SEASONAL AND LACTATIONAL INFLUENCES ION BOVINE-MILK COMPOSITION IN NEW-ZEALAND

This study was designed to evaluate the respective influences of stage of lactation (SOL) and time of year on the seasonal variation in milk composition for pasture-fed dairy cows in New Zealand. Four herds of similar to 20 Friesian cows were used, one herd calving in a 6 week pe riod beginning in each of January, April, July and October. Cows graze d rye-grass-white clover pasture only, except during June when all cow s received supplementary pasture silage. Milli samples were collected from each cow in milk on four occasions during the gear (September, De cember, March and June), to give a total of three samples per cow (ear ly, mid and late lactation; about 30, 120 and 210 d respectively after calving). Samples were analysed for a detailed range of components. C oncentrations of many milli components (e.g. total protein, fat, casei n and whey protein) increased as lactation progressed; the extent of t hese increases depended on the time of year. These results indicated t hat spreading calving throughout the year would lessen seasonal variat ions in the gross composition of milk supplied to factories, leading t o a more even distribution of product yield across the year. Despite t his, variations in some important manufacturing properties were affect ed by time of yeas but not by SOL. Ratios of protein:fat and casein:wh ey protein were not significantly affected by SOL, but were affected b y time of year. The solid fat content of milk was also affected by tim e of year. Seasonal variations in the manufacturing properties of mill i may be reduced but not eliminated by changing the time of calving.