Factors affecting the yield and quality of milk. II. Variations in successive lactations

1. Milk and butterfat production rise to about the sixth lactation, but beyond the fifth lactation the increase may be neglected. 2. The lactation of maximum production may be taken as the fifth which corresponds to 7 years of age. 3. Beyond the fifth lactation the milk and butterfat yields vary irregularly but show a downward tendency. 4. The fat yield shows either a fall or only a very small increase in the second lactation. 5. The fat percentage is highest in the first lactation, drops markedly in the second, remains fairly steady for a few lactations, though with a slight downward tendency, and then falls off in the higher lactations. 6. Results obtained by applying correction factors for either age or lactation are quite comparable and choice in the method to be used will depend largely on the original data available.