The effects of live weight and body condition score (CS) at calving on milksolids production has been well documented. The effects of CS at calving on detailed composition of milk, the physical characteristics, or, product quality are largely unknown. Milkfat yield, fatty acid profile (FAP), solid fat content (SFC) of milkfat during the first 10 weeks of lactation were monitored for Friesian heifers with a pre-calving CS of 4.1 (LCS) or 5.2 (HCS). Once calved, all heifers were grazed as one herd and offered pasture ad libitum. Composition and characteristics of milkfat were determined at four, six, and 10 weeks postpartum. At four weeks postpartum the HCS heifers, produced 23% more (P<0.001) milkfat, lost 59% more (P<0.001) live weight and CS declined 30% more (P<0.05) than did LCS heifers. Associated with these changes were compositional changes in milkfat. CS at calving had no effect on milkfat concentration. HCS heifers produced milkfat with lower concentrations (P<0.01) of de novo synthesised fatty acids (C4:0-C15:1) and higher (P<0.01) concentrations of preformed fatty acids. Concentration (mg/100 mg milkfat) of oleic acid (C18:1) was higher: 27.4% c.f. 24.8%, s.e.d. 0.94 (P<0.001) for HCS compared with LCS heifers. Milkfat from HCS heifers was softer, SFC at 5°C (55% c.f. 59%, s.e.d. 1.2, P<0.01) than from LCS heifers. Six weeks postpartum, the differences in FAP persisted but changes in live weight and CS were small compared with the first four weeks of lactation. Ten weeks post calving both groups were gaining live weight, but although differences (P<0.01) in live weight, CS, and daily milkfat yield persisted, no differences in FAP or SFC were found. These results show that differences in pre-calving live weight and CS cause differences in the composition and characteristics of milkfat. Milkfat from HCS heifers was more desirable (softer and higher in unsaturated fatty acids) than from LCS heifers. These differences disappeared once the two heifer groups were in positive energy balance by 10 weeks postpartum.
Proceedings of the New Zealand Society of Animal Production, Volume 62, Palmerston North, 38-41, 2002
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