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Ovarian Follicular Populations at Two Stages of an Estrous Cycle in Heifers Given High Energy Diets^1,2,

Agriculture Canada Research Station, Lennoxville, Quebec, J1M 1Z3

Abstract

Sixteen crossbred heifers were assigned at random on d 3 of an estrous cycle to continue to receive the medium quality hay (control diet) or to be flushed with a grain ration (high-energy diet) until slaughtered on d 12 or 16. The ovarian follicles were counted and measured using routine histological techniques. Both the nonatretic and atretic (more than four pyknotic bodies) antral follicles (>.14 mm) were separated into six classes according to diameter size. The mean number of nonatretic follicles measuring .29 to .67 mm and .68 to 1.57 mm in heifers on a high-energy diet increased from 30.8 and 8.1 on d 12 to 57.5 and 15.5 on d 16, whereas in heifers on a control diet it decreased from 43.1 and 13.0 to 28.1 and 6.5, respectively (energy in diet x day of estrous cycle interaction, P<.05). The mean number of nonatretic follicles measuring 3.68 to 8.56 mm decreased from 2.1 to 0 between d 12 and 16 in heifers on a high-energy diet while it remained the same at 0 and .5, respectively, for heifers on a control diet (energy in diet x day of cycle interaction, P<.05). Of the total number of follicles (mean = 161) 38.2% were atretic. Mean number of atretic follicles measuring 1.58 to 3.67 mm in heifers on a high-energy diet increased from 14.8 at d 12 to 31.0 at d 16 whereas in heifers on a control diet it decreased from 27.4 to 13.3, respectively (energy in diet x day of cycle interaction, P<.05). Results of the present study suggest that the high-energy diet increased the pool of follicles measuring .29 to .67 mm and .68 to 1.57 mm and limits growth to follicles measuring 3.68 to 8.56 mm by increasing atresia of follicles measuring 1.58 to 3.67 mm.

¹ Contribution no. 159.

² The authors wish to thank D. Pitman, I. Kirby, A. Bouchard, A. Belleau and L. Boisvert for their contributions.

³ Dept. de Biol., Faculté des Sciences, Université de Sherbrooke, Sherbrooke, Québec, J1K 2R1.

⁴ Agriculture Canada Research Station, Lennoxville, Quebec, J1M 1Z3.

⁵ Present address: Département de Zootechnie, Faculté des Sciences de l'Agriculture et de l'Alimentation, Université Laval, Québec G1K 7P4.

⁶ Send reprint requests to this author.