Nitrogen use efficiency and feed conversion efficiency of lactating dairy cows grazing simple and diverse pastures

Abstract

Two experiments were conducted at the Lincoln University Research Dairy Farm from the 25th of March to the 10th of April to examine whether NUE and FCE were affected by pasture type. In experiment 1, 20 Holstein Friesian × Jersey cows were fed either a simple (S) pasture containing ryegrass and white clover, or a diverse (D) pasture containing ryegrass, white clover, lucerne, plantain and chicory. Apparent intake and nutritive value of was determined and NUE was estimated using N intake and N excreted in milk. In experiment 2, 8 cows from experiment 1 were grazed individually on either S or D pastures. Energy intake was determined for each cow based on disappearance. FCE was estimated using heart rate (HR) to determine heat production (HP). Differences in cow activity were recorded using activity monitoring ear tags to examine differences in energy expenditure. Milk production and composition were unaffected by diet in experiment 1. There was a tendency for higher fat and protein yields (P<0.10), however this was not significant. Average milk yield ranged from 14 to 14.9 L/cow/day. Average N intake ranged from 510 to 541 g N/cow/day and was unaffected by pasture type (P>0.05). NUE was not affected by diet ranging from 21.4 to 22.7%. There was a tendency (P<0.10) for cows grazing the S pasture to excrete more N in milk, though this did not result in a higher NUE. FCE in experiment 2 was not affected (P>0.05) by diet, and ranged from 0.37 to 0.46 MJ NE/MJ ME intake. ME intakes were similar (average 155 MJ ME/cow/day) and total NE retained averaged 56.7 MJ NE (milk and live weight gain). Cows grazing D pastures spent 417 mins/day ruminating compared with 383 mins/day for cows grazing S pastures (P=0.001). This difference was not reflected in their HR (average 82.5 beats/min) or estimated HP (80.3 MJ heat/cow/day) (P>0.05). Feeding D pastures that do not differ in their ME content or N content did not influence NUE or FCE of lactating dairy cows in this experiment.