Most of the beef breeds in the United States currently have national genetic evaluation programs that have the potential to provide genetic values on over 600,000 registered beef animals each year (Benyshek et al., 1988). Predictions of genetic values obtained in these evaluations are reported as Expected Progeny Differences (EPD) for growth traits by the purebred associations in their sire evaluation reports (BIF, 1990). However, these evaluations account only for additive genetic effects and can not be used to compare bulls in multibreed populations, where the genetic value of a sire depends on additive and nonadditive genetic effects (Elzo, 1983). In addition, sire evaluation reports contain genetic evaluations for the complete preweaning growth period only, i.e., growth from birth to weaning. However, bulls to be used in crossbreeding programs for a particular year are selected when the current calf crop is between one to three months of age. Genetic evaluations at earlier ages than weaning would help to make such selection decisions based on the current calf crop data feasible. Furthermore, traits that could be measured simultaneously with weight at weaning (e.g., macrominerals) could also be measured at these earlier ages. Macrominerals such as calcium, phosphorus and magnesium are reasonable choices to be used in these evaluations because of their known physiological and biochemical relationship to growth (Littledike and Goff, 1987; Arnaud and Sanchez, 1990). Consequently, the objectives of this study were: 1) to obtain predictions of additive and nonadditive genetic effects for weight at 120, 150 and 205 d of age when a macromineral was used in the genetic evaluation, and 2) to determine changes in the ranking of sires across sire breed groups across the ages mentioned above.

Materials and Methods

Description of data

Data from 380 calves born during 1989 and 1990 were collected from birth to weaning. These animals form part of the Angus-Brahman multibreed herd of the University of Florida, located at the Pine Acres Research station in Citra, and they were produced by the mating of six sire breed groups consisting of Angus (A), Brahman (B), .75A .25B, .5A .5B, .25A .75B, and Brangus (.625A .375B), to the same breed groups of dams except that .25A .75B were unavailable. The distribution of sires and dams per breed group are presented in Table 1. Between two (.75A .25B in 1989) and five (Brangus in 1990) sires were available per breed group per year, for a total of 28. To ensure connectedness in the data set, between one (.5A .5B) and three (A) sires per breed group were represented in both 1989 and 1990. A total of 243 dams were used. There was a minimum of 14 dams (.75A .25B in 1990) and a maximum of 65 dams (A in 1989) per breed group per year. The distribution of progeny by mating subclass in both years is presented in Table 2. There were between three (.5A .5B sires mated to .75A .25B dams) and forty (B sires mated to B dams) calves per mating subclass.

effects for weight and macrominerals at 120, 150, and 205 d. For this study, fixed and random effects for two traits were computed using a sire-maternal grandsire model. The fixed effects were contemporary groups, sex of calf H dam age group subclass, additive genetic groups (sire group, maternal grandsire group, maternal granddam group), nonadditive genetic groups (sire group H dam group subclass, maternal grandsire group H maternal granddam group subclass). Contemporary group included the effects of year and management group within year. There were three sex classes (bull, heifer and steer) and three dam age classes (under 3 yr of age, 3-5, and over 5 yr). The random effects were sire additive direct genetic effects, sire nonadditive direct genetic effects (sire H breed group of dam subclass), and residual effects. These random effects were assumed to be uncorrelated among themselves. Sire additive genetic effects were assumed to have mean zero and covariance matrix Aσ_s² , where σ_s² is the sire additive genetic variance and A is the additive genetic relationship matrix (Henderson, 1976). The sire additive genetic variance represents .25 the additive genetic variance. Sire nonadditive direct genetic effects and residuals were assumed to have mean zero and each with a common variance. Additive and nonadditive maternal genetic effects were not included in these evaluations because their variances and covariances could not be estimated in this data set (27% of the calves had an unknown maternal grandsire, and most of the known maternal grandsires were mated to dams of the same breed group as the maternal grandsires). Table 3 presents the values of additive and nonadditive direct genetic variances and covariances used for these genetic evaluations. These variances and covariances were assumed to be the same for all breed groups of sires.

For this study, the predicted genetic values for weight were expressed as additive expected progeny differences (AEPD) and nonadditive expected progeny differences (NEPD). The sum of AEPD and NEPD was defined as multibreed expected progeny differences (MEPD). Correlations between AEPD and NEPD across ages were computed using the genetic values obtained in evaluations that involved the same traits. The ranking of sires across sire breed groups was determined for AEPD and MEPD for weight based on a two-trait multibreed genetic evaluation procedure where the other trait involved was either serum Ca, P or Mg. Rank correlations between AEPD and NEPD across ages were computed for all the evaluations.

Results and Discussion

Prediction of Genetic Effects when a Macromineral is Involved in the Evaluation

To investigate changes in genetic values that may occur over time, predictions of sire additive and nonadditive genetic values for weight at 120, 150 and 205 d for the sires used in the Angus-Brahman multibreed herd are presented in Table 4, Table 5 and Table 6 when serum Ca, P or Mg was involved in the evaluation, respectively.

