VARIATION IN THE CHARACTERISTICS OF MILKFAT FROM PASTURE-FED DAIRY-COWS DURING LATE SPRING AND THE EFFECTS OF GRAIN SUPPLEMENTATION

Sources of variation in milkfat characteristics were determined after approximately 115 days-in-milk for Friesian cows that had calved in sp ring and grazed ryegrass-white clover pastures. Three groups each of 1 5 multiparous cows were managed as separate herds. Each herd was subje cted to one of 3 management strategies: pasture only, pasture only dur ing the experimental period but previously supplemented with 2-3 kg/co w/day maize grain, and pasture plus 3-4 kg/cow/day maize grain. During a 3-week period beginning in November 1994, milk samples were collect ed twice-weekly from individual cows and analysed for total milkfat, p rotein and lactose, fatty acid profiles, and solid fat content (SFC). Repeat analyses were conducted on 39 samples to provide an estimation of analytical variation for fatty acid profiles. Estimates of between- cow variation were greater than those of within-cow variation for all parameters except fatty acids C-6:0, C-14:0, and C-18:2. Analytical va riation was small. SFC at 10 degrees C of milkfat from individual cows was correlated significantly with total milkfat concentration, the pr otein:fat ratio, and the proportions of fatty acids C-16:0, C-17:0, C- 17:1, C-18:1, and conjugated C-18:2 Supplementing pasture with maize g rain had minor effects on the fatty acid. profile of milkfat, but had no significant effect on SFC. These data will enable increased efficie ncy of experimental design in this research area, and detail the exten t of the natural variation in milkfat characteristics.