constant weight for an 84 day experimental period. Milk production was estimated by the weigh-suckle-weigh technique and foals were allowed to nurse every 4 hr in trial I and every 3 hr in trial II to obtain daily milk yields...