Adjusting for Heterosis

Part 3 of a Series

In the current Simmental evaluation, heterosis is controlled by grouping calves according to percent Simmental. Calves from the same herd and management are split further by grouping 50%, 75%, and 87.5% and higher calves separately into different management groups. For the purebred Simbrah evaluation, records from purebred Simbrah calves are used only if their parents range from 3/8ths to 3/4ths Simmental. Only performance records of purebred Simbrah calves are used however, the expected heterosis of the calves can vary by the parents used to produce the purebred calves. By limiting the variation in percentage Simmental and Brahman of the parents, the range in expected heterosis is held under some control.

The Multiple-Breed Evaluation (MBE) system places all calves from the same herd in the same contemporary group as long as they have been managed alike and are the same sex. The restriction on percent Simmental is no longer a part of the contemporary group definition in the MBE analysis. By placing calves with different genetic backgrounds in the same contemporary group, the MBE system estimates breed differences and direct and maternal heterosis effects. For the traits included in the evaluation, heterosis provides an increase in performance that cannot be attibuted to the animal's breeding value.

The primary benefit from crossbreeding is the effect of heterosis on the performance of animals produced by crossing parents of different breeds. For beef cattle, two types of heterosis are important: heterosis expressed in the calfs performance (direct) and heterosis expressed in the crossbred dam (maternal). Heterosis results from the interaction of genes coming from parents of different breeds. The interaction among genes producing heterosis can be classified as dominance, the interaction of genes, or epistasis, the interaction of gene complexes. Studies at Montana State and the USDA indicate that heterosis in beef cattle traits is due mostly to the interaction of genes (dominance). The heterosis effects in the MBE system are proportional to the chance of getting genes from different breeds at a locus. In the MBE system, a direct heterosis effect is included for birth weight, weaning weight, and postweaning gain. A maternal heterosis effect is used for weaning weight.

With 63 different breeds, there are 1,953 different F1 combinations ignoring reciprocal crosses. For the vast majority of these combinations, there are very little if any data from which to estimate these different heterosis effects. To alleviate this problem, breeds were grouped into four classifications: British (B), Continental (C), Zebu (Z), and Other (O). This grouping provides ten combinations: B x B, B x C, ..., O x O. For example, the heterosis expressed by purebred Simbrahs would be represented in the C x Z group since purebred Simbrahs are 62.5% Simmental - 37.5% Brahman.

For each animal in the ASA data base, the fraction of expected heterozygosity is calculated as the product of the breed fractions represented in the parents. The percentage of heterozygosity is expressed relative to the fraction of heterozygosity expected in the F1 calf (100%). As an example, if a breeder mated a purebred Simmental bull that was 15/16ths Simmental and 1/16th Angus to a crossbred cow that was 3/4ths Angus and 1/4th Simmental, the heterozygousity of the calf is calculated as the product of gene fractions from different breeds. Because each parent has Simmental and Angus genes, the fraction of genes coming different breeds will be less than 100% since Simmental or Angus genes from each parent are considered the same for determining heterosis.

Table One:

In this example, the calf would retain 71.9% of the heterozygosity expressed in the F₁ British x Continental cross calf. The expected heterosis of this calf would be .719*h_BC where h_BC is the pounds of heterosis in a F₁ mating of British and Continental breeds. The combination of Simmental genes or Angus genes from both parents contributes nothing to the calfs heterosis since the genes are from the same breed.

A popular mating for producing 1^st generation purebred Simbrah uses a 3/4 SM 1/4 BR sire and a F₁ SM-BR dam. In this mating, the purebred Simbrah calf is 50% heterozygous for Simmental and Brahman genes. (Table 2):

Table Two: 

All the heterozygous gene pairs contain Continental and Zebu genes so the calf retains 50% of the expected Continental x Zebu heterosis.

