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Bureau of Agricultural Economics
Bureau of Dairy Industry, United States Department of Agriculture
ABSTRACT
From a group of 15,442 lactation records obtained from D.H.I. Associations in 12 States, 1,139 comparisons were made of the same cows having lactation records following dry periods of different lengths.
These records were converted to 4 per cent fat corrected milk and adjusted to maturity basis. They were then expressed as percentages of the average yield of 4 per cent fat corrected milk so as to reduce them to a common basis. Finally they were adjusted for differences in calving interval.
It was found that cows dry 1–2 months gave 9.2 per cent more milk than when dry 0–1 month; cows dry 2–3 months gave 4.3 per cent more milk when dry 1–2 months; and that cows dry 3–4 months gave 1.4 per cent more milk than when dry 2–3 months.
The exact date of drying off was known for 200 of the 1,139 cows. Dry periods of 0–1 month dry averaged 22 days, 1–2 months 47 days, 2–3 months 75 days, and 3–4 months 104 days. The application of these figures to the records of the 1,139 cows made it possible to represent quite accurately the influence of dry period on milk production by the equation Y = 105.28 – 33.98(.9667)x, where Y equals the per cent of average yield of 4 per cent milk, mature basis, and X equals number of days dry period.
A table for converting yields of milk to a 55-day period basis is presented.
A dry period of 55 days was found to be the optimum length for cows yielding 10,000 pounds and calving at 12-month intervals. Either a longer or a shorter dry period reduces the milk yield, the longer because more milk would be lost in the current lactation than would be gained in the following lactation, the shorter because more milk would be lost in the following lactation than would be gained in the current lactation.
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