Relationship between milk composition and some nutritional parameters in early lactation

Ten sets of data from four experiments with individual measurements on 236 cows for one to four weeks in each of second and third months of lactation were used to determine relationships between milk composition and nutritional parameters. Effects of breed and parity of cow as well as experimental rations were removed in analysis. The estimated balance between energy intake and energy requirement for milk and maintenance was calculated from intake of roughage and concentrates together with their chemical composition and in vitro digestibility, milk yield and milk composition. Adjusted mean estimated energy balances ranged from −17.65 to 14.35 MJ NEl per day; dry matter intake from 16.1 to 20.1 kg per day and % dietary roughage from 36.6 to 67.5% across data sets. Mean milk fat % was between 3.76 and 4.11% while milk protein % was 2.79–3.15% across data sets. Dry matter intake and % dietary roughage explained relatively small amounts of variation in milk fat %, protein % or the ratio of these two components after effects of ration were removed in data analysis. Estimated energy balance was negatively correlated with milk fat % (−0.07 to −0.65), positively correlated with milk protein % (0.12 to 0.47) and negatively correlated with ratio of milk fat to protein (−0.36 to −0.74) across data sets. Estimated energy balance increased the proportion of explained variation of fat:protein ratio by an average 21% to a total of 43%. In the reverse situation, fat to protein ratio increased coefficients of determination of energy balance by 19 to 52%. The ratio of contents of milk fat and protein was a more sensitive and consistent indicator of changes in nutritional variables and also a better predictor of energy status of the cow than either component by itself.