Responses to abomasal infusions of casein plus methionine in lactating ewes fed fresh pasture

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Abstract
Twin-bearing Romney ewes were fed fresh pasture of 80% organic matter digestibility (11.4 MJ ME per kg DM) and 3.5% N ad libitum for 6 weeks during early lactation. Eight of the 18 ewes were given a daily abomasal infusion of 95 g sodium caseinate and 2 g l-methionine. Voluntary pasture DM intake did not significantly differ between control and infused ewes. The infusions had no effect on milk secretion at peak lactation (week 1), but the decline in net rates of protein and fat secretion as lactation progressed was less for infused than for control ewes. Wool production rates were low, and were unaffected by the infusion treatment. The quantity of amino acid N estimated to have been absorbed from the small intestine was respectively 85% and 110% of that required to sustain the levels of milk and wool production that were actually recorded in control and infused ewes. The absence of a difference in milk production between the two groups of ewes at peak lactation may have been due to greater mobilisation of body protein reserves by the control ewes. The higher plasma concentration of growth hormone observed in the infused ewes was probably implicated in their improved lactation persistency.