[Physiological basis of high milk production in cattle]

Milk yield is determined by number, size, synthetic and secretory capacity of mammary alveolar cells and low amounts of residual milk after milking. Initial milk yield and persistency of lactation are important. Important factors to achieve high milk yield are: optimal mammary development and preparation to lactation; maintenance and possibly increase in number and of synthetic capacity of alveolar cells during established lactation; sufficient availability of substrates for milk synthesis through high feed intake, readiness to mobilize body reserves (especially depot fat) and increased mammary blood flow; optimal milk let-down; regular and possibly more than twice milking/day. Favorable for a high milk yield are high circulating concentrations of somatotropin, possibly associated with increased production and levels of the insulin-like growth factor I, in the presence of low concentrations of and reduced sensitivity or responsiveness of target organs to insulin, reduced circulating levels of thyroid hormones and possibly enhanced conversion of thyroxine to triiodothyronine in the mammary gland. Enhanced sensitivity of depot fat to lipolytic hormones is favorable for high milk production.