This experiment tested the hypothesis that the form of surplus nitrogen (ammonia or amino acids) affects nutrient partitioning in lactating ruminants. Nine, abomasally-cannulated, 4 year old Coopworth ewes (66.4 ± 5.9 kg liveweight), suckling twin lambs were offered a basal diet designed to meet their requirements for ME and MP in 12 equal portions at 2 h intervals for two periods of five weeks. During both periods, ewes were allocated to one of three groups; continuous abomasal infusion of 2 l water/day (control), infusion of 150 g sodium caseinate (20g N) dissolved in 2 l water or 85 g ammonium bicarbonate (15 g N) in 2 l water. Liveweight change of ewes and lambs, ewe milk production and composition and nitrogen balances were measured. Treatments had no significant effect on ewe dry matter intake, in vivo dry matter digestibility or ewe liveweight change. Rela-tive to control, casein infusion increased milk production (33%) and lamb liveweight gain (by 26% from 132 to 166 g/day) but had no effect on milk composition. In contrast ammonia infusion decreased milk production (15%), reduced milk protein concentration (from 63 to 56 g/kg), increased milk fat concentration (from 66 to 75 g/kg) and reduced lamb liveweight gain (from 132 to 66 g/day). The N apparently utilised (dietary N + infusate N faecal N urine N) was apportioned (milk N : body N) in ratios of 0.86 : 0.14, 1.0 : 0 and 0.52 : 0.48 for control, casein and ammonia infusions respectively. It is concluded that excess N in the form of ammonia diverted protein from milk production to body retention, thus the form of surplus N may affect nutrient partitioning in lactating ewes.
Proceedings of the New Zealand Society of Animal Production, Volume 59, , 158-161, 1999
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