Heritability estimated for direct and maternal in this study tended to be high under the different models used. These appear to be due categorization of genetic groups into only three groups to have a reasonable number of observations in each category. The results showed that model choice had a prominent role in genetic improvement for obtaining reliable genetic parameters. This study has also shown higher genetic parameters estimated when models accounting for the covariance of direct maternal genetics. The direct heterotic gain was high for all growth traits, except for BW and WW. The estimates of direct heterosis obtained in the present study were positives for most growth traits. Thus, it is advisable to cross Horro with Friesian and Jersey in a low input management system. Mostly maternal additive effects for growth traits are positive, and it is preferable to use crossbred cows resulting from purebred dams instead of crossbred cows from crossbred dams.
A breeding plan and implementation of a selection program require knowledge of the genetic parameters of growth traits. On the other hand, the biased estimation of breeding values was incorrect (co)variance components (13). Besides, Haile et al. (6) reported that for the planning of a sound crossbreeding program, the information on the relative breed’s performance was needed under various environmental conditions. Furthermore, accurate estimation of breed additive and nonadditive effects and separation of their causal components is essential for the design of breeding programmed, which fully exploits the value of crossbreeding (14). Indeed, previous studies indicated that reasonable genetic improvement using selection for moderate and high heritability traits(6, 15, 16). In agreement with these findings, the high heritability of one-year weight (0.72 ± 0.05) and two-year weight (0.73 ± 0.06) of Ethiopian Boran was previously described (6). In contrast, Demeke et al. (3) found that a low heritability (0.13) value was estimated for purebreds and crossbreds in Ethiopia. Consistent with these results, high heritability has been reported for the population consisting of 15 cattle breeds due to the multibreed composition of the herd (16, 17). In the current study, a high level of genetic variability within a group and the estimated heritability is inflated due to categorized genetic groups into only three groups to have a reasonable number of observations in each category (9). These results might be explained by Rodriguez-Almeida et al. (18), who suggested that quite variable heritability estimates in different combinations of herd lines results were inclusion of nonadditive genetic variances like dominance and epistatic effects. Bennett and Gregory (19) and Tosh et al. (20) also reported that high heritability estimates for early growth traits in composite populations. Following previous observations of Haile et al. (6), the larger discrepancies in the estimates of heritability for Boran cattle were related to data size in the genotype. Even though, heritability values were varied based on differences in the type of data record analyses, method of estimation, and models used for the analysis as cited by Haile et al. (6). In the present results, the inclusion of the covariance between direct and maternal genetics effects increased the estimate of direct heritability for one-year weight from 0.77 (model 4) and 0.77 (model 6), which was supported by our previous reports on pre-weaning weight (9). Comparable to these results, inclusions of covariance between direct and maternal genetic effects were increasing the direct heritability of birth weight due to a small number of observations (16).
As illustrated in Table 1, maternal heritability estimates for one-year weight ranged from 0.00 to 0.26. Furthermore, Pico (21) found that maternal genetic effects might be important in one year's age. Surprising results were observed in the present study; because the calves were reared by artificially suckling and no longer depend on their dam, these results may indicate the carry-over effects of weaning weight. On the other hand, the maternal and permanent environment heritability of growth traits of Boran cattle is comparable to direct heritability estimates, while maternal and permanent environment heritability of Ethiopian Boran cattle crosses were close to zero, which is suggested that the importance of maternal effects in indigenous cattle (6). These results might be explained by Meyer(22), who reported that maternal effects gradually decreased with increasing calf age and were still important at 700 days of age. Furthermore, Haile et al. (6) reported that maternal heritability diminished with age. However, relatively high estimates of maternal heritability were reported by Mackinnon et al.(23) for weights beyond one year of age. Nevertheless, Demeke et al. (3) reported that none of the maternal association effects on one year weight of the mixed population of purebreds and crossbred cattle in artificially reared calves. Moreover, Vostry et al. (24) reported that maternal effects were as important as direct effects during the early pre-weaning development and that they got smaller as the calf was growing up. The significant direct maternal effects are inconsistent with the reports of Kahi et al. (25) and Thorpe et al. (26). Therefore, pre-weaning growth is a direct indicator of the dam’s milk production and mothering ability in suckled calves (25). Meyer (27) reported that milk production is the major component of maternal effects for pre-weaning growth of beef calves and breed differences in maternal effects contributable to differences in milk yield. In this study, bucket feeding is expected to have contributed to the lack of maternal effects for one-year weight and post-weaning average daily gain.
