A. Yilmaz
M. E. Davis1
The Ohio State University
Department of Animal Sciences
R. C. M. Simmen
University of Florida
Department of Animal Science
1 For more information contact at: The Ohio State University, 2027 Coffey Road, 221 Plumb Hall, Columbus, OH, 43210; 614-292-4984; fax: 614-292-2929; e-mail: davis.28@osu.edu.
This study was designed to examine the effects of selection for high vs. low blood serum IGF-I concentration on reproductive performance of spring- and fall- calving Angus cows. Data were obtained from an ongoing experiment involving 100 spring-calving (50 high- and 50 low-line) and 100 fall-calving (50 high- and 50 low- line) purebred Angus cows. Effects of year-line-season and age of dam on calving rate were highly significant. Calving rate was slightly higher in high IGF-I line cows (P = 0.07), but did not differ between spring- and fall-calving cows (P = 0.47) when only natural service matings were considered. Postweaning IGF-I concentrations of cows that had data for calving rate were significantly higher in high-line cows, but did not differ between spring- and fall-calving cows. As heifers, IGF-I concentrations averaged 38.5 ± 8.9 ( P= 0.0001), 42.7 ± 9.9 (P = 0.0001), and 34.8 ± 10.2 (P = 0.0007) ng/mL, more IGF-I for high-line than low-line at days 28, 42, and 56, respectively, of the postweaning period among those females that had data for calving rate. Mean IGF-I was 38.6 ± 9.4 (P = 0.0001) ng/mL higher in the high-line. Therefore, the slightly higher calving rate found in the high-line may have been associated with higher IGF-I concentrations. Mean age of heifers at first calving did not differ between the high and low IGF-I line heifers (P = 0.54). Spring-born heifers were significantly older than fall-born heifers at first calving. Relationships (as indicated by regression coefficients) between age of heifers at first calving and IGF-I concentrations were negative, but nonsignificant for all measures of IGF-I. Thus, selection for higher blood serum IGF-I concentration improved calving rate of Angus cows and had a small, albeit desirable, effect on age at first calving. Season had a large influence on age of heifers at first calving but not on calving rate.
Number of calves obtained per cow mated is of great economic importance in beef cattle herds. Animals that have low reproductive efficiency are culled from the herd, resulting in economic loss. Due to its influence on reproductive functions of animals, insulin-like growth factor I (IGF-I) concentration is an important candidate for selection to improve reproductive efficiency in a herd.
Seasonal changes in temperature is one of the factors that influences fertility in a cattle population (De Kruif, 1978). Ingraham et al. (1976) reported a significant correlation between conception rate and temperature-humidity index on the second day prior to breeding in Holstein cows. They found a decline in conception rate from 66 to 35% as the temperature-humidity index increased from 68 to 78. They also reported variation in the sensitivity of conception rate to heat stress on different days of the estrous cycle prior to breeding.
The IGFs play a role in cell proliferation and differentiation, and may play a role in conception by changing the nature and level of expression of uterine proteins at the time of implantation (Pope 1988). Similarly, IGFs have been postulated to mediate communication between fetal and maternal tissues and are involved in the initiation, development, and maintenance of pregnancy (Simmen et al., 1993). Davis and Bishop (1991) detected earlier calving dates in identical twin heifers with high concentrations of serum IGF-I. Because effects of IGF-I on reproductive functions of animals have been well-documented, it is of interest to examine whether selection for high vs. low blood serum IGF-I concentration affects calving rate and age of heifers at first calving.
The objectives of this study were, therefore, to examine the effects of season and IGF-I selection line on calving rate and age of heifers at first calving.
Selection for high vs. low blood serum IGF-I concentration was initiated in 1989 using 100 spring-calving (50 high-line and 50 low-line) and in 1990 using 100 fall-calving (50 high-line and 50 low-line) purebred Angus cows with unknown IGF-I levels located at The Ohio State University's Eastern Ohio Resource Development Center (EORDC). The 1989 spring and 1990 spring and fall calf crops were produced using different sets of four bulls with unknown IGF-I values. In subsequent years, the four bulls that had the highest or lowest blood serum IGF-I concentrations, adjusted for age of calf and age of dam, were saved for breeding within the respective selection lines. Selection of bulls was done on a within-season basis. To create ties between the EORDC herd and other herds contributing to North Central Regional Project NC-196, "The Genetics of Body Composition in Beef Cattle," 10 cows per line were randomly chosen and artificially inseminated using semen from an Angus reference sire. Selection of bulls and heifers was based on serum IGF-I samples collected at days 28, 42, and 56 of postweaning test. IGF-I concentrations measured at days 28, 42, and 56 are abbreviated as IGF28, IGF42, and IGF56, respectively.
