Effects of body condition at calving and peri-parturient nutrition on the composition of milkfat produced by grazing dairy cows in early lactation

Two grazing experiments, in which the body condition score (BCS) of dairy cows and their nutrition either before (high energy or high protein concentrates) or after (levels of high energy concentrates) calving were varied, were conducted at Kyabram, in northern Victoria, during the spring of 2001 (Experiment 1) and 2002 (Experiment 2). In each year, 72 cows calved between August and early September, and milk samples were collected in mid-late September and mid-late October to determine treatment effects on some aspects of milkfat composition. In experiment 1, increasing BCS at calving decreased (p<0.05) medium chain saturated fatty acids (C-10:0 - C-14:0), palmitic acid (C-16:0) and total saturated fatty acids and increased (p<0.05) oleic acid (C-18:1) in September. In October, increasing BCS at calving decreased (p<0.05) palmitic acid (C-16:0) and the long chain saturated fatty acids, and increased (p<0.05) stearic acid (C-18:0) and oleic acid (C-18:1). Increasing the amount of concentrates fed in early lactation resulted in a reduction (p<0.05) in the proportions of short and total long chain fatty acids, and an increase (p<0.05) in the medium and a number of the C-18 unsaturated fatty acids, including conjugated linoleic acid (CLA) and the total trans C-18:1 fatty acids in September. In October, the effects of concentrates were the same for the short and medium chain fatty acids, otherwise there were less consistent effects. In experiment 2, precalving nutrition had no influence on any of the fatty acids in milkfat at either sampling time. Body condition at calving had similar effects on fatty acid composition in September as those recorded in experiment 1, with the exception that linolenic acid (C-18:3) and CLA were significantly (p<0.05) reduced as body condition increased. Unlike experiment 1, the only effect of BCS on fatty acid composition in October in experiment 2 was that the proportions of medium chain fatty acids increased (p<0.05) as BCS increased. In terms of physical characteristics (only measured in experiment 2), milkfat was softer earlier in lactation than subsequently, and thin cows produced milk with higher solid fat content than fat cows at 10 degrees C (p<0.05). The implications of these effects on milkfat composition for nutritional and physical functionality are discussed.