The current results showed that the individual breed additive contribution of Jersey to the BW, WW, YW, DG, and PDG of calves were 35% (p < 0.01), 27% (p < 0.01), 4% (p < 0.05), 17% (p < 0.05), and 27% (p < 0.05), respectively. On the other hand, the individual breed additive contribution of Holstein Friesian to the BW, WW, YW, DG, and PDG of calves were 67% (p < 0.01), 47% (p < 0.01), 12% (p < 0.05), 26%(p < 0.05) and 29%( p < 0.05), respectively. These comparisons are revealed that Holstein Friesian and Horro crosses have high growth performance than the Jersey and Horro crosses under Bako environmental conditions. These results are by Demeke et al. (3), who reported the breed additive contribution of Friesian was more than the additive contribution of Jersey to the BW, WW, DG, and YW of Boran cattle. In the present results, the lower additive contribution of the Holstein Friesian and Jersey to the YW and PDG traits might be due to low management and genotype-environment interactions (6, 10). In contrast, Schoeman and Jordan(16) reported that the higher additive contribution in a multibreed beef cattle herd to the YW and PDG. Besides, Haile et al. (6) reported that the individual additive breed differences between Boran and Friesian breeds were positive and significant for growth performance in central Ethiopia. The current study was observed that all growth traits except post-weaning average daily gain had significant positive additive effects. However, Demeke et al. (3) reported that negative additive effects of Jersey breeds on all growth traits in Boran cattle, which is suggested that no weight advantage when crossing with a larger zebu breed. This is consistent with the results of Haile et al. (6), which demonstrated that the significant additive breed effect for birth weight, weaning weight, and pre-weaning average daily gain in a crossbreeding studies involving Boran and Friesian breeds in central Ethiopia. Furthermore, a significant additive breed effect for birth weight, weaning weight, and pre-weaning average daily was reported in crosses of Jersey with Ghana Shorthorn and Sokoto Gudali cattle (28). Contrary to these reports, Kahi et al. (25) elucidated that the nonsignificant individual additive genetic effect in Ayrshire and Sahiwal breeds for birth weight, weaning weight and pre-weaning average daily gain was associated with the level of management of calf feeding in the farm. In addition, Rege et al. (28) reported that negative additive effects on growth traits for the Jersey breed compared with the Ghanaian Gudali breed and a positive contribution of the Jersey breed when crossing it with a smaller African Bos Taurus breed of the Ghanaian Shorthorn.
As shown in Table 4, the maternal additive breed effect positive small values were estimated for birth weight but large positives values for the other traits of Holstein Friesian crosses. However, negative small values were estimated for the maternal additive effect of birth weight but large positive values for the other traits of Jersey crosses. Correspondingly, Skrypzek et al. (29) found that negative values of maternal additive effects for Simmental and Hereford beef cattle. Indeed, Kahi et al. (25) reported that maternal breed additive effects nonsignificant for pre-weaning growth of crosses of Ayrshire, Brown Swiss, and Sahiwal cattle in the lowland tropics of Kenya. Apart from these works, Kahi et al. (25) indicated that lack of the maternal additive breed effects in bucket feeding and calf house rearing for pre-weaning traits of crosses of Ayrshire, Brown Swiss and Sahiwal cattle in the lowland tropics of Kenya. The presence of the maternal additive breed effects at one year weight in the present results was suggesting that maternal additive breed effects should not be ignored in defining the results from crossbreeding studies that are conducted in the environment of Horro cattle occupied and in making recommendations to farmers.
