Production and metabolic aspects of nonprotein nitrogen incorporation in lactation rations of dairy goats

Abstract

Two lactation trials were conducted to evaluate the effects of dietary urea on milk production and metabolic criteria in dairy goats. In Experiment 1, 18 primiparous Alpine does (average BW 42kg) were fed one of three diets (65% TDN): NPC1, negative protein control (9.5% CP); PPC1, positive protein control (14% CP); and NPND, nonprotein N diet (14% CP, urea contributed 33% of the total dietary N). The lactation trial consisted of a 3 week adaptation and a 5 week collection period. Mean ADG, DM intake, milk yield, milk fat, and milk N were not affected by diet ( P > 0.10); however, milk noncasein N ( P < 0.02) and serum-derived protein N ( P < 0.05) were greater in the does fed the NPND diet compared with the does fed the NPC1 diet. Ruminal fluid ammonia N ( P < 0.01) and plasma urea N ( P < 0.001) were increased by dietary N level and NPN incorporation, and ruminal acetate:propionate molar ratio was lower ( P < 0.02) in does fed NPND compared with does fed PPC1. In Experiment 2, 20 multiparous Alpine does (average BW 55 kg) were fed one of four diets (65% TDN): NPC2, negative protein control (12.5% CP); PPC2, positive protein control (17% CP); LNPN, low nonprotein N (17% CP, urea contributed 25% of the total dietary N); and HNPN, high nonprotein N (16% CP, urea contributed 50% of the total dietary N). The lactation trial lasted 10 weeks following a 3 week adaptation period. Mean ADG, DM intake, milk yield, and milk fat were not affected by dietary treatment ( P > 0.10). Total milk N was affected ( P < 0.002) by diet. The NPN fraction was lowest ( P < 0.04), and the true protein N ( P < 0.005) and casein N ( P < 0.001) fractions highest in milk from does fed NPC2 compared with does fed PPC2. In addition, milk true protein N and casein N fractions were lowest in does fed the HNPN diet ( P < 0.005). Increasing dietary N levels and incorporation of NPN resulted in elevated ruminal fluid pH ( P < 0.10), ammonia N ( P < 0.02), and free, nonionized ammonia N ( P < 0.03). Ruminal fluid acetate:propionate molar ratio was lowest ( P < 0.001), and plasma ammonia N ( P < 0.04) and urea N ( P < 0.01) were elevated in does fed the HNPN diet. Plasma glucose, insulin, glucagon, and cortisol were not affected ( P > 0.10) by diet in either experiment. Incorporation of NPN in goat lactation diets can sustain milk yield during mid-lactation without the development of clinical hyperammonemia; however, milk N fractions, and hence, the milk processing properties may be affected at the higher levels of NPN incorporation.