Effects of oscillating the crude protein content in dairy cow rations
Overfeeding crude protein (CP) is a common practice in the dairy industry to reduce the risk of a loss in milk; however, overfeeding CP increases costs and negatively impacts the environment. We hypothesized that oscillating dietary CP concentrations to equal the average concentration of a diet limited in metabolizable protein (MP) for lactating dairy cows will improve milk protein yield and milk N efficiency because oscillating CP should stimulate nitrogen recycling to the rumen. Twenty-one Holstein dairy cows averaging 123 DIM were randomly assigned to a treatment sequence in seven 3x3 Latin Squares with 28-day periods. The control diet contained 16.4% CP (MP allowable milk = 47 kg/d), the low protein diet contained 13.4% CP (MP allowable milk = 31 kg/d), and the oscillating treatment consisted of a diet with 10.3% CP fed for 2 d followed by a diet with 16.4% CP fed for 2 d repeated over the 28 d period to average 13.4% CP. The cows were fed once daily and milked twice daily. Cows on the low protein diet had greater DMI than cows on the oscillating treatment (24. 8 kg/d vs 24.3 kg/d; P = 0.04) but were similar in DMI compared to cows on the control diet (24.8 kg/d). There was no treatment difference (P > 0.05) for milk yield (avg. 34.6 kg/d), feed efficiency, milk fat yield, milk fat content, and milk protein content. Milk protein yield showed a treatment effect; however, treatment contrasts showed no difference between the oscillating treatment and low treatment and between the high and low treatments. Milk urea nitrogen (MUN) increased with protein content of the diet (14.5 mg/dl vs. 9.1 mg/dl; P < 0.0001) with no difference in MUN between cows fed the low and oscillating treatments except that MUN followed a cyclic pattern for oscillating cows. Given that cows on the low diet produced more milk than MP allowable milk, cows on the low protein diet used nitrogen more efficiently than expected or the NRC overestimated the requirements. Although milk and milk protein production were not affected by the oscillating treatment, very little research has been done on oscillating CP content of dairy cows rations; therefore, this study is a first step in understanding how oscillating protein affects dairy cows.
Keywords: dairy cow, metabolizable protein, oscillating protein