Effect of litter size, dietary protein content, ewe genotype and season on milk production and associated endocrine and blood metabolite status of ewes
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In a series of experiments, ewe milk production and associated plasma hormone and blood metabolite status were investigated.Separate comparisons were made between ewes suckling either single (S ewes) or twin (T ewes) lambs, ewes lambing in either January or April, ewes fed either 150 g (low) or 210 g (high) of crude protein/kg DM in the diet and between ewes of the East Friesland (EF) and Scottish Blackface (SBF) genotypes.In all comparisons milk yields, live weight, body condition score changes, fat, protein and ash contents and energy values of milk were determined weekly. Blood samples were collected on one day each week, at 20 minute intervals for 2 hours, prior to feeding. Samples were pooled within each week and each animal. Plasma glucose, non- esterified fatty acids (NEFA), 3-hydroxybutyrate (3-OHB), urea, albumin, protein, insulin, growth hormone (GH), cortisol, prolactin, triiodothyronine (T3) and thyroxine (T4) concentrations were determined. During weeks 2, 4 and 10 (and 14; genotype comparison only) of lactation blood samples were collection at 20 minute intervals for 8 hours and individually assayed for plasma insulin, GH, cortisol and prolactin concentrations.Ewes rearing twin lambs had higher milk yields than those rearing single lambs. This was associated with higher NEFA, 3-OHB, GH and cortisol concentrations and lower insulin concentrations in the plasma of twin-rearing ewes. There were no consistent differences in prolactin or thyroid hormone concentrations of the plasma of single and twin rearing ewes. The decline in milk production with advancing lactation was associated with an increase in the post prandial insulin:GH and insulintcortisol ratios and T4 levels and a decrease in prolactin levels. Feeding was followed by significantly higher insulin levels and slightly higher GH levelsEwes fed the high protein diet had higher milk yields and milk protein contents compared with ewes fed the low protein diet. This, however, was not apparently associated with a higher degree of adipose tissue mobilisation. There were no significant differences between the two protein treatments in any of the plasma hormones measured. Change in milk production with stage of lactation was associated with an increase in the insulin:GH and insulinrcortisol ratios and in circulating T4 levels, and a decrease in plasma prolactin levels. During early lactation, feeding was followed by a rise in insulin and GH levels but during late lactation only insulin levels increased.In contrast to previous observations there was no difference in milk yield or pattern of milk production between the ewes of the EF and SBF genotypes. SBF ewes produced milk of higher fat content compared with EF ewes. All ewes gained similar amounts of live weight throughout lactation. However, plasma albumin and protein levels were lower in EF compared with SBF ewes; plasma insulin, cortisol, prolactin and T3 levels were consistently lower in EF than in SBF ewes. The decline in milk production in late lactation was associated with an increase in the insulin:GH ratio and in T4 levels while prolactin levels decreased.Feeding was followed by increased insulin levels and cortisol levels (EF ewes only) and lower GH levels (during late lactation).The role of these hormones in the control of milk production is discussed and in particular the hormonal inter-relationships in relation to level and pattern of milk production and associated nutrient status throughout lactation.