Impact of time of milk storage in the udder on fat
Résumé
Our objective was to study the effect of duration of milk storage on milk fat globule (MFG) secretion to better understand relationships between milk yield, milk fat and MFG secretion. Four milking frequencies were studied in 6 dairy cows averaging 118 ± 22 dim: 2 milkings/d separated by 11- and 13-h intervals (2M11-13) or by 4- and 20-h (2M4-20) and 1 milking/d (1M24). The experimental trial was a double Latin square 3 × 3 with 2 wk periods. In post-experiment, milking frequency of 36-h (1M36) was repeated twice. Compared with 2M11-13, 1M24 reduced milk and milk fat yields and increased fat content, without any effect on the size of MFG which agrees with previous research. 2M4-20 had no significant effect on milk fat yield and content but tended to increase the size of the MFG. Lipolysis, measured on morning milk, was weaker with 1M24. Milk fatty acid composition was not modified by milking frequency. When data were analyzed according to kinetics of milk storage duration (from 4 to 36-h), the highest fat content and the largest diameters of MFG were obtained on milks from 4- and 36-h milking (respectively 62.8 g/kg and 4.15 µm and 57.7 g/kg and 4.09 µm). Such observations could have 2 origins: the richness in residual milk of the 4-h milk and the coalescence of MFG related to the long storage duration in the 36-h milk. Independently, for each duration of milk storage, there was a relationship between MFG size and fat yield (R2 from 0.31 to 0.81). Conversely, the relationship between MFG size and fat content was confirmed whatever duration of milk storage (R2 = 0.55). Speed of secretion of milk fat (storage of 4 h excluded) was also well correlated with MFG size (R2 = 0.62). For the 36-h milk, this relationship was also observed but with a significantly different slope suggesting a phenomena of MFG coalescence in response to the increased intra-mammary pressure. Duration of milk storage induces changes in MFG size under factors which interact.