Uterine inflammation is dynamic during a timed AI protocol, and its resolution before AI is associated with pregnancy in young primiparous Bos indicus beef cows

Abstract Uterine PMN proportion is dynamic during a timed AI (TAI) protocol, and its resolution by AI may be associated with restore fertility in postpartum beef cows. Our objective was to characterize changes in uterine polymorphonuclear neutrophils (PMN) proportion between D0 and D8 of a TAI protocol and to determine the association between uterine status and fertility in young primiparous (YP) Bos indicus beef cows. The YP cows (~ 26 mo; n = 107) underwent a TAI protocol, with uterine cytobrush samples collected on D0 and D8. Cows were classified by the proportion of uterine PMN on D8 as Low (L; PMN ≤ 5%; n = 77) or High (H; PMN > 5%; n = 30), and by their D0-to-D8 transition: HH (n = 8), HL (n = 11), LL (n = 66), and LH (n = 22). Uterine status changed between D0 and D8 in 33 of 107 cows (30.8%; HL, n = 11; LH, n = 22). Pregnancy per AI (P/AI) was higher in cows classified as L PMN than in cows classified as H PMN on D8 (50.7% vs. 30.0%; p = 0.02). Among transition groups, P/AI was 51.5%, 45.5%, 31.8%, and 25.0% for LL, HL, LH, and HH cows, respectively. Cows that resolved inflammation during the protocol (HL) attained pregnancy rates similar to cows that remained with Low (L/L), whereas cows classified as H PMN on D8 (LH or HH) had lower fertility. Uterine status at the end of the TAI protocol rather than at protocol onset was associated with pregnancy.