How much heterosis is retained in a purebred Simbrah calf produced by mating purebred Simbrah parents? A purebred Simbrah calf will have some heterozygous gene pairs on its chromosomes since it is the product of mating parents with Simmental and Brahman genes. To determine the percent heterozygousity in the multi-generation purebred Simbrah calf, consider the example in table 3.

Table Three:

A purebred Simbrah calf retains 46.8% of the heterosis seen in a F₁ Simmental-Brahman calf if the parents are purebred Simbrahs.

Just like the breed of founder effects, prior values are used to help estimate the direct and maternal heterosis effects in the MBE system. The prior values used in the MBE research run and the heterosis solutions are listed in table 4. The priors were estimated from a review of the literature in the same analysis that produced the breed of founder priors. For Birth Weight, Weaning Weight, and Postweaning Gain, you will notice very little difference between the priors and the solutions. The direct heterosis priors were given much more emphasis relative to the data since in previous research runs, negative estimates were obtained for the B x C direct heterosis effect for some traits. For maternal milk heterosis, the data were allowed to have some influence on the final solutions of maternal heterosis. As shown by the difference between the priors and solutions for MMK, the estimates for maternal heterosis were less than the literature values except for the C x Z group and the Z x Z group. The Z x Z group estimate was equal to its prior since there were not any cows that contributed to the Zebu x Zebu maternal heterosis group in the data.

(1) B = British, C = Continental, Z = Zebu, O = Other

How much heterosis can a breeder expect by mating a purebred Simbrah sire to a F₁ Simmental x Angus cow (Table 5)? From this mating, the calf would be 56.25% SM, 25% AN, and 18.75% BR. In the diagram below, the crossbred calf retained 68.9% of the heterozygousity of the F₁ calf. The total heterozygousity of the calf is the sum of the three different heterosis combinations present when its parents are mated: 31.3% B*C + 18.8% C*Z + 18.8% B*Z.  

Tabel 4

The expected direct heterosis for weaning weight from this mating would be .313*(14.6) + .188*(40.3) + .188*(40.9) or 19.8 lb. of added weaning weight due to heterosis. In the MBE, the calf's deviation is adjusted for direct heterosis by subtracting 19.8 lb. from the calf s record. If the adjustment was not made, the added weaning performance due to heterosis would be used to predict the EPDs of the calf and its parents thus biasing the EPDs.

When using records from crossbred calves, the differences in performance are due to differences among the breeds represented in the cross and the effects of heterosis. The calf's performance is influenced by heterosis in two ways: 1) heterosis expressed by the calf (direct) and 2) heterosis expressed by the dam (maternal). Heterosis results from the interaction of genes coming from different breeds represented in the calf's parents. If the calf was a product of mating a Hereford bull to an Angus cow, the calf would 100% heterozygous since every gene pair would have an Angus gene and a Hereford gene. If a calf was produced by mating a PB Simmental bull to a half-blood Simmental-Angus cow, 50% of the calf's gene pairs would be heterozygous for Simmental and Angus genes.

The MBE system calculates the heterozygousity of every calf and its dam in the Simmental data set. Since the data does not allow the expected heterosis to be determined for every F₁ breed combination in the data, breeds are placed into four groups: British, Continental, Zebu and Other. Using these four biological types, ten groups are created to represent the combination of genes coming from different biological types.

As with the breed of founder effects, prior values are used to help estimate direct and maternal heterosis effects. For the direct heterosis effects for birth weight , weaning weight, and postweaning gain, the priors were given much greater emphasis compared to the data. The data were allowed to have greater influence on the estimates for maternal milk heterosis.

For calves of different genetic backgrounds in the same management groups, it is very important to adjust for heterosis differences. The added performance due to heterosis cannot be transmitted to the next generation since it results from the interaction of genes. Since heterosis is not part of the animals breeding value, the calf's record needs to be adjusted for the increased performance due to direct and maternal heterosis. The heterosis adjustment is analogous to the adjustment for age of dam. If differences due to heterosis remained part of the animal's own record, the animal and its parents would be given credit for superiority that would bias the EPDs.