Regulation of Factors Affecting Milk Yield

The heritability of milk yield is 0.25, which indicates that only 25% of the difference between milk yield in cattle is due to their genomic makeup while fully 75% is due to the environment the cow is in, which includes all management, environmental, and physical factors impacting cows during their lactation. Primary genetic factors influence the amount of secretory tissue and the endocrine regulation of lactation and metabolism. Mammary gland development and lactation are under endocrine regulation, and the process of lactation is also considered to be homeorhetic in nature as the metabolism of the animal is coordinated to meet the metabolic demands of milk synthesis for the developing neonate. Key hormones regulating the process of growth and lactation involve steroid and protein hormones and growth factors. Three management tools consistently increase milk yield in lactating dairy cows during established lactation: administrating exogenous bovine somatotropin (bST), increased milking frequency, and increased photoperiod. Short-day length during the dry period also increases milk yield postpartum. Season of year affects both yield and composition of milk. Seasonal variables known to impact milk and composition yield are photoperiod and thermal environmental variables such as temperature, wind speed, solar infrared load, and humidity. Milk composition can also be markedly altered by nutritional management. This permits the opportunity to improve the nutritional value of milk by altering milk composition through dietary management.