Effect of follicle size on oocytes recovery rate, quality, and developmental competence in cows.

The objective of the present study was to determine the effect of follicle size on recovery rate, quality, and developmental competence of oocytes in cows. The ovaries (n = 507) of cows having age of 5-8 years, with mixed parity, BCS 2.75 ± 0.25, and clinically normal reproductive tracts were collected from the local abattoir. The follicles on the ovaries were divided into two groups based upon their size; 1) ≥6 mm diameter, and 2) <6 mm diameter. After initial evaluation of quality of the oocytes, the COCs were matured, fertilized, and cultured to determine the developmental competence. The oocyte recovery, quality, maturation, cleavage, 4-cell, 8-cell, and 16-cell stages were analyzed using PROC GLIMMIX procedure of SAS. However, the number of oocytes recovered per ovary was analyzed using MIXED procedure of SAS. Results revealed that the recovery of oocytes (LSM ± SEM) derived from the follicles having size <6 mm per ovary was greater (1.02 vs. 3.14 ± 0.13; < 0. 0001). However, the percentage (n/n) recovery [69.8 (474/679) vs. 62.7% (1454/2320); = 0.01] and grade I_+_II oocytes [68.4 (324/474) vs. 57.9% (842/1454); < 0.0001] was greater in ≥6 mm as compared with <6 mm group, respectively. However, maturation rate did not differ [92.9 (288/310) vs. 92.2% (296/321); = 0.98] between the groups. In contrast, cleavage rate [58.1 (180/310) vs. 47.4% (152/321); = 0.01], the 4-cell [34.5 (107/310) vs. 18.7% (60/321); = 0.0003], 8-cell [15.5 (48/310) vs. 7.8% (25/321); = 0.008], and 16-cell [8.7 (27/310) vs. 2.1% (7/321); = 0.004] stage embryos were greater in ≥6 mm group. It can be concluded that oocytes derived from follicle ≥6 mm have better developmental competence based on embryonic conversion in cows.