Approximately eight cows were culled from each line, each year, due to poor reproductive performance, oldest age, or health problems. Cows were culled if they were open for two consecutive years. All heifers were mated and selection was performed among heifers that conceived. Approximately eight pregnant heifers with the highest or lowest serum IGF-I concentrations were used to replace cows that were culled from each selection line.
Spring-born calves were reared by their dams until weaning at seven months of age. During an adjustment period of two weeks and a 140-day postweaning test period, bull calves were fed a corn-soybean meal based concentrate diet and, from 1989 through 1993, heifer calves were fed corn silage. Bulls were kept at the EORDC and heifers were transferred to the North Appalachian Experimental Watershed, Coshocton, OH, for the postweaning test. Beginning in 1994, heifers were kept at EORDC and were fed a corn-soybean meal concentrate diet. Bulls and heifers were given ad libitum access to feed.
Fall-born calves were fed a growing diet that was designed to yield gains of 1.98 lbs/day during a 112 day-growing period in drylot after weaning, at approximately 140 day of age. Following the growing period, bulls were kept at EORDC, and from 1990 through 1993, heifers were transferred to Coshocton, OH. Heifers were generally fed corn silage. However, due to unavailability of corn silage, a soybean meal-based concentrate diet was used for feeding the fall 1990 heifers beginning between days 42 and 56 of the postweaning test and for feeding the fall 1992 heifers beginning between days 84 and 112. Heifers born in fall 1991 were fed a corn-soybean meal concentrate diet during the entire postweaning test period due to drought conditions and the unavailability of corn silage. Beginning in 1994, fall-born heifers were kept at EORDC following weaning and were fed the same diet as spring-born heifers.
Approximately 25 milliliters (mL) of blood were collected into sterile glass tubes at days 28, 42, and 56 of the postweaning test, allowed to clot for 24 hours at 4 degrees C, and centrifuged. Serum was drawn off and frozen at -20 degrees C, until it was assayed.
Procedures previously described by Bishop et al. (1989) were followed to determine IGF-I concentrations using radioimmunoassay.
All data were analyzed using PROC GLM procedures of SAS (SAS,1992). Year, line, and season were combined into one variable to obtain a unique identification for the nested effect of sire. Effects of age of dam, year-line-season, and sire nested within year-line-season, were included in the analysis of calving rate and age at first calving. In the analysis of age at first calving, on-test age of calf was added to the model for adjustment purposes. Orthogonal linear contrasts were used to compare calving rate, age at first calving, and IGF-I concentrations of spring- vs. fall-born calves and high vs. low IGF-I line calves. Sire nested within year-line-season was used as the error term to test year-line-season effects and to obtain significance levels for contrasts of high- vs. low-line and spring vs. fall means. Sire nested within year-line-season was not included in the analysis in which only A.I. matings were considered because there was only one A.I. sire. Data obtained from A.I. sires were included in the regression analyses, but not in the linear contrast analyses, because doing so would have reduced the divergence between the IGF-I lines. In a second analysis, IGF28, IGF42, IGF56, and mean IGF-I were included one at a time in the models as independent variables (i.e., covariates) to calculate coefficients for the linear and nonlinear (i.e., quadratic) regression of age at first calving on these measures of IGF-I. The nonlinear terms were found to be nonsignificant (P = 0.83, 0.67, 0.13, and 0.66 for IGF28, IGF42, IGF56, and mean IGF-I, respectively) and therefore were deleted from the models.
