Protected canola meal increases milk protein concentration in dairy cows fed a grass silage-based diet

Low concentrations of protein in milk occur during the summer-autumn in south-west Australia. This is the period, on dryland farms, when the diet of lactating cows typically consists of grass silage and a mixture of crushed lupins and cereal grain. This experiment was conducted to test the hypothesis that supplying protected canola meal would increase the protein concentration of milk and, possibly, milk yield in cows fed grass silage and a lupin-cereal concentrate. Sixty Holstein cows in mid lactation were allocated to 2 equal-sized dietary treatment groups: control (lupin) or protected canola meal. The control diet consisted of 14.5 kg DM grass silage (annual ryegrasses-subterranean clover) and 5.4 kg DM of crushed lupins and barley (4:1) per head per day. For the protected canola meal diet, 2.15 kg DM protected canola meal replaced 2.15 kg lupins. The protected canola meal was produced by treating solvent-extracted canola meal with formaldehyde, to produce a product with an in sacco fractional degradability of 0.29 at a rumen fractional outflow rate of 0.08/h. The equivalent degradability of untreated canola meal was 0.80 and of lupin was 0.83. Cows were individually fed the concentrate ration twice daily, after each milking, then were managed as a single herd in dry lots and fed grass silage. By the end of 8 weeks, cows fed the protected canola meal diet had higher milk protein concentrations (30.7 v. 29.2 g/L; P<0.05) and higher liveweights (604 v. 593 kg; P<0.05). Milk yield (L/day) was increased by 1 L/day, but this effect was not significant (P>0.10). Fat concentration was unaffected by diet (P>0.05). Since the only difference in treatment was the replacement of a portion of lupins with protected canola meal, the results indicate that a deficiency of metabolisable amino acids contributes to the low milk protein concentrations recorded during summer-autumn in south-west Australia. Whether this was acting primarily through a stimulus of appetite, or directly on milk components, could not be determined because silage intakes were not recorded.