Management of Effective Mating and Crossbreeding Systems Chapter

Management of Effective Mating and Crossbreeding Systems Chapter 27

Phenotype = Genotype + Environment n Phenotype is the level of performance (quantification) of any trait n Genotype has two components: q Breeding Value – singular effect of individual genes q Gene Combination Value – effect of gene pairs at a locus

Selection & Mating Systems n Selection is the process of deciding which animals become parents. Effective selection is based choosing animals with the best Breeding Value. n Mating is deciding which bulls will be bred to which females. Mating decisions determine Gene Combination Value. Mating decisions involve the choices we make to crossbreed or inbreed.

Effects of Inbreeding n An increase in homozygosity results in: q An increase in the number of homozygous loci… both dominant and recessive q An increase in the frequency of homozygosity results in more prepotent breeding stock n Deleterious (often lethal) recessive alleles often surface q Inbreeding does not create deleterious recessive alleles, but rather allows them to be seen q Each animal likely has between 5 and 20 of these mutationswhen we inbreed, we’re more likely to see these defects

Measurement of Inbreeding coefficient (F) % increase in homozygosity above the average of the breed… If F =. 25 … 25% of the animal’s genome is estimated to be identical by descent

Inbreeding Depression

Breed Formation n we inbreed to form breeds! n we form breeds to reduce genetic variation n we form breeds to “fix” favorable alleles and run the risk of fixing unfavorable alleles

Genetic Management q Avoidance of inbreeding n Alleviate incidence of genetic defects n Likely improve performance q Crossbreeding: breed complementarity and heterosis

Genetic Management q Purebred Seedstock operations have a responsibility to improve breeding value in traits of economic importance. This responsibility may involve some inbreeding. q Commercial Beef Operations can crossbreed to maximize breed complementarity and hybrid vigor (Gene Combination Value). q Effective Crossbreeding Systems require purebred animals.

Benefits of Crossbreeding n Heterosis (Hybrid Vigor) Individual (expressed in the calf) Maternal (expressed in the cow) Paternal (expressed in the bull) Breed Complementarity combine breeds the strengths of the parent

Heritability and Heterosis Inverse Relationship We can make changes in lowly heritable traits QUICKLY by crossbreeding Traits associated with reproduction have 5 x more $$ to commercial beef production than those associated with growth or milk (Trenkle & Willham, 1977)

Calculating Heterosis

Heterosis vs. Breed Complementarity Heterosis

Maternal Heterosis n If you only take advantage of heterosis in one place, do it in the cows! Heterosis largely impacts fertility and longevitythese gains are not at the expense of much larger cows and higher feed costs!

Types of Mating Systems n Terminal q Maximum use of breed complementarity q No replacement females retained-all progeny should be sold n Rotational q Replacement females raised within the system q Females rotated between pastures n Combination Systems q Combine rotational and terminal systems into one enterprise

Crossbreeding Systems

2 -Breed Rotation n Crisscross, 67% retained heterosis n 2 breeding pastures q Use of AI can decrease this somewhat q Min. 25 cows in each pasture (50 cows) n Must ID breed of sire to ensure proper matings q Ear tