Effect of level of oil inclusion in the diet of dairy cows at pasture on animal performance and milk composition and properties

The effect of including additional oil, incorporated as whole rapeseeds, in the diet of 64 Holstein-Friesian dairy cows (32 mid- and 32 late-lactation) at pasture on animal performance and milk fat composition and properties was followed over a continuous trial of 20 weeks duration. Within two stages of lactation (mid, 130 (plus-minus) 16.2 days, or late, 231 (plus-minus) 58.9 days), cows were allocated to concentrate treatments representing four levels of rapeseed oil inclusion, 0 (control), 200, 400 and 600 g oil day^-1. Oil inclusion had little effect on milk yield but decreased milk fat content significantly (P < 0.01), with a mean depression of 0.40% at the highest level of oil inclusion. The content of milk protein also decreased with increasing addition of oil, but the decrease was smaller than the milk fat depression and was not statistically significant. Increasing the level of rapeseed oil in the diet to 600 g oil day^-1 resulted in linear changes in milk fat and protein concentrations which were described by regression equations. For each 100 g of rapeseed oil added to the diet, milk fat content decreased by 0.068% in mid-lactation cows and 0.061% in late-lactation cows, while protein content decreased by 0.026% in mid-lactation cows and 0.028% in late-lactation cows. Total unsaturated fatty acid content of milk fat also increased in a linear fashion with increased level of oil addition, from 345.7 g kg^-1 total fatty acids in control milk fat to 459.3 g kg^-1 total fatty acids at 600 g oil day^-1, while total saturated fatty acids decreased in the same milk fats from 640.7 to 522.2 g kg^-1 total fatty acids. These changes were reflected in lower solid fat contents (SFC) in the milk fat at the lower temperatures of measurement, eg 41% SFC at 5 °C at the highest level of oil inclusion compared with 52% in the control milk fat. However, SFC at 20 °C showed little difference with increasing level of dietary oil addition, an important factor in maintaining product integrity at room temperatures. The relatively high content of the monounsaturated fatty acid C18:1 (345.5 g kg^-1 total fatty acids at 600 g oil day^-1) and low content of polyunsaturated fatty acids (total C18:2 and C18:3 <40 g kg^-1 total fatty acids at 600 g oil day^-1) ensured that the oxidative stability of the treatment and control milk fats did not differ significantly. Stage of lactation had an unexplained effect of consistent magnitude on milk fat composition throughout the trial period, with late-lactation animals producing milk fats containing a significantly (P < 0.001) higher proportion of unsaturated fatty acids than the mid-lactation animals. Changes in the proportions of unsaturated fatty acids in milk fat, as reflected by changes in iodine value, were established within 2 weeks of the trial commencing and persisted over the 20 weeks of the trial duration. No adverse effect on animal health from this type of dietary manipulation was identified.