Lambs in pens were infused duodenally with increasing amounts of casein or a mixture of four essential amino acids (EAA) to examine effects upon N retention. Eight Dorset Down x Coopworth lambs (39+/-1.1 kg) each fitted with rumen and duodenal cannulae were housed indoors and offered freshly cut ryegrass/white clover pasture at a rate of 470g fresh herbage/kgW0.75(MW)/d. The metabolizable energy intake from herbage was approximately 870 kJ/kg(MW)/d. Experimental treatments were duodenal infusions of water (control), 3, 6, 9 and 12g casein/kgMW/d and three levels of a mixture of AA (methionine, lysine, histidine and arginine) in amounts equivalent to the supply of these AA in 3, 6 and 9g casein. Treatments were allocated in an 8 x 8 latin square design. Duodenal non-ammonia N (NAN) flow indicated that 33% of forage protein N intake was lost across the rumen, the mean flow being 1.46gNAN/kgMW/d. Infusions of casein resulted in significant linear increases (P<0.01) in N balance up to 6g casein/kgMW/d, the casein N supply at this level being equal to the N lost across the rumen from the basal forage. The increase in N balance with infusion of 6g casein was equivalent to an additional 96% of the N balance observed with the control treatment. No further significant change in N balance was observed with casein infusions greater than 6g/kg/MW/d. A small non-significant response to infusions of 4 AA was observed indicating that these 4 essential AA were not solely responsible for the response. These results indicate that liveweight gain responses can be expected if the losses of protein that occur in the rumen of lambs grazing temperate pastures can be reduced.

AR, Bray, RN Burton, MA Murray, and DJ Saville

Proceedings of the New Zealand Society of Animal Production, Volume 49, , 303-306, 1989
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