424. A study of lactation as affected by heredity and environment

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Abstract
1. A study has been made of the interrelationships between persistency and maximum yield as components of the lactation curve, and of the importance of environment and heredity in determining the shape of the lactation curve of Ayrshire cows.2. Persistency was defined as the linear constant of the exponential curve expressed as a percentage, maximum yield as the highest 30-day yield as calculated from the records, and total yield as the yield of milk from 15 days to 8½ months after calving.3. The study was based on milk records computed by the official method of the Scottish Milk Records Association. The data used in much of the work was obtained from five herds of Ayrshire cows and involved 2392 lactations during the period 1920–39.4. In an exploratory test involving 100 lactations it was found that the exponential curve fitted ninety-five of the lactation curves and that the fit was improved in forty-eight of these ninety-five curves by using the parabolic exponential curve. Five of the lactations were too erratic to be fitted by any simple mathematical curve.