Abstract:
2A THE EFFECT OF PROTECTED AND UNPROTECTED CASEIN SUPPLEMENTS ON THE PERFORMANCE OF LACTATING COWS FED PASTURE I.M. BROOKES * There is conflicting evidence in the literature as to whether the yield of cows fed pasture is limited by the supply of amino acids reaching the small intestine (Brookes 1982). In this study, four pairs of identical twin cows were housed indoors and offered ryegrass- white clover pasture ad libitum from 2 weeks after calving. The pasture contained 14SMJ digestible energy(DE) and 36g nitrogen (N)/kg dry matter (DM). A daily supplement of lkg casein (887gDM/kg and 149gN/kgDM) was mixed with the pasture. The casein offered to one twin in each pair was treated with a 10% solution of formaldehyde at a rate of O.lg/kg casein. The extent to which this treatment afforded protection against rumen degradation was determined by suspending polyester bags containing casein in the rumen of a grazing cow. After 8 hours, 5% and 88% of the N was lost from the treated and untreated casein respectively. milk Feed intake and milk production data are given in Table 1 and the results of a IO-day faecal and urine collection on individual cows are shown in Table 2. TABLE 1 Mean daily DM intakes and milk production of 3 sets of twin cows for 29 days in early lactation Plasma urea and insulin concentrations were determined on blood samples taken hourly throughout the day. These did not differ significantly between treatments (Urea - 0.48 and 0.52 mg/ml and insulin - 3.0 and 2.7 ng/ml for the protected and unprotected casein groups respectively)* The supplementation of high yielding dairy cows(over lkg milkfat daily) with protected casein for approximately one month in early lactation did not significantly increase milk yield -or N output in milk,compared with those fed unprotected casein. This suggests that the supply of amino acids reaching the small intestine from a pasture diet was not limiting the production of these cows. BROOKES, I.M.(1982) Dairy Production from Pasture:p 211. N.Z.S.A,P, Hamilton, N.Z. * Department of Animal Science, Massey University, Palmerston North, N.Z.