The rate of decline in milk yield after peak lactation is about 10% per month, for cows in New Zealand grazing pasture diets, and less than 4% per month in concentrate-fed animals in northern hemisphere environments. Much of the decline in milk yield after peak lactation is associated with a similar proportional loss of secretory cells from the udder and, once lost, these cells cannot be replaced until the dry period before the next lactation. Thus, we have found (unpublished data) that from peak lactation (October) to late-lactation in New Zealand, there was a loss of over 50% of mammary DNA. Such a loss was also reflected in changes in udder volume (Davis et al., 1985) and also in changes in milk storage capacity in the udder (Davis et al., 1987). Differences in lactational persistency between concentrate and pasture-fed animals suggest that nutrition exerts a long-term effect on milk production through maintenance of mammary secretory cell structures, as well as function. In the present experiment, our hypothesis was that cows fed concentrate diets would maintain milk production better than cows fed pasture over the summer months and that this would be reflected in the functional udder capacity (FUC). FUC is a measure of mammary cell numbers and is highly correlated with milk production (Davis et al., 1987). Further work attempted to correlate changes in FUC with concentrations of blood hormones sensitive to diet.
Proceedings of the New Zealand Society of Animal Production, Volume 60, Hamilton, 275-276, 2000
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