Some effects of post-oestrous hormonal therapies on conception rates and resubmission rates in lactating dairy cows

A range of hormonal therapies has been evaluated to potentially improve the reproductive performance of lactating dairy cows. Early lactation treatments with gonadotrophin releasing hormone (GnRH) or prostaglandin F2∝(PGF) may reduce the interval to first insemination or increase the conception rate to first insemination, but mainly in cows which have had a difficult pueperium or which are in herds with low conception rates. These two hormones, as well as progesterone and oestradiol benzoate (ODB) are commonly used either singly, or in combination (GnRH + PGF; progesterone + ODB + PGF) to synchronise the oestrus preceding first inseminations. None of these synchrony treatments is associated with increased conception rates. Extensive series of trials have been completed to identify post-oestrous or post-insemination hormonal therapies which could increase conception rates to the preceding insemination. The wide variation in results has precluded any being commonly regarded as sufficiently reliable for routine use. Nonetheless, meta-analyses have shown that GnRH treatment at insemination or in late dioestrus (11 to 13 day post-first insemination) can significantly increase “the risk of pregnancy”. Insemination treatments have been most effective with repeat breeders (+22.5%), whereas late dioestrous treatments (10%) may be dose and analogue specific (10 μg buserelin). Although metoestrous supplementation with progesterone can stimulate early embryonic development, the associated reduction in oestrous cycle length also reduces conception rates in heifers. Late dioestrous use of GnRH can prevent both of these negative effects. Early dioestrous supplementation with progesterone may enhance production of interferon tau, but this potentially beneficial effect has not been able to be reliably translated into increased conception rates. Many of these hormonal therapies are associated with altered patterns of ovarian follicle development which are similar to those in some synchrony treatments preceding first insemination. Recent studies have indicated that OBD and progesterone can be used to synchronise returns to service and increase the submission rate for second inseminations made about 3 weeks after first inseminations. This can make the non-return rate a more accurate measure of the response to a hormonal treatment and potentially overcome confusing impressions created when oestrous detection rates may be around 50%. Even if effective hormonal therapies are successfully developed, the results may be compromised by environmental factors such as heat stress, energy balance or energy partitioning for lactation. These factors may reduce oocyte quality, fertilization rates or normal uterine secretion patterns. Reduced conception rates associated with high daily milk yields in early lactation may not be able to be remedied simply with hormonal supplementation or by altering patterns of ovarian follicle development. Under these circumstances, controlling the inter-service interval could reduce the impact of the lowered conception rates.