Differences in AEPD from 120 to 205 d were found for most of the sires through all the evaluations with macrominerals. The largest changes through all these evaluations were for the B sire 10 (.61 kg at 150 d to 1.18 kg at 205 d, Table 4; .48 kg at 150 d to 1.30 kg at 205 d, Table 5; .58 kg at 150 d to 1.02 kg at 205 d, Table 6). Differences among NEPD occurred through all evaluations for all breed groups of dams and for most of the sires across ages. The largest changes were found in 5.A .5B dams that were mated to B sires (.29 kg at 120 d to .79 kg at 205 d, Table 4; 1.06 kg at 150 to 53 kg at 205 d, Table 5; 1.02 kg at 150 d to .69 kg at 205 d, Table 6). However, correlations among predictions of genetic effects across ages were high and important (P < .01, Table 7). Correlations among predictions of AEPD across ages for all macrominerals ranged from .83 to .99 (Table 7). This indicates that no major differences existed for sire additive genetic effects during the growth of the animals in this multibreed herd at the ages studied here. Correlations among predictions of NEPD at different ages for all the macrominerals ranged from .80 to .99 (Table 7). Thus, the combining ability of sires when mated to dams of a particular breed group had no significant differences through the growth of the animals. These results suggest that evaluating bulls when their calves are younger (e.g., at 120 d of age) would identify the best bulls at weaning fairly accurately. This is an advantage for producers who want to use the evaluation of their bulls using the current calf crop to choose those bulls to be used in the breeding season to produce the next calf crop.

Sire Rankings for Weight at Several Calf Ages

The ranking of sires across sire breed groups for weight at 120, 150 and 205 d when Ca, P or Mg are involved in the evaluations are presented in Tables 5-8, 5-9, and 5-10, respectively. There were differences in sire rankings through all ages for AEPD through all macrominerals. However, the ranking of the three top sires was essentially the same across calf ages when Ca or P were involved in the evaluation (Tables 5-8 and 5-9, respectively). For Mg, only the B sire 10 was ranked the same across calf ages (Table 10). Rank correlations among predictions of AEPD across ages were high (>.95) and important (P < .01) for all macrominerals. Therefore, the ranking of sires for additive genetic effects was not affected notably by changes in the prediction of AEPD. Moderate differences in sire rankings occurred for MEPD across all breed groups of dams and all ages (Tables 5-8, 5-9 and 5-10). These differences were due to changes in the NEPD. Rank correlations among predictions of MEPD across ages within each breed group of dams were high (>.83) and important (P < .01). Thus, sire rankings for genetic effects had no significant changes across ages in this multibreed herd. Animals that were ranked highest in earlier stages of growth had similar rankings at weaning, which is the usual age were selection or culling decisions are made. The advantage of having evaluated these animals prior to the breeding season is may be reflected in more appropriate sire selection.

Implications

Prediction of additive and nonadditive genetic effects at earlier stages of growth were similar to predictions of genetic effects at weaning, the age at which producers usually make selection or culling decisions. The small differences in the predictions did not affect the ranking of sires across sire breed groups for additive and nonadditive genetic effects at earlier stages. These results suggest that selection or culling decisions may be feasible at earlier stages than weaning (e.g., at breeding time). This could be particularly valuable for selection purposes within herds because producers could use these earlier evaluations based on the current calf crop for selection of bulls to breed the next calf crop. A more effective breeding program can be achieved.

Literature Cited

Arnaud, C. D. and S. D.Sanchez. 1990. Calcium and phosphorus. In: M. L.Brown (Ed.). Present Knowledge in Nutrition. p 212. Int. Life Sci. Inst., Washington, DC.

Benyshek, L. L., M. H. Johnson, D. E. Little, J. K. Bertrand, and L. A. Kriese. 1988. Applications of an animal model in the United States beef cattle industry. J. Dairy Sci. 71:35.

BIF. 1990. Guidelines for uniform beef improvement programs. North Carolina State Univ., Raleigh.

Elzo, M. A. 1983. Multibreed sire evaluation within and across countries. Ph.D. Dissertation. Univ. of California, Davis.

Henderson, C. R. 1976. A simple method for computing the inverse of a numerator relationship matrix used in prediction of breeding values. Biometrics 32:69.

Littledike, E. T. and J. Goff. 1987. Interactions of calcium, phosphorus, magnesium and vitamin D that influence their status in domestic meat animals. J. Anim. Sci. 65:1727.

Table 1. DISTRIBUTION OF SIRES AND DAMS BY YEAR ACCORDING TO BREED GROUP COMPOSITION

Sires

Dams

Breed Group^a

Total

1989

1990

1989 &

1990^b

Total

1989

1990

1989 &

1990^c

.75A.25B

.5A.5B

.25A.75B

BRANGUS

Total

243

Introduction

Materials and Methods

Implications

Literature Cited