Abstract
Photoperiod manipulation during the lactation cycle alters milk yield, with long days (LDPP) increasing yield in lactation and short days (SDPP) in the dry period improving subsequent yield. Circulating prolactin (PRL) is directly related to day length, with LDPP increasing and SDPP decreasing PRL, respectively. Two blocks of 24 multiparous Holstein cows were used during two consecutive years to test the hypothesis that the mammary response to SDPP is the result of decreased concentrations of PRL in the circulation relative to LDPP. Cows were randomly assigned to one of three treatment groups during the dry period: SDPP, LDPP, or SDPP+PRL. Cows were returned to ambient photoperiod at calving and milk yield and DMI recorded for 120 d and 42 d, respectively. Mammary biopsies were obtained to determine rates of [3H]-thymidine incorporation into DNA in vitro. Treatment of SDPP cows with PRL caused a rapid increase in systemic PRL that reached concentrations similar to cows under LDPP. The periparturient PRL surge was similar for LDPP and SDPP+PRL cows, but those groups had greater surge concentrations versus SDPP. Cows exposed to SDPP produced more milk than LDPP cows, and there was a trend for SDPP+PRL cows to produce more milk than LDPP cows. Milk production was inversely related to the periparturient PRL surge. There was a trend for a treatment effect on mammary cell proliferation with greater proliferation in mammary tissue of SDPP cows relative to LDPP or SDPP+PRL on day −20 relative to parturition. Replacement of PRL to cows on SDPP when dry resulted in milk yield intermediate to cows on SDPP or LDPP, supporting the concept of a link between dry period PRL and yield.
Highlights
IntroductionThe dry period is an essential part of the lactation cycle to maximize yield
In dairy cows, the dry period is an essential part of the lactation cycle to maximize yield
Because milk production potential is a function of the number of mammary epithelial cells in the gland, as well as the secretory activity of those cells [4,5,6], increased milk yield can be observed under conditions that enhance mammary cell number and mammary cell activity
Summary
The dry period is an essential part of the lactation cycle to maximize yield. The mammary gland undergoes an extensive process of cellular remodeling which is essential for achieving optimal milk production performance in the subsequent lactation [1,2,3]. Any factors involved in the regulation of these processes can directly impact mammary function and milk yield [4]. The dry period presents an opportunity to impose management strategies that can influence mammary remodeling and enhance milk yield in the subsequent lactation. One example of such a management tool is manipulation of photoperiod
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