Published online by Cambridge University Press: 02 September 2010
Multiparous cows were assigned before calving to three calving to first insemination intervals. Records of cows conceiving at first or second insemination, were used to construct a model of the lactation curve which incorporated peak production and the effect of progressing pregnancy. The model was used to simulate milk yield during a 4-year period for three production levels and five calving intervals. The model separated the descending part of the lactation curve into a linearly and an exponentially declining component, with the latter becoming distinct at about 20 weeks after conception. Peak yield was negatively correlated with the slope of the linear decline. Within a simulated 4-year period, cumulative milk yields at fixed time periods after calving depended upon the period chosen and the calving-to-conception interval of the cow. Late conceptions resulted in higher cumulative yields at the end of the 1st year, and in lower yields at the 2nd year end, with respect to early conceptions. Smaller differences were found between the intermediate calving intervals. During the 3rd and 4th years the early conceptions had a distinct advantage. Different rates of the linear decline, obtained for the different production levels, changed the magnitude of the yield differences between the calving intervals but not their relative ranking. The model presented offers a means for the suitable choice of the calving cycle according to the length of the period for which a cow is expected to remain in the herd.
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