Milk progesterone on day 5 following insemination in the dairy cow: associated metabolic variables and reproductive consequences.

Discipline: reproduction; Key words:conception rate, dairy cow, leptin, metabolic variables, milk progesterone.

Failure of the corpus luteum to produce sufficient progesterone represents a major cause of early embryo loss and reproductive problems in the dairy cow. During early pregnancy, progesterone influences the secretory function of the uterus, including nutrients, growth factors, immunosuppressive agents, enzymes, ions and steroids that are essential for embryo development. Studies have demonstrated a close link between progesterone concentration after mating and early embryo development as well as the outcome of pregnancy. In both lactating and non-lactating dairy cows, a poor post-ovulatory rise in progesterone, and more specifically, poor progesterone concentrations between days 4 and 7 after mating, have been associated with poor embryo development and low pregnancy rates. This provides evidence that progesterone production is key to the success of early pregnancy in the dairy cow and milk progesterone concentration at approximately day 5 following mating can be used to monitor reproductive function during the post-calving period. However, many questions remain unanswered surrounding the way in which level of progesterone secretion is controlled during this important time. Therefore, in the present study, Dr L.Y. Yan and colleagues have investigated the relationship between milk progesterone on day 5 following insemination and a variety of metabolic variables as well as measures of reproductive performance in lactating dairy cows. Their results were published in the South African Journal of Animal Science Volume 48 of 2018, page 361 to 368. The title of the paper is: Milk progesterone on day 5 following insemination in the dairy cow: associated metabolic variables and reproductive consequences.

Data collected across a number of complimentary studies were compiled to produce a single database of 168 lactating Holstein Friesian dairy cows maintained under commercial conditions. In all animals a number of variables were measured during the insemination period and related to milk progesterone measured on day 5 following the first artificial insemination (AI). Overall, 44% of the cows conceived at first AI and, whereas the mean day 5 progesterone was not significantly higher in these cows, there was a significant increasing and then levelling off relationship between milk progesterone concentration and conception rate. A number of factors showed some association with progesterone concentration. However, the only one showing a strong and repeatable relationship was the plasma concentration of the hormone leptin. Interesting enough, contrary to expectation and previous studies milk yield showed no relationship whatsoever.

                                                                                                                                                                            It was concluded that adequate but not excessive progesterone levels on day 5 bring about better fertility, and plasma leptin concentration may be a much better indicator of metabolic status in lactating dairy cows.