The Variations in milk yields caused by season of the year, service, age, and dry period, and their climination: Part III. Age

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Abstract
Age, it will be remembered, is here measured in lactations, but little error is involved if the true age in years, at the commencement of the lactation, is taken as two more than the number of the lactation—e.g. a cow, on the average, calves her third calf when she is roughly 5 years old.