A reliable estimation of crossbreeding parameters has been required for effective designing of crossbreeding system (30). Furthermore, Theunissen et al.(31) reported that the indicus x sanga and indicus x taurus direct heterosis effects of growth traits were greater than either the taurus x sanga or taurus x taurus effects; suggesting that greater genetic distances between breeds have resulted in greater heterosis than the more closely related breeds. Furthermore, Leal et al. (32) reported that the effects of individual and maternal heterosis were consistently greater in indicine × taurine crosses than in crosses among taurine breeds. Estimated direct heterosis effects of BW and WW for crosses of Holstein Friesian and Jersey with Horro breeds were significant (P < 0.01) and negative. From this study I reported that the average individual heterosis estimated within Holstein Friesian and Jersey were − 2.6 ± 0.49 and − 3.8 ± 0.86 kg for BW, -0.59 ± 1.3 and − 1.8 ± 2.6 kg for WW, 26.04 ± 3.9 and 21.6 ± 6.7 kg for YW 28.9 ± 15 and 27.4 ± 26 gm for DG and 61.8 ± 9.9 and 68.8 ± 16.6 gm for PDG, respectively. Similar results have been reported, individual heterosis effects were significant for weight at birth, weaning, six months, and one year of age for Friesian, Jersey, and Simmental breeds (6). Compared with these results, Demeke et al. (3) reported that heterosis was an important genetic factor influencing primary growth performance in crossbreed animals in central Ethiopia. The present results show that the negative heterosis estimates for the Holstein Friesian and Jersey crosses for birth and weaning weight, which might be explained by the fact that lower performance calves than the average of the parents(3). Comparable to these results, the individual heterosis influence on YW, ADG, and PDG was positive and significant(3). Indeed, Theunissen et al. .(31) examined that both effects of direct and maternal heterosis were positive for all growth traits. On the other hand, previous studies showed that negative heterosis for birth weight in the crossbreeding of Bos indicus breeds as a dam line (28). In agreement with these results, Cunningham (33) suggested that a significant difference between the crosses of Bos taurus and Bos indicus was determined by the interactions of genotype and environment, which is explained that in a poor environmental production is highly influenced by heterosis (34), while in a good environmental production is largely determined by breed additive effects and small heterosis effects (25). Furthermore, Kahi et al. (25)reported that lack of heterosis was determined under good nutrition level in crosses of Ayrshire, Brown Swiss and Sahiwal cattle breeds and the significant high heterosis effects in crosses of Ayrshire, Brown Swiss and Sahiwal cattle breeds in sub-optimal nutrition(26). In line with this result, the absence of major heterosis results in the cross of Ayrshire and Brown Swiss was expected (30), because the wider genetic divergence of the phenotypic difference between parental breeds was the greater the heterosis expressed(30). Comparable to these findings, Mendonça et al. (34) have shown that greater mature weight and maturing rate in crossbred cows were due to heterotic effects that might depend on the distance between the parental breeds.
On the other hand, the average maternal heterosis estimated within Holstein Friesian and Jersey were 1.3 ± 0.23 and 1.9 ± 0.43 kg for BW, 0.3 ± 0.64 and 0.89 ± 1.0 kg for WW, -13.0 ± 1.9 and − 10.8 ± 3.3 kg for YW, -14.5 ± 7.5 and − 13.7 ± 13 gm for DG and − 30.9 ± 4.9 and − 34.4 ± 8.3 gm for PDG, respectively. Indeed, Haile et al. (6) reported that the maternal heterosis effects were significant for birth weight (2.5 kg) and weaning weight (3.0 kg) for crosses of Friesian and Boran breeds in central Ethiopia. As previously described, significant influences of maternal heterosis on birth weight in the crosses of Bos taurus and Bos indicus in Bangladesh(35). Consistent with these results, maternal heterosis effects were of a higher magnitude and the opposite sign has been reported by Kahi et al. (25)for pre-weaning traits of crosses of Ayrshire, Brown Swiss, and Sahiwal cattle in the lowland tropics of Kenya. Furthermore, the maternal heterosis expressed by taurine indicine was greater than taurine-taurine maternal heterosis (32), due to the greater genetic distance between taurine and indicine breeds (36).
The direct heritability of one-year weight of Horro and their crosses were high. The maternal additive effect for growth traits was mostly positive, therefore, it could be preferable to use crossbred cows resulting from purebred dams instead of using crossbred cows from crossbred dams. The significantly high heterosis and additive parameters in Holstein FriesianམHorro crosses indicate that crossbreeding of Horro with Holstein Friesian may be advisable in areas of Horro breeds occupied.