Effect of dietary protein and rumen degradable organic matter on milk production and efficiency in heat-stressed goats

Abstract

Four Israeli mix-breed Saanen dairy goats were used in a 2×2 factorial design to evaluate the effect of the level of dietary crude protein (CP) and ruminally degradable OM (RDOM) concentration on the production, composition and efficiency of milk production under hot ambient conditions. Daily ration comprised of 30% hay and 70% concentrates was divided into 12 equal meals. Treatment factors were 18% (HCP) or 15% (LCP) of CP in the diet DM, and 56% (HRDOM) or 50% (LRDOM) of RDOM. Goats were housed in metabolic crates in air-conditioned room. Air temperature ranged between 25 °C and 36 °C, and the thermal humidity index (THI) between 74 and 90 units. Daily amplitude of goat's body temperature was 1.2 °C and was unaffected by treatments. The DM intake, milk yield, and concentrations of protein, fat, and lactose, as well as total tract apparent digestibility of DM and OM were similar among treatments (P>0.1). Digestibility of CP was higher in the HCP diets (P<0.01), and digestibilities of NDF was higher in the LRDOM diets (P<0.01). Digestibility of non-structural carbohydrates was higher in the high RDOM diets and high CP diets (P<0.1). Blood glucose and insulin concentrations were similar among treatments (P>0.1), while NEFA level was higher in LRDOM (P<0.1). Rumen N-ammonia (P<0.05) and milk N-urea (P<0.01) were lower, and plasma N-urea tended to be lower in LCP diets (P<0.15). Overall, these data indicate that N utilization was lowered in goats consuming the high CP diets. It was concluded that diets comprising 15% of CP in DM are adequate for dairy goats exposed to daily high environmental temperatures, and moderate temperatures at night. Though varying dietary RDOM levels did not affect milk N efficiency, the higher NSC digestibility, and the concomitant lower blood NEFA suggest a reduced heat increment in dairy ruminants consuming HRDOM diets.