Effect of cow diet on free fatty acid concentrations in milk

Two experiments were conducted: one during summer 1999 (Experiment 1, 180-200 days in milk (DIM)) and one in autumn 1999 (Experiment 2, 220-250 DIM), to examine the effect of pasture allowance on the formation of free fatty acids (FFA) in milk from multiparous Jersey cows. In Experiment 1, three treatments were investigated (n = 15): fully fed on pasture (FFP) (pasture allowance 36 g DM/cow per day), restricted pasture (RP) (pas re allowance 23 kg DM/cow per day), and RP plus 5 kg DM/cow per day of pasture silage (RP + S). Treatments were applied for 21 days. Experiment 2 ran for 14 days and comprised two treatments, (n = 15) FFP (pasture allowance 38 kg DM/cow per day) and a more restricted pasture allowance (RP), of 16 kg DM/cow per day. Milk samples for FFA analyses were collected by hand milking each cow during the morning milking. The sample was split into three subsamples: Sample 1 was fixed to prevent any further lipolysis (L0), Sample 2 had no additive (L24), and for Sample 3 air was drawn through the sample for 4 min (D24). All samples were stored at 4 degrees C for 24 h before being analysed for FFA concentrations by the Bureau of Dairy Industries (BDI) method. A restricted pasture allowance (RP) reduced milk yield by 14 and 36% in Experiments 1 and 2, respectively. The feeding of silage to cows on a restricted pasture allowance had no effect on milk yield. Pasture allowance or supplementary feeding in Experiment I had no effect on total FFA concentrations in L0, L24, or D24 samples. However, the increase in FFA during 24 h, storage (L24-L0) was greater (P < 0.001) for the RP and RP+S than the FFP treatments. The restricted pasture allowance in Experiment 2 caused an increase in total FFA concentrations in the L24 (P < 0.001) and D24 (P < 0.05) samples. The increase in FFA concentration during storage (L24-L0) was greater (P < 0.001) in milk from the RP treatment. In both trials, the increase in FFA concentrations over 24 h between damaged and undamaged milk (D24-L24) was similar for each treatment. The greater increase in FFA concentration was in undamaged milk (L24-L0) from the RP treatments. From this observation it is hypothesised that the increase in FFA concentrations in milk due to underfeeding resulted from an increase in the activity of lipoprotein lipase and not from an increased susceptibility of the milk fat globule membrane to damage. Overcoming a restricted pasture allowance by supplementary feeding of silage had little effect on reducing the impact of a restricted availability of summer pasture on milk yield or FFA concentrations.