Means ± standard errors for calving rate are presented in Table 1. The mean natural service calving rate was 78.3%. Calving rate was slightly higher in the high IGF-I line (P = 0.07), but did not differ between spring- and fall-calving cows (P = 0.47) when only natural service matings were considered. Year-line-season and age of dam effects on calving rate were highly significant. Calving rate increased from age of dam 2 to age of dam 5 to 9 and then decreased. Azzam et al. (1989) reported that age, but not season, influenced first-service conception rate in beef cattle.
| Table 1. Contrasts and Year-Line-Season and Age of Dam Means ± Standard Errors for Calving Rate. | ||||||
|---|---|---|---|---|---|---|
| n | Natural calving rate | n | Natural + A.I. calving rate | n | A.I. calving rate | |
| Year-line-season | P = 0.0001 | P = 0.001 | P = 0.16 | |||
| 1990 high-spring | 50 | 0.83 ± 0.05 | 50 | 0.83 ± 0.05 | ||
| 1990 high-fall | 42 | 0.58 ± 0.14 | 42 | 0.58 ± 0.14 | ||
| 1990 low-spring | 47 | 0.83 ± 0.05 | 47 | 0.83 ± 0.05 | ||
| 1990 low-fall | 23 | 0.42 ± 0.20 | 23 | 0.42 ± 0.20 | ||
| 1991 high-spring | 45 | 0.76 ± 0.06 | 56 | 0.73 ± 0.05 | 9 | 0.45 ± 0.13 |
| 1991 high-fall | 42 | 0.74 ± 0.06 | 49 | 0.67 ± 0.06 | 7 | 0.28 ± 0.17 |
| 1991 low-spring | 43 | 0.57 ± 0.09 | 53 | 0.60 ± 0.08 | 9 | 0.61 ± 0.14 |
| 1991 low-fall | 37 | 0.89 ± 0.06 | 43 | 0.87 ± 0.06 | 6 | 0.79 ± 0.18 |
| 1992 high-spring | 64 | 0.66 ± 0.06 | 74 | 0.64 ± 0.06 | 7 | 0.38 ± 0.14 |
| 1992 high-fall | 46 | 0.81 ± 0.05 | 55 | 0.78 ± 0.05 | 9 | 0.46 ± 0.15 |
| 1992 low-spring | 57 | 0.53 ± 0.09 | 68 | 0.57 ± 0.07 | 11 | 0.60 ± 0.13 |
| 1992 low-fall | 37 | 0.73 ± 0.06 | 46 | 0.76 ± 0.05 | 9 | 0.75 ± 0.14 |
| 1993 high-spring | 53 | 0.65 ± 0.09 | 61 | 0.66 ± 0.08 | 8 | 0.55 ± 0.15 |
| 1993 high-fall | 43 | 0.68 ± 0.06 | 52 | 0.72 ± 0.05 | 9 | 0.67 ± 0.15 |
| 1993 low-spring | 52 | 0.69 ± 0.09 | 62 | 0.73 ± 0.08 | 10 | 0.78 ± 0.14 |
| 1993 low-fall | 34 | 0.64 ± 0.07 | 43 | 0.64 ± 0.06 | 9 | 0.45 ± 0.15 |
| 1994 high-spring | 61 | 0.86 ± 0.05 | 71 | 0.80 ± 0.04 | 8 | 0.52 ± 0.12 |
| 1994 high-fall | 57 | 0.79 ± 0.09 | 62 | 0.82 ± 0.04 | 5 | 0.77 ± 0.19 |
| 1994 low-spring | 60 | 0.64 ± 0.09 | 73 | 0.61 ± 0.07 | 12 | 0.52 ± 0.12 |
| 1994 low-fall | 37 | 0.73 ± 0.06 | 49 | 0.68 ± 0.05 | 12 | 0.33 ± 0.13 |
| 1995 high-spring | 65 | 0.77 ± 0.05 | 65 | 0.77 ± 0.05 | ||
| 1995 high-fall | 68 | 0.81 ± 0.04 | 68 | 0.81 ± 0.04 | ||
| 1995 low-spring | 70 | 0.61 ± 0.08 | 70 | 0.61 ± 0.08 | ||
| 1995 low-fall | 50 | 0.89 ± 0.05 | 50 | 0.89 ± 0.05 | ||
| 1996 high-spring | 68 | 0.71 ± 0.04 | 68 | 0.71 ± 0.04 | ||
| 1996 high-fall | 65 | 0.66 ± 0.05 | 65 | 0.66 ± 0.05 | ||
| 1996 low-spring | 69 | 0.52 ± 0.08 | 69 | 0.52 ± 0.08 | ||
| 1996 low-fall | 55 | 0.87 ± 0.05 | 55 | 0.87 ± 0.05 | ||
