Abstract
The aim of this study was to investigate the effect of different feeding proportions of forage ―grazing vs. silage― on milk fatty acids (FA) profile and conjugated linoleic acid (CLA) content of autumn calving Holstein-Friesian cows (n = 61) at CIAM (Galicia, NW Spain). Three treatments (S, 100% silage; G/S, 50% grazing + 50% silage; G, 100% grazing) were set and milk FA profile of dairy cows was determined by gas chromatography-mass spectrometry. The G group showed a decrease in short (p < 0.05) and medium chain FA (p < 0.001), with an increase in long chain FA (p < 0.001) in comparison to the G/S and S groups, which showed the lowest levels (p < 0.001) of mono- and polyunsaturated FA. The CLA content in milk fat increased (p < 0.001) linearly in relation to the increased proportion of fresh grass in the diet of dairy cows from 0.49 and 0.82 to 1.14 g/100 g FA for the treatments S, G/S and G, respectively. During spring and summer, the levels of CLA were three times higher (p < 0.001, +0.76 g/100 g FA) in milk from dairy cows at the G group than in cows at the S group and twice higher (p < 0.001, +0.40 g/100 g FA) than in cows at the G/S group. High proportion of grass in the diet of cows increased CLA content, with the highest levels of unsaturated FA and the lowest levels of saturated FA, increasing the added value of milk on grazing systems using available farm resources.
Highlights
The majority of milk conjugated linoleic acid (CLA) content is derived by de novo synthesis and it is rich in milk from grazing animals or from those fed with high fat diets (Khanal & Olson, 2004)
In humid areas as Galicia (NW Spain) where sustainable milk production systems using available farm resources mainly fresh grass is a common practice by dairy farmers, it is expected that the highest CLA content in milk fat of Holstein-Friesian cows will be basically found on long periods of pasture feeding
The largest seasonal variation on milk fatty acids (FA) profile was mainly attributed to the diet, significantly greater amounts of CLA were found in summer months when cows fed outdoors fresh grass than in winter when dried or ensiled forage was fed indoors by dairy cows (Thorsdottir et al, 2004)
Summary
Pasture DM intake (kg DM cow–1 day–1) was higher in the G group than in the G/S (p < 0.001, +7.7) and S (p < 0.001, +17.3) groups, according to the grazing proportion of fresh grass in the three forage diets. The main result of our study pointed that high grazing proportion of fresh grass in the diet of dairy cows was the main responsible of the highest levels of CLA, linolenic acid and PUFA in milk fat and it highlighted that appropriate grassland management practices are necessary to be implemented at farm leave for increasing sward quality.
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