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 spring and grazed ryegrass-white clover pastures. Three groups each of 15 multiparous cows were managed as separate herds. Each herd was subjected to one of 3 management strategies: pasture only, pasture only during the experimental period but previously supplemented with 2-3 kg/cow/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 collected twice-weekly from individual cows and analysed for total milkfat, protein 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 variation was small. SFC at 10 degrees C of milkfat from individual cows was correlated significantly with total milkfat concentration, the protein: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 grain had minor effects on the fatty acid. profile of milkfat, but had no significant effect on SFC. These data will enable increased efficiency of experimental design in this research area, and detail the extent of the natural variation in milkfat characteristics.