Distribution of follicle numbers in dairy cows aged from 2 to 11 years

Distribution of follicle numbers in dairy cows aged from 2 to 11 years F. Mossa1, S.Butler2, P.Duffy1, F. Jimenez-Krassel3, J Folger3, G.W. Smith3, P. Lonergan1, J.J. Ireland3, A.C.O. Evans1 1School of Agriculture Food Science and Veterinary Medicine, University College Dublin, Belfield, Dublin 4, Ireland, 2 Teagasc, Moorepark Dairy Research Centre, Fermoy, Co. Cork, Ireland, 3Department of Animal Science, Michigan State University, East Lasting, Michigan 48824,USA (E-mail Francesca.Mossa@ucd.ie) Introduction. The number of antral follicles in cattle is highly repeatable in individuals, while being highly variable between animals (Burns et al., 2005; Ireland et al., 2007). Furthermore, a high antral follicle number is associated with a greater number of transferable embryos following superovulation (Singh et al, 2004; Ireland et al, 2007). However, the relation between age of the cow and follicle numbers has not been investigated. Furthermore, the ability to phenotypically classify cows with respect to follicle number based on a single rather than multiple ultrasound examinations would be desirable to rapidly identify cattle with high responsiveness to superovulation. The aim of this study was to investigate the accuracy of a single ultrasonographic examination, performed on a random day of the oestrous cycle, and to assess the variability in the number of follicles in dairy cows of different ages. Materials and Methods. Transrectal ultrasonography was performed on 116 dairy cows, aged from 2.6 years to 10.8 years, to assess the number of ovarian follicles ≥3 mm in diameter on a random day of the oestrous cycle. Based on this scan, a subset of animals was selected as follows: cows with less than 15 antral follicles ≥3 mm in diameter (Low group n= 8) and those with more than 30 follicles≥3 mm in diameter (High group, n=10). Both groups were synchronized with 2 injections of PGF2α (Estrumate®, Loughrea, Co., Galway, Ireland) 11 days apart and the number of follicles ≥ 3 mm in diameter was counted daily during an entire oestrous cycle. The correlations between follicle number and animal age and between a single ultrasound examination and the maximal number of follicles during follicular waves were analyzed using ANOVA. Results. Amongst the 116 animals, the number of antral follicles ≥3mm in diameter per animal ranged from 4 to 44, with a mean (±SEM) of 17.99±0.78 (Fig.1). Cow age was not related to the number of follicles detected on a random day of the oestrous cycle (R2 = 0.0086; P=0.31). Amongst the subsets of animals, the mean (±SEM) number of follicles was 35.9±1.50 in the High group and 8.13±0.71 in the Low group. No difference (P=0.89) was detected between the two groups in mean age (High=5.28±0.43 years, Low=5.38±0.62 years). Numbers of follicles at the initial random scan were highly correlated with the maximal number of follicles observed during follicular waves of a complete oestrous cycle (R2 =0.7756; P <0.0001). Conclusion. The number of antral follicles in the ovaries of dairy cows is highly variable among cattle but apparently not linked with cow age. Cows with consistently high or low numbers of follicles can be reliably identified through transrectal ultrasonography performed on a single random day of the oestrous cycle References. Burns DS, Jimenez–Krassel F, Ireland J, Knight PG, Ireland JJ. Numbers of antral follicles during follicular waves in cattle: evidence for high variation among animals, very high repeatability in individuals, and an inverse association with serum-follicle-stimulating hormone concentrations. Biol. Reprod. 2005; 73:54-62. Ireland JJ, Ward F, Jimenez-Krassel F, Ireland JLH, Smith GW, Lonergan P, Evans ACO. Follicle numbers are highly repeatable within animal but inversely correlated with FSH concentrations and the proportion of good quality embryos after superstimulation in cattle. Human Reprod. 2007 in press. Singh J, Dominguez M, Jaiswal R, Adams GP. A simple ultrasound test to predict the superstimulatory response in cattle. Theriogenology. 2004; 62: 227-243.