|Spicer, L - OKLAHOMA STATE UNIVERSITY|
|Rutter, L - UNIVERSITY OF FLORIDA|
Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 22, 2001
Publication Date: March 1, 2002
Interpretive Summary: In order for cows to produce a calf each year it is important that estrous cycles begin soon after calving due to a gestation length of about 284 d. Hormone changes have been studied extensively to understand the initiation of estrous cycles following calving. One hormone that has been shown to change after calving is insulin-like growth factor-I (IGF-I). Previous studies have shown IGF-I to be influenced by factors such as genotype, bod condition score (BCS), nutrient intake, and energy balance. Because IGF-I has direct effects on brain (hypothalamic and pituitary) and ovarian functions, IGF-I may be a potential regulator of return to estrous cyclicity in post-calving cows. Therefore the objective of the present study was to determine whether differences in ovarian function post-calving among various genotypes of beef cows are associated with differences in systematic concentrations of IGF-I. Purebred (Angus [AxA], Brahman [BxB], Charolais [CxC]) and crossbred (AxB, BxC, AxC) cows were used. Serum con- centrations of IGF-I, BCS, and BW were determined between wk 2 and 9 after calving. Ovarian function was assessed using ultrasonography. Genotype and week postpartum influenced BCS, BW, and serum levels of IGF-I. From wk 3 to 6 after calving, BCS decreased. Serum levels of IGF-I were greatest in BxB cows; lowest in AxA, CxC, and AxC cows; and intermediate in AxB and BxC cows. Serum levels of IGF-I did not change post-calving in CxC, AxA, and AxC cows, but increased in BxC, AxB, and BxB cows. Serum levels of IGF-I were associated with days to first large follicle and with BCS. In con- clusion, systematic levels of IGF-I vary with genotype and BCS in beef cows after calving and may hold potential as an indicator for improved repro- ductive performance in beef cows after calving.
Technical Abstract: The objective of this study was to determine the effect of genotype and week postpartum on serum concentrations of insulin-like growth factor-I (IGF-I), body condition score (BCS), body weight (BW), and ovarian function in beef cows. Cows from the following genotypes were utilized in two repli- cations 1 year apart: Angus (AxA; n=9), Brahman (BxB; n=10), Charolais (CxC; n=12), AxB (n=22), BxC (n=19) and AxC (n=24). Serum concentrations o IGF-I, BCS, and BW were determined between wk 2 and 9 postpartum. Rectal ultrasound was used to determine days postpartum to first medium (FME; 6 to 9 mm) and first large (FLA; greater than 9 mm) follicle, and to first corpus luteum (CL). Genotype (P < .001) and week postpartum (P < .01) influenced BCS, BW, and serum IGF-I concentrations. Averaged across geno- type, BCS decreased (P <.05) from wk 3 to wk 6 postpartum. Averaged over year and week postpartum, serum IGF-I concentrations were greatest (P <.05) )in BxB cows (46 ng/mL); lowest in AxA (12 ng/mL), CxC (13 ng/mL), and AxC cows (18 ng/mL); and intermediate (P < .05) in AxB (28 ng/mL) and BxC (26 ng/mL) cows. Serum IGF-I concentrations did not change with week postpartum in CxC, AxA, and AxC cows, but increased (P < .05) between 2 and 7 wk post- partum in BxC, AxB, and BxB cows. Average interval to FME (16 d) and FLA (35 d) follicle did not differ among genotypes. Serum IGF-I concentrations correlated (P < .01) with days to FLA (r=.29) and BCS (r=.59). Averaged across genotypes, cows that lost BCS postpartum had lower (P < .01) serum IGF-I concentrations and a greater interval to first CL than cows that maintained BCS. In conclusion, concentrations of IGF-I in serum differed among genotypes and were associated with BCS and days to first large follicle and CL in postpartum beef cows.