The effect of environment and heredity on lactation. II. Persistency of lactation

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Abstract
1. A statistical study of factors affecting variation in persistency of lactation has been made using 5000 lactation records from twelve leading herds of Ayrshire cattle in south-west Scotland for the period 1930–9. 2. It is suggested that a satisfactory numerical expression for the shape of the lactation curve could be obtained from the formula, Persistency = where A is the milk yield during the first 180 days and B is the initial milk yield, namely, the milk yield during the first ten weeks of lactation. 3. The interrelationships of persistency, initial milk yield and 180-day milk yield showed that these three characteristics are positively correlated with one another on a between cows within herd basis. This meant that it should be possible to combine high initial milk yield with high persistency to obtain increased total production. 4. The effect of month of calving on persistency of lactation varied significantly between herds. On the average, the highest persistency was attained by cows calving in the winter, and the lowest by summer calvers. 5. The variation of persistency with age showed that it is necessary to correct for the high persistency of first calvers when comparing cows with different lactations. 6. The average repeatability of persistency was 0.242 in the present material, while heritability was of the order of 0.10–0.15. 7. It has been concluded that improved feeding and management would bring about the greatest returns in the direction of improved persistency.