Effect of protein and energy intake on digestion and nitrogen metabolism in wethers and on ovulation in ewes

Two experiments were conducted to investigate the basis for an increase in ovulation rate in ewes given improved nutrition prior to mating.Sixteen wether sheep were used in the first experiment in order to screen physiological and nutritional variables likely to be affected by protein and energy intake. They were given low protein (LP) and high protein (HP) diets (19·1 and 35·2 g nitrogen per kg dry matter (DM)) each at four levels of energy intake (range 7·67 to 27·06 MJ gross energy per day). Effects of treatment on digestibility and partition of digestion between the rumen and intestines were small but increased nitrogen intake increased both nitrogen retention and the concentration of some essential amino acids (EAA) in plasma. The dietary treatments imposed on the wethers had been given to 800 ewes previously in a separate trial designed to predict ovulatory response to dietary and energy intakes. When the variables measured in the wether sheep were compared with predicted ovulation for ewes given the same treatments, the strongest relationship was with plasma concentrations of branched-chain amino acids (BCAA; r = 0·95; P < 0·001). Correlations with other EAA were weaker (r = 0·61; P < 001) and were not significantly related to concentrations of non-essential amino acids (NEAA) in plasma (r = −0·21).Eighty ewes in experiment 2 were given an LP diet, blood samples were taken and ovulation rate determined by laparoscopy. One group of 20 ewes was maintained on the diet and another group of 20 was given an HP diet starting on day 7 of the oestrous cycle. Blood samples were taken 5 days later and numbers of ova determined during the next cycle. There were no changes in the number of ova present in ewes maintained on the LP diet, but a change to the HP diet increased ovulation rate from 1·47 to 1·89 eggs per ewe. Ewes which had single or multiple ova when given the LP diet (pre-treatment) had similar concentrations of amino acids in their blood, but those given the HP diet increased blood concentrations of BCAA by proportionately 0·58 and other EAA by 0·29 whilst NEAA were unaffected. Ewes which increased their ovulation rate when given the HP diet had a greater increase in blood BCAA concentration than those which did not increase their ovulation rate. Both the concentration in blood and the magnitude of increase in BCAA concentration appear to affect the number of eggs released. This concept is supported by evidence that BCAA are able to exert regulatory effects on mammalian protein metabolism under some circumstances.