The results in this field over the last 10 years at Ruakura and relevant international data is reviewed. The nutrients involved have been defined and a likely mechanism of action indicated. Pasture allowance experiments indicated effects of pasture quality and a series of pellet feeding trials defined effects of both energy (linear) and protein (threshold) intakes. The threshold effect of protein was confirmed by the abomasal infusion of protein. Analyses of changes in blood concentrations of amino-acids indicated the involvement of the branched-chain amino-acids (BCAA) and this has been confirmed by an increased ovulation rate after intravenous infusion of BCAA. It is concluded that the static and dynamic liveweight and the immediate nutrient effects are the same response measured at different times. The rate at which the response develops is dependant on the type of feedstuff. Studies into the mechanism of action in both intact and ovariectomised ewes have ruled out effects on pituitary release of FSH either directly or via interference with negative feedback influences. This indicates that the effect is at the ovarian level and is mediated either by direct action of the BCAA or indirectly via a metabolic hormone.

AL, Parry, DK Edmonds, TW Knight, and GJ Hamilton

Proceedings of the New Zealand Society of Animal Production, Volume 50, , 477-478, 1990
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