154 Characterization of anovular phenotypes in postpartum beef cows

Distinct anovular phenotypes have been proposed based on the largest size attained by anovulatory follicles that reflect the physiological mechanisms during specific stages of antral follicle development. Anovular phenotypes have been characterised in dairy cows but have yet to be assessed in beef cows. The objective of this study was to evaluate the prevalence and characteristics of anovular phenotypes in postpartum beef cows. Angus-cross postpartum beef cows (n = 589) from four herds were evaluated using ultrasonography on Day −10 and Day 0 (onset of synchronization for AI). At each exam, corpus luteum (CL) presence/absence and size of the largest follicle were recorded. Cows were classified as cyclic (CL presence) or anovular (absence of CL). Anovular cows were further classified based on maximal follicle size as follows: Type I (<8.5 mm; non-ovulatory follicle–deficient FSH); Type II (8.5–11.7 mm; small ovulatory follicle–deficient LH pulses); Type III (11.8–16.0 mm; average ovulatory follicle–absence of LH surge); Type IV (16.1–24.0 mm; large ovulatory follicle–absence of LH surge); and Type V (≥25 mm; cystic follicle). At calving, age, body condition score (BCS; scale 1 to 9) and calf weight were recorded while cow weight was recorded for a subset (n = 289). On Day −10 BCS, weight and a blood sample for nonesterified fatty acids (NEFA) were collected. Data were analysed by generalized linear mixed models and pairwise comparisons were conducted using Tukey’s test. Results are shown in Table 1. Prevalence of anovulation was 45%. Distribution among phenotypes was Type I 1.5%, Type II 12.1%, Type III 64.5%, Type IV 21.5%, and Type V 0.7%. Due to reduced frequency (n = 6), Type I and V cows were removed from further analysis. Maximal follicle size differed (P < 0.05) between phenotypes, while calf weight and NEFA did not differ (P > 0.05). Type II and III anovular cows were younger (P < 0.05) than Type IV and cyclic cows. Postpartum interval (PPI) was greater (P < 0.05) for cyclic than Type II and III cows, while Type IV cows were intermediate. Calving and Day −10 BCS were greater (P < 0.05) for cyclic than anovular cows regardless of type. Furthermore, cyclic cows gained more BCS between calving and Day −10 than anovular cows, regardless of type (P < 0.05). Cyclic cows were heavier (P < 0.05) than Type III cows at calving, while Type II and IV cows were intermediate. Cyclic cows were heavier at Day −10 than Type IV cows which in turn were heavier than Type II and III cows (P < 0.05). As expected, cyclic cows differ in multiple traits from anovular cows. Intriguingly, the majority of anovular cows were Type III, and there were minor differences in cow traits among anovular phenotypes.