In lactating dairy cows, exposure to a long-day photoperiod is associated consistently with an increase in prolactin concentration and milk production. In contrast, cows exposed to a long-day photoperiod during the dry period produced less milk in the following lactation than those exposed to a short-day photoperiod.
Exogenous melatonin may represent an alternative to a short-day photoperiod when photoperiod of the dry cow cannot be housed separately from the lactating cows. The objective of this study was to determine the effect of photoperiod and melatonin feeding during the last 2 months of gestation on the milk production in the subsequent and complete lactation and on prolactin concentration in dairy heifers and cows.
The results of this experiment confirm that a short-day photoperiod during the dry period enhances milk production in multiparous cows in the following lactation. However, this effect is gradually lost and cannot be mimicked by feeding melatonin.
Photoperiod treatments during the last 2 months of gestation do not appear to affect the following lactation in primiparous heifers, although feed efficiency was improved by precalving exposure to short-day photoperiod; however, this effect is different from those found in previous studies in heifers and will need to be confirmed by additional experimental data.