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Production level of dairy cows affects the extent of diet-induced milk fat depression
The interaction between diet and milk production level on the risk of diet-induced milk fat depression (MFD) was evaluated in a crossover design experiment with a covariate period. Thirty-two mid- and late- lactation multiparous Holstein cows (14 rumen cannulated and 18 non-cannulated), with a wide range and uniform distribution of milk yield (25 to 60 kg/d), were assigned randomly to treatment sequence within level of milk yield. Treatment diets and composition (% DM) were 1) control diet (CON), containing 24% starch, 33% NDF, 22% forage NDF, and 3% fatty acids (FA); and 2) MFD-inducing diet (MFDI), containing 30% starch, 28% NDF, 17% forage NDF, and 4% FA. Treatment periods were 28 d with the last 7 d for data and sample collection. The statistical model included the random effect of cow, fixed effects of period, treatment, cannulation block, and the interactions of cannulation block and treatment, and treatment and period. Linear and quadratic effects for the interaction between covariate milk yield and treatment were added to evaluate responses to treatment by level of milk yield. MFDI decreased milk fat concentration (3.31 vs. 3.69%, P<0.01) and tended to decrease milk fat yield (1.21 vs. 1.28 kg, P<0.10) and 3.5% fat corrected milk (35.6 vs. 37.0 kg, P<0.10) compared with CON. There was no main effect of treatment on dry matter intake (DMI) but treatments interacted with covariate milk yield (P<0.01); MFDI decreased DMI for higher producing cows but increased DMI for lower producing cows. Linear interactions (P<0.10) were detected between treatment and covariate milk yield for milk fat content and yields of milk fat and 3.5% fat corrected milk; compared with CON, MFDI decreased content and yield of milk fat and 3.5% fat corrected milk to a greater extent in higher producing cows than lower producing cows. The MFDI treatment decreased mean ruminal pH compared with CON (6.00 vs. 6.13, P<0.01), and treatments interacted with covariate milk yield (P<0.10), with less difference between CON and MFDI in higher producing cows than lower producing cows. Compared with CON, MFDI increased body condition score change (0.18 vs. 0.11, P<0.01) but response was not related to covariate milk yield. In conclusion, higher producing cows were at higher risk for milk fat depression induced by a high starch, low forage NDF diet containing supplemental fat.
Keywords:
Milk fat depression
Rumen pH
Production level