Sources of variation underlying the production of lactose by dairy cows

This study explored variability in the production of lactose by dairy cows, and the factors underlying it, using herd testing data from New South Wales, Australia. The dataset spanned 14 years (2008-2022) and comprised 393,772 records from 33,280 cows across 85 herds, alongside meteorological and dairy sire genetic data. Variables included milk yield, composition, and quality; reproductive data; breed information; and environmental factors. We found significant variation in lactose yield (LY), lactose percentage (LP), and milk solids yield (MSY) across breed, parity, and stage of lactation. Holsteins had the highest LY and MSY and the lowest LP, whereas Jerseys produced more MSY per unit of LY (MSY:LY ratio). We observed a negative correlation between LY and milk composition variables (fat and protein percentages) and environmental factors such as the temperature-humidity index. Heritability estimates indicated a moderate genetic influence on LY, LP, and MSY:LY (0.24-0.33). The identification of cows producing milk with a consistently lower LP or higher MSY:LY ratio highlights the potential for selective breeding against lactose output to increase the production of milk components and suppress milk volume. These findings highlight the potential to modulate lactose synthesis that could enhance the milk production efficiency of dairy cows, and potentially reduce their environmental impact.