Abstract:
EFFECTS OF TREATMENT OF LACTATING EWES WITH ALLOXAN ON YIELD AND FAT CONTENT OF MILK D. LEENANURAKSA and G.H. MCDOWELL Feeding rations containing high concentrate/low roughage to ruminants increases propionate production and decreases the ratio of acetate: propionate in the rumen, and increases glucose production in the body. Rations of this type commonly induce the 'low milk fat syndrome'. McClymont (1951) first drew attention to the possibility that increased propionate production may stimulate insulin secretion and thereby reduce the availability of precursors for milk fat synthesis. We have tested effects of ablation of insulin and subsequent replacement with exogenous insulin to maintain a hyperglycaemic state as yield and fat content of milk in lactating ewes. c Four lactating crossbred ewes, - 8 weeks post-partum and 60 kg liveweight, were kept in metabolism cages from parturition. Lambs had been removed at birth and ewes were milked by hand twice daily. They were fed continuously 2.5 kg/d a mixed ration (9.6 MJ ME and 218 g crude protein per kg dry matter) and feed refusals were recorded. Insulin secretion was ablated by treatment with alloxan (50 mg/kg liveweight) and subsequently ewes were given continuous intravenous infusions of insulin (at rates varying from 0.7-l-7 U/h) to maintain a mild hyperglycaemia, Measurements were made, over periods of 3d, before alloxan treatment and when plasma glucose concentration had stabilised (c 1 wk after alloxan) - see Table. Yield and fat content of milk, feed intake and plasma glucose before alloxan . and after stable glucose levels had been achieved. Values are means for 4 ewes In spite of maintenance of high concentrations of plasma glucose after alloxan, milk fat content was significantly higher and milk yield unchanged than before alloxan treatment. This contrasts with the situation in intact animals where it has been found that intravenous infusion of glucose to raise plasma levels induces the 'low milk fat syndrome' in lactating cows (McClymont and Valiance 1962) and ewes (Aryajasseet al. 1980). The present results support the suggestion of McClymont (1951) that insulin is a key factor in the aetiology of the 'low milk fat syndrome' ARYAJASSE, Aust. McCLYMONT, McCLYMONT, S., GOODEN, J.M. and MCDOWELL, G.H. (1980). Proc. Nutr. Soc. 5: 158, E.L. (1951). Aust. J. Agric. Res. 2: 158. G.L. and VALLANCE, S. (1962). Pro:. Nutr. Soc. 21: xii. -- Dairy Research Unit, Department of Animal Husbandry, University of Sydney, Camden, New South Wales 2570
