Short-term effects of level of dry matter intake upon milk yield, fatty acid composition of milk fat, and some blood constituents of Hereford cows

Abstract

The effect of two levels of feed intake (lucerne hay with mean digestible organic matter intakes (DOMI) of 6.1 and 9.1 kg/animal/day) on milk yield, calf growth rate, fatty acid composition of milk fat, and blood metabolites (non-esterified fatty acids and glucose) of suckled Hereford cows in their second and third month of lactation was measured to determine the most accurate indicators of intake of nutrients. Milk yields of cows averaged 5.0 and 6.1 kg/day on low and high intakes respectively, but production was extremely variable and poorly correlated with DOMI (r = 0.33). Calf growth rates were correlated with the milk yield of dams (r = 0.41) and with DOMI (r = 0.37). The fatty acid composition of milk fat provided the most repeatable (coefficient of variation of 6%) and reliable index of the feed intake, correlations being 0.71 with C4–C16 acids and –0.65 with oleic acid. Dry matter restriction decreased the proportion of C4–C16 acids (68.5% to 62.3%), while the proportion of oleic acid increased (17.0% to 21.9%). Cows responded rapidly to the level of feeding imposed, and no residual effects of previous feeding treatments were measured after 14 days. Non-esterified fatty acid levels in blood increased with restriction of feed (244–377 µ-equiv.11) and were poorly correlated (r = –0.16) with the intake, while the glucose content did not vary (mean of 70.6 mg/100 ml). It was concluded that the fatty acid composition of milk fat provides a reliable index of variation in the level of nutrient intake by suckled beef cows and could be used in short-term grazing studies.