Abstract #T329

# T329
Ovarian follicular dynamics in early- and late-maturing Brahman cows.
Rui A. d'Orey Branco*1,2, Thomaz H. Welsh2, Don Neuendorff1, Ron D. Randel1, 1Texas A&M AgriLife Research, Overton, TX, 2Department of Animal Science, Texas A&M University, College Station, TX.

Age at first calving is greater in Bos indicus than in Bos taurus cattle. Two different populations of Brahman cows exist based on age at first calving. Specifically, cows that are capable of calving by 26 mo of age are defined as early maturing (EM). Conversely, those cows that must be over 33 mo of age to calve are considered late maturing (LM). The objective of this trial was to compare follicular dynamics of EM and LM cows. This study’s herd (located near Overton, Texas) was comprised of 120 pure bred Brahman cows. The EM group (n = 7) calved before 26 mo of age and the LM group (n = 7) calved at 33 mo of age or later. Cow BCS was between 4.5 and 7.5 and all of the cows calved at least 60 d before the trial began. Follicular evaluation was performed by transrectal ultrasound (US; Medison Co. SA-600; 7.5 mHz probe) and began after a naturally occurring estrus detected by a heat check bull. Cows were examined daily by US until signs of a new estrus were detected. During this second estrus the cow was examined by US every 4 h. Data were analyzed using PROC GLM of SAS. The EM and LM groups did not differ (P > 0.1) with respect to the diameter of the pre-ovulatory follicle or the time elapsed from the onset of estrus to ovulation. There were 3 follicular waves in 85.7% (6/7) of the LM and 14.3% (1/7) in EM cows (chi-squared; P < 0.05). The size of the largest subordinate follicle on the day before estrus was greater (P < 0.01) in EM (10.42 ± 2.38 mm) than in LM (5.23 ± 2.31 mm) cows. The number of subordinate follicles (>4mm) one day before estrus in EM cows (4.28 ± 2.55) was greater than in the LM cows (1 ± 0.76; P < 0.01). The total of follicular diameters one day before estrus was greater (P < 0.01) in EM (32.43 ± 5.63 mm) than in LM (7.43 ± 5.63 mm) cows. These results suggest that early and late maturing cows have differing aspects of follicular dynamics such as the number of follicular waves, the number of subordinate follicles, and the diameter of the second largest follicle. Further studies to determine whether this phenotypic difference is present in cows of other breeds are warranted.

Key Words: follicular dynamics, early maturing, late maturing