The effects of the addition of either 5 or 10% ‘oleic acid’ (78% pure) or 10% of a mixture of saturated fatty acids (64% palmitic acid, 31% stearic acid) to the dietary concentrate mixture on the yield and composition of the milk and milk fat and on the pattern of fermentation in the rumen were investigated in a feeding experiment with 8 cows in mid-lactation. The concentrate mixtures were given with a high-roughage diet that supplied 9·1 kg of hay/day.

The addition of 5% ‘oleic acid’ to the concentrate mixture resulted in increased yields of milk and solids-not-fat (SNF); the percentage of fat in the milk was decreased but the yield of milk fat was unaltered. The addition of 10% ‘oleic acid’ to the concentrate mixture decreased both the yield and percentage of fat in the milk. In contrast, the concentrate mixture containing 10% of the mixture of saturated fatty acids increased the yield of milk fat.

When the concentrate mixture containing 5% ‘oleic acid’ was given to the cows, the yields and percentages of the fatty acids from 4:0 to 16:0 (except 12:0) in the milk fat were decreased, but the yields and percentages of 18:0 and 18:1 were increased. Similar but more pronounced effects on the yields and percentages of the fatty acids from 4:0 to 16:0 (except 12:0) in the milk fat were observed when the cows were given the concentrate mixture containing 10% ‘oleic acid’, but under these dietary conditions the yield and percentage of only 18:1 in the milk fat were increased. The addition of the mixture of saturated fatty acids to the concentrate mixture decreased the percentages of the fatty acids from 4:0 to 14:0 (except 12:0) in the milk fat but decreased the yields of only 10:0 and 14:0; the yields and percentages of 16:0 and 18:1 were increased.

When the cows were given the concentrate mixture containing 5% ‘oleic acid’ there was a small but significant decrease in the acetic acid:propionic acid ratio in the rumen liquor. A similar but more pronounced change in the acetic acid:propionic acid ratio in the rumen liquor was observed when the cows were given the concentrate mixture containing 10% ‘oleic acid’, but in this instance there was a significant reduction in the concentration of total volatile fatty acids in the rumen liquor. Apart from a small increase in the relative proportion of propionic acid, the addition of the mixture of saturated fatty acids to the concentrate mixture had no effect on the concentrations of volatile fatty acids in the rumen liquor.