Concentrations of follicle stimulating hormone associated with follicle selection, number of follicles, and ipsilateral vs contralateral relationships in mares

Diameter deviation or follicle selection during a follicular wave begins with continuous growth rate of F1 (dominant or selected follicle; DF) and decreasing growth rate of F2 (largest subordinate). Intraovarian patterns based on presence or absence of the DF and CL are DF−CL, DF, CL, and devoid (neither DF nor CL). The DF and CL relationships in a pair of ovaries are ipsilateral (DF−CL pattern and devoid pattern) and contralateral (DF pattern and CL pattern). The effects of patterns and relationships on F1, F2, and FSH during deviation were determined in 21 mares. Diameters of F1 and F2 at expected beginning of deviation (F1, 22.5 mm; day 0) defined deviation classifications of conventional (F2 ≥ 18.5 mm on day 0), F2-undersized (F2 < 18.5 mm), and F1,F2-switched (F2 larger than F1 on day −1 or 0). During the decline in the FSH surge, an interaction (P < 0.001) of deviation classification and day indicated that FSH concentration was greater (P < 0.05) in undersized than in conventional deviations on days −3 and −1 and intermediate in switched deviation. Low FSH during days −3 to −1 in switched deviation began to increase on day −1 during switching in diameter rank between F1 and F2. The number of subordinate follicles per wave that attained a maximum of 11 to <16 mm was greatest in the DF−CL and devoid patterns and in ipsilateral (4.2 ± 0.5 follicles) than in contralateral (2.5 ± 0.7) relationships (P < 0.03). Concentration of FSH was greater (P < 0.04) on days −4 to 5 in ipsilateral (5.3 ± 0.2 ng/mL) than in contralateral (4.2 ± 0.4 ng/mL) relationships consistent with more follicles 11 to < 16 mm. In a survey (N = 63), an ipsilateral vs contralateral relationship was associated with 17 vs 17 conventional deviations and 7 vs 22 nonconventional deviations (P < 0.04), and conventional deviation (54%) was more frequent (P < 0.0001) than undersized (21%) or switched (25%) deviations. Results supported the hypotheses that (1) diameter deviation can be classified in mares into distinct categories based on diameter and rank of the future largest subordinate at expected deviation, (2) the number of subordinate follicles is influenced by ipsilateral vs contralateral relationships of DF and CL, and (3) conventional deviation is more common when the relationship between DF and CL is ipsilateral. Knowledge on follicle diameters for the different deviation classifications should be considered in studying follicle selection especially when terminal follicle sampling is done before expected deviation.