| Age of dam at calving | P = 0.0001 | P = 0.0001 | P = 0.0003 | |||
| 2 | 393 | 0.65 ± 0.02 | 400 | 0.64 ± 0.02 | 6 | 0.03 ± 0.19 |
| 3 | 243 | 0.72 ± 0.02 | 253 | 0.71 ± 0.02 | 9 | 0.47 ± 0.14 |
| 4 | 170 | 0.76 ± 0.01 | 206 | 0.77 ± 0.03 | 33 | 0.76 ± 0.07 |
| 5a | 614 | 0.78 ± 0.01 | 668 | 0.78 ± 0.01 | 51 | 0.80 ± 0.06 |
| 10b | 213 | 0.73 ± 0.03 | 255 | 0.73 ± 0.02 | 41 | 0.73 ± 0.06 |
| Contrasts c | ||||||
| High minus low line | P = 0.07d | P = 0.14d | P = 0.19d | |||
| 0.05 ± 0.02 | 0.04 ± 0.02 | -0.09 ± 0.07 | ||||
| Spring minus fall | P = 0.47e | P = 0.58e | P = 0.88e | |||
| -0.02 ± 0.02 | -0.01 ± 0.02 | -0.01 ± 0.07 | ||||
| a Cows 5- to 9-years-old at calving. b Cows that were 10-years-old and older at calving. c Number of observations for natural matings was 1,633; for A.I. and natural matings combined was 1,782; and for A.I. matings was 149. d P is the level of statistical significance for the contrast of high- vs. low-line. e P is the level of statistical significance for the contrast of spring- vs. fall-calving. | ||||||
Mean calving rate was 77.6% when A.I. and natural service matings were combined for analysis. Year-line-season and age of dam effects on calving rate were highly significant. Calving rate did not differ between high- and low-line cows (P = 0.14) or between spring- and fall-calving cows (P = 0.58). Calving rate increased from age of dam 2 to age of dam 5 to 9 and then decreased.
Mean calving rate for A.I. matings was 70.4%. High- vs. low-line and spring vs. fall differences in calving rate were not significant (P = 0.19 and 0.88, respectively). Age of dam effects on A.I. calving rate were highly significant. Very low calving rates occurred in yearling heifers (age of dam = 2-years-old at calving). However, this result was based on only six observations.
Among those females that had data for calving rate, IGF-I concentrations as heifers averaged 38.5 ± 8.9 (P = 0.0001), 42.7 ± 9.9 (P = 0.0001), and 34.8 ± 10.2 (P = 0.0007) ng/mL more IGF-I for high-line than low-line at days 28, 42, and 56, respectively, of the postweaning period. Mean IGF-I was 38.6 ± 9.4 (P = 0.0001) ng/mL higher in the high-line. Therefore, the slightly higher calving rate found in the high-line may have been associated with higher IGF-I concentrations.
In contrast to our results, Kroonsberg et al. (1989) found no difference in conception rate of mice selected for high vs. low blood plasma IGF-I concentrations. However, they detected increases in litter size, fetal weight, total placental weight, and mammary gland weight in high IGF-I line mice. Although they observed an increased number and weight of fetuses in the high IGF-I line, these differences were not significant when adjusted for maternal body weight.
