Changes in milk citrate concentration during lactation in the goat

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Abstract
In goats suckling their young, milk citrate concentration decreased exponentially from .apprx. 30 days post partum. An exponential relationship fitted to the mean data from the 2nd sampling to the final sampling gives r = -0.93 (P < 0.001) and the approximate time for half-maximal concentration to be reached was 125 days of lactation. A similar pattern of change was evident in hand-milked goats and an exponential relationship gave r = -0.68 (P < 0.001) and an approximate time for half-maximal concentration to be reached of 145 days of lactation. Since all animals were not sampled at all stages of lactation, and since milk yield varies between animals, it is difficult to relate absolute milk yield to milk citrate concentration. With yield per unit volume of mammary tissue plotted against citrate, there was a significant, positive linear correlation (r = 0.609, P < 0.001). The following series of changes is suggested as a possible explanation to account for the decrease in milk citrate concentration during lactation. In early lactation, milk yield is high and ruminants are in a state of negative energy balance and fat stores are mobilized. This supply of long-chain fatty acids (akin to feeding a high-fat diet) is associated with a relatively low rate of de novo fatty acid synthesis within the mammary gland in relation to the total secretory rate of milk fat:milk citrate concentration is therefore high. As lactation advances, milk yield begins to decline and there is a shift in metabolism to use substrates derived entirely from the diet rather than from body stores. The proportion of milk fat derived from de novo synthesis then increases and milk citrate concentration is decreased. Such changes in milk fatty acids were observed in the cow.