Short Communication: Influence of estrus activity and reproductive tract size and position scores on fertility in Bos indicus and Bos taurus suckled beef cows

Abstract

The primary objective of this study was to determine if estrus activity and reproductive tract size and position score (SPS) are associated with pregnancy outcomes in Bos indicus (Nelore) and Bos taurus (Angus) beef cows. In study 1, multiparous Nelore cows (n = 1,280) were artificially inseminated at a fixed time (FTAI, day 0) using an estradiol and progesterone (P4)-based estrus synchronization protocol. In study 2, multiparous Angus cows (n = 764) were artificially inseminated at a fixed time (FTAI, day 0) using a gonadotropin-releasing hormone and P4-based estrus synchronization protocol. Estrus activity was assessed using Estrotect heat detector patches and scored on day 0 using the following scoring system: 0 (patch was lost, most likely due to repeated mounting), 1 (<25% activation), 2 (≥25%, <50% activation), 3 (≥50%, <75% activation), or 4 (>75% activation) where patch scores of 1 and 2 signified no or limited estrus activity, whereas scores of 0, 3, and 4 had increased estrus activity. Reproductive tract SPS were assigned on day 0 as SPS1: small and compact resting within the pelvic cavity; SPS2: intermediate, resting partially outside the pelvic cavity; and SPS3: larger and resting outside the pelvic cavity. Pregnancy diagnosis was performed by ultrasound on day 30 and 100 after FTAI. Cows were determined as undergoing pregnancy loss if a viable embryo with heartbeat was detected at day 30 but was no longer present at day 100. Pregnancy rate at day 30 was influenced by estrus activity and SPS in both Nelore (P = 0.004) and Angus (P = 0.009) cows. Specifically, cows with smaller reproductive tracts (SPS1) had greater (P < 0.001) pregnancy rate when estrus was expressed before FTAI. There was no effect of estrus activity nor reproductive tract size on pregnancy loss between day 30 and 100 for both breeds. In summary, estrus activity before FTAI may influence reproductive outcomes differently depending on size and position of the reproductive tract at time of breeding.

Keywords: estrus; pregnancy; reproductive tract size and position score.

Figure 1.

Influence of estrus activity and reproductive tract SPS on FTAI day 30 pregnancy rates in Nelore and Angus cows. Nelore cows with smaller reproductive tract (SPS1) had greater (P < 0.05) pregnancy rate with increased estrus activity compared to cows of similar reproductive tract size without estrus activity (69% vs. 39%; SPS1—estrus n = 54; no estrus n = 328; SPS2—estrus n = 28; no estrus n = 108; SPS3—estrus n = 10; no estrus n = 36). Similarly, Angus cows with reproductive tract SPS1 had greater pregnancy rate associated with estrus activity, but no difference among cows with larger reproductive tract (SPS2 and SPS3; SPS1—estrus n = 58; no estrus n = 57; SPS2—estrus n = 106; no estrus n = 152; SPS3—estrus n = 26; no estrus n = 35). Different letters represent statistical difference across means (a, b for study 1; e, f for study 2). No estrus, no or little estrus activity (patch scores of 1 and 2); estrus, increased estrus activity (patch scores of 0, 3, and 4). Abbreviations: FTAI, fixed-time artificial insemination; SPS, size and position score.