EFFECTS OF BODY CONDITION, REPRODUCTIVE STATUS AND BREED ON FOLLICULAR POPULATION AND OOCYTE QUALITY IN COWS

This study was designed to evaluate the influences of body condition score (BCS), reproductive status and breed on the number of ovarian follicles and the quality of oocytes developing in them in 449 cows at slaughter. Each cow was given a body condition score using a 1-to-5 scale. After slaughter, reproductive status (cycling or pregnant) was determined. Ovarian follicles were classified as small (1 to 4 mm), medium (5 to 9 mm) or large (greater than or equal to 10 mm), and were counted and aspirated. Oocytes were defined as normal or abnormal, based on morphology of the oocyte/cumulus complex; the proportion of normal oocytes was estimated for each follicle size. The presence of a CL did not affect follicle number between or within cows. Pregnant cows showed fewer medium (P<0.05) and large (P<0.001) follicles than cyclic cows, whereas development of small follicles is not impeded during pregnancy. Cows with condition scores of 3 to 5 had more small follicles than those with 1 or 2 (P<0.01) Cyclic and early-pregnant cows also showed an effect of BCS on large follicles (P<0.05). Cows of European breeds exhibited significantly more large follicles than Zebu or crossbred cows. The proportion of normal oocytes decreased with increasing follicular size (P<0.001), and it increased with condition score in small (P<0.001) and large follicles (P<0.05). In conclusion, while pregnancy seems to affect the number of medium and large follicles, and breed may influence large follicle population, the mechanisms of impaired fertility in cows in poor body condition appear to be related to effects on both follicle number and oocyte quality.