Effects on milk production of increased dietary crude protein by feeding nitogen-fertilised turnips or lupins to dairy cows in mid-lactation

This experiment investigated whether increased crude protein concentration in turnips fertilised with nitrogen would improve milk production. Dairy cows were offered a basal diet of silage, hay and 5 kg dry matter barley supplemented with 5 kg dry matter turnips fertilised with nitrogen (n = 12) or 5 kg dry matter control turnips (n = 12) over 14 days. The effect of providing a diet isonitrogenous with the nitrogen-fertilised turnip diet and with rumen degradable protein on milk production was also investigated by replacing 1 kg barley with 1 kg lupins; 12 cows were offered 4 kg (dry matter) barley, 1 kg lupins with 5 kg control turnips. The control cows (offered control turnips with barley) consumed 17.8 kg dry matter, 2.34 kg crude protein and 187 MJ metabolisable energy. In comparison with the control cows, cows offered nitrogen-fertilised turnips with barley had lower (P<0.05) daily dry matter and metabolisable energy intakes and higher (P<0.05) crude protein intake whereas cows offered control turnips with barley and lupins had similar daily dry matter intake and higher (P<0.05) intakes of metabolisable energy and crude protein. The cows offered nitrogen-fertilised turnips with barley produced similar yields of milk and milk constituents to those of the control cows (19.6 L milk with 855 g fat, 625 g protein and 951 g lactose daily), reflecting the lower intakes of dry matter and metabolisable energy by the cows offered nitrogen-fertilised turnips with barley compared with that of the control cows. Cows offered control turnips with barley and lupins had a greater (P<0.05) daily yield of milk fat and protein than did the other groups of cows; milk yield was increased by 0.5 L when 1 kg dry matter barley was replaced with 1 kg dry matter lupins. It is economical for farmers to feed turnips in conjunction with barley and lupins to overcome summer shortages of high-quality feeds.