Long-acting progesterone supplementation at early diestrus in beef cattle: fertility responses in TAI and TET programs

In dairy and beef cattle, the circulating progesterone (P4) concentrations at early diestrus are positively associated with pregnancy outcome. Recently, many studies using different hormonal strategies were performed to increase the P4 concentrations during early and mid diestrus with the purpose of enhancing fertility in timed-AI (TAI) and timed-ET (TET) programs. Supplemental P4 using intravaginal P4 devices during early diestrus stimulates conceptus growth, but may not be practical for use in large cattle operations. Therefore, we have performed sequential studies aiming to evaluate the use of an injectable, single-dose long-acting P4 treatment as a novel strategy to supplement P4 during diestrus and potentially improve fertility in TAI and TET in beef programs. In our first trial, doses of 150 or 300 mg of long-acting P4 were given on Day 2 or 3 post-ovulation and efficiently increased the plasma P4 concentrations for at least 3 days in non-lactating beef cattle. This strategy did not affect initial luteal development nor vascularization, but accelerated the onset of functional and structural luteolysis on about 3 days in non-inseminated cows (Reprod.Dom.Anim.49,85-91, 2014). Despite this paradoxical effect, in a subsequent study (Theriogenology.85,1239-1248, 2016) we tested and confirmed the hypothesis that the supplementation with 150-mg long-acting P4 at Day 4 post-TAI improves fertility in lactating beef cows submitted to TAI. Pregnancy rate (P/TAI) was similar (P > 0.1) between the control and P4-treated cows (53.2%; 209/393 vs. 56.2%; 219/390), but when the P/TAI was evaluated according the size of the CL, cows with a CL < 0.9 cm2 benefited from the P4 treatment and showed P/TAI 35% greater than non-supplemented cows. This result indicated a positive effect of P4 supplementation in those cows with impaired corpus luteum (CL) function. We also observed that the P4-stimulated embryotrophic effects increased in 20% the pregnancy/TAI in lactating anestrous beef cows submitted to the same TAI protocol. In high producing dairy cattle, greater increases in P/TAI were obtained using 900 mg long-acting P4. However, additional trials from our group using 150 mg long-acting P4 at Day 4 post-TAI did not result in improvements on fertility of crossbreed and Nelore beef cows at early postpartum, indicating that further understanding of potential interactions between the reproductive status of the cow and the supplemental P4 dose are needed. In TET programs, our preliminary data is indicating a potential increase of 7 to 12% on P/TET in crossbreed beef recipients for supplementation with 150 mg P4, on Day 4 or Day 7 (day of embryo transfer). In summary, the use of long-acting P4 at early diestrus is a potential strategy to favor embryo development and maintenance. Although fertility responses varied among TAI studies, the expressive increase on P/TAI in lactating anestrous cows and in cows with a smaller CL indicate a potential use of long-acting P4 supplementation for fertility improvement in TAI and TET programs. For additional advances or understanding of the inconsistency in the fertility’s trails, the causes of early luteolysis and the new combinations of timing and dose of supplemental P4 for cows under different environmental and physiological conditions need to be explored in further studies.