Follicle Size and Volume is Less Indicative of Development of a Persistent Follicle in Beef Heifers.

We have established that feeding 0.5 mg/h/d of melengestrol acetate (MGA) for 14 d (< 1 ng/ml of P4) develops persistent follicles with increased follicle size, follicular fluid volume, theca cell weight, and follicular fluid androstenedione concentration in mature cows. Therefore, we hypothesized that similar parameters would be developed in heifers that were 2 years of age or younger. Mature cows and heifers received either: 1) prostaglandin F2alpha (PG) on d0 and d7 and MGA for 14 d; or 2) GnRH on d 7 and a CIDR for 7 days. On d14, all cows received PG and CIDRs were removed. Ovariectomies were conducted 36 h post-PG and granulosa cells, theca cells, follicular fluid, and blood were collected for analysis. Data were analyzed using proc mixed of SAS with means separation by LSD to evaluate differences between treatment, age, and follicle status. Interestingly, MGA treatment did not result in a persistent (P) dominant (DOM) follicle diameter or follicular fluid volume in heifers (2-years-old or younger) that were as large as the P-DOM in mature cows. Mature cows had P-DOM and control (C) subordinate (SUB) follicles that were 40% (P < 0.006) and 45% (P < 0.009) larger in diameter than the same follicles in 2-year-olds. Furthermore, the mean follicular fluid volume of C-SUB follicles was 200% greater (P < 0.05) in mature cows and the mean diameter (P < 0.09) and the theca cell weight (P < 0.07) of C-DOM follicles tended to be larger in mature cows compared to the 2-year-olds. No differences were identified between mature cows and 2-year-olds in granulosa cell pellet weight or in the follicular fluid concentration of E2 and P4 for any of the follicle types; suggesting that even though the follicles were not the same diameter they still had characteristic steroid and theca cell weights of persistent follicles. Gene expression profiles for mRNA of genes involved in follicle development such as vascular endothelial growth factor A_164 (VEGFA_164) were also not different between mature cows and 2-year-olds. However, VEGFA_165B mRNA was more abundant (P < 0.03) in C-DOM follicles from 2-year-olds compared to mature cows. Furthermore, mRNA levels for both anti-Mullerian hormone (P < 0.08) and CART prepropeptide (P < 0.03) in SUB follicles were more abundant in mature cows than in 2-year-olds. Therefore, we can conclude from these data that follicle size may not be indicative of whether a follicle is persistent or not, especially in beef heifers. In addition, follicular fluid steroid hormone profiles and gene expression may be more accurate measures of whether a follicle is persistent.