Seasonal changes in the fatty acid composition of ewe-milk fat.

Although it has long been known that there are seasonal variations in the chemical characteristics of butterfat, it has as yet not been possible to assess the relative importance of nutritional and lactational influences. Moreover, in the Northern Hemisphere the problem is further complicated by the climatic effects, which necessitate the maintenance of cows indoors during the colder months, thereby causing marked changes in the composition and properties of milk fats. It has been shown that the composition of New Zealand butterfat does not vary greatly with locality, but undergoes regular seasonal changes (Cox & McDowall, 1948; Hansen & Shorland, 1952). In a series of papers on the factors causing variations in the yield and composition of the milk ofNew Zealand Romney ewes grazing under conditions comparable with those of dairy cows (Barnicoat, Logan & Grant, 1949a, b; Barnicoat, Murray, Roberts & Wilson, 1956) it has been shown that the lactation curves for yield and composition of ewe's milk are similar to those of the dairy cows, when allowance is made for the different lengths of lactation, which are approximately 12 and 20 weeks respectively. Ewe's milk yields attained a peak at about 3 weeks, cow's at about 8 weeks, and the fat, protein and solids-not-fat contents of milks of both species, after slight initial declines, showed gradual rises which inclined steeply towards the end of the lactation periods. The lactation curve for the ewe is therefore of similar form to that of the dairy cow, but of only about one-third the duration. The circumstance that sheep and cattle are both pasture-fed throughout the year in New Zealand has permitted us to compare the chemical compositions of their milk fats at similar dates and at equivalent stages of lactation, and to produce evidence in support of a theory that the diet is the most important factor responsible for seasonal changes in the composition of their milk fats.