Mean age of heifers at first calving was 730 days. Means ± standard errors for age of heifers at first calving are presented in Table 2. Year-line-season and age of dam effects on age of heifers at first calving were important (P = 0.02 and P= 0.13, respectively). Age at first calving did not differ between high and low IGF-I line heifers (P = 0.54). Spring-born heifers were significantly older than fall-born heifers at their first calving.
|
Table 2. Contrasts and Year-Line-Season and Age of Dam Means ± Standard Errors for Age of Heifers at First Calving. | ||
|---|---|---|
| Year line-season | n | Age at first calving, days P = 0.02 |
| 1990 high-spring | 8 | 734.6 ± 5.8 |
| 1990 high-fall | 11 | 699.0 ± 6.3 |
| 1990 low-spring | 9 | 743.3 ± 5.6 |
| 1990 low-fall | 5 | 697.0 ± 8.8 |
| 1991 high-spring | 7 | 736.7 ± 7.1 |
| 1991 high-fall | 2 | 719.3 ± 12.1 |
| 1991 low-spring | 7 | 752.2 ± 7.1 |
| 1991 low-fall | 2 | 709.7 ± 11.8 |
| 1992 high-spring | 8 | 727.7 ± 6.3 |
| 1992 high-fall | 11 | 717.5 ± 6.1 |
| 1992 low-spring | 6 | 742.4 ± 7.6 |
| 1992 low-fall | 2 | 713.1 ± 11.5 |
| 1993 high-spring | 8 | 756.9 ± 6.7 |
| 1993 high-fall | 7 | 721.6 ± 7.2 |
| 1993 low-spring | 10 | 740.0 ± 6.3 |
| 1993 low-fall | 8 | 726.1 ± 6.8 |
| 1994 high-spring | 12 | 732.7 ± 4.9 |
| 1994 high-fall | 8 | 727.3 ± 6.0 |
| 1994 low-spring | 8 | 740.5 ± 6.3 |
| 1994 low-fall | 6 | 713.1 ± 7.4 |
| 1995 high-spring | 12 | 729.2 ± 5.9 |
| 1995 high-fall | 7 | 714.4 ± 7.3 |
| 1995 low-spring | 11 | 733.5 ± 5.5 |
| 1995 low-fall | 7 | 726.4 ± 6.9 |
| Age of dam at calving | P = 0.13 | |
| 2 | 36 | 732.7 ± 3.6 |
| 3 | 30 | 725.0 ± 3.6 |
| 4 | 19 | 728.1 ± 4.7 |
| 5a | 78 | 727.9 ± 2.4 |
| 10b | 19 | 722.6 ± 4.8 |
| Contrasts c | ||
| High- minus low-line | P = 0.54d | |
| -1.7 ± 2.9 | ||
| Spring minus fall | P = 0.0001e | |
| 23.7 ± 4.4 | ||
| a Cows 5- to 9-years-old at calving. b Cows that were 10-years-old and older at calving. c Number of observations was 182. d P is the level of statistical significance for the contrast of high- vs. low-line. e P is the level of statistical significance for the contrast of spring- vs. fall-calving. | ||
The IGF-I concentrations of heifers that had data for age at first calving were significantly higher in high-line heifers, but did not differ between spring- and fall-born heifers. Schillo et al. (1983; 1992) reported that fall-born beef heifers attained puberty at younger ages than spring-born heifers. The authors implied a role for photoperiod in determining age at puberty.
Coefficients for the regression of age of heifers at first calving on IGF28, IGF42, IGF56, and mean IGF-I are presented in Table 3. All regression coefficients were nonsignificant. However, all coefficients were negative, indicating that heifers that had higher IGF-I concentrations during the postweaning period tended to be younger at first calving.
| Table 3. Regression Coefficients for Regression of Age of Heifers at First Calving on IGF-I Concentrations. a | ||||
|---|---|---|---|---|
| IGF28 b | IGF42 | IGF56 | Mean IGF-I c | |
| Age at first | -0.06 ± 0.05 | -0.06 ± 0.04 | -0.02 ± 0.04 | -0.08 ± 0.05 |
| calving | P = 0.21 | P = 0.11 | P = 0.58 | P = 0.14 |
| a Unit of measurement for the regression of age of heifers at first calving on IGF-I concentrations is day x ng -1 mL -1. b Number of observations for IGF28, IGF42, and IGF56 was 165. c Number of observations for mean IGF-I was 182. | ||||
Calving rate tended to be higher in high IGF-I line cows. Therefore, postweaning IGF-I concentration may be a useful physiological indicator trait for calving rate. Selection for increased blood serum IGF-I concentration also had small, but desirable effects on age of heifers at first calving.
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