The effect of diet in late pregnancy on progesterone concentration and colostrum yield in ewes

In the ewe, high level feeding is associated with low levels of circulating progesterone and that progesterone withdrawal is a prerequisite for lactogenesis and the onset of copious milk secretion. A total of 108 ewes was allocated to a 3 x 2 factorial experiment to determine the effect of diet on serum progesterone (P4) concentration and colostrum yield. On D 96 of pregnancy, the ewes were offered either grass silage, molassed sugar beet pulp (beet pulp) silage or grass silage supplemented with beet pulp. All ewes were given ad libitum access to silage. From D 126 of pregnancy, half the diets were supplemented with soybean meal with the aim of achieving a total crude protein intake of 220 g/ewe/day. Blood samples were taken on D 142 of pregnancy and at 1, 10 and 18 h post lambing for P4 determination. Metabolizable energy intakes (Mega Joules/ewe/day) of 9.1, 11.2 and 12.1 (SEM 0.37) and crude protein intakes (g/ewe/day) of 142.8, 167.4 and 162.0 (SEM 3.46) were recorded for grass silage, beet pulp ensiled and beet pulp supplemented silages, respectively, from D 126 to D 147 of pregnancy. Soybean meal supplementation increased the crude protein intake from 97.1 to 217.0 g/ewe/day (SEM 2.86) and Metabolisable Energy intake from 9.0 to 12.6 Mega Joules/ewe/day (SEM 0.31). Supplementation with soybean meal decreased P4 concentration (ng/ml) on D 142 (P < 0.05) and at 1 h (P < 0.01), 10 h (P < 0.01) and 18 h post lambing (P < 0.01). Ewes offered grass silage had higher P4 concentrations on D 142 (P < 0.01) and at 1 h after lambing (P < 0.05) than the beet pulp-supplemented ewes. There were negative linear relationships between P4 concentration at D 142 and colostrum yield at 1 h (P < 0.01) and between P4 concentration at 1 h and colostrum yield at 1 h (P < 0.001) and total yield at 18 h (P < 0.001). In conclusion, ewes which were underfed had higher P4 concentrations at all times, and there were negative linear relationships between colostrum yield and P4 concentrations on D 142 of pregnancy and at 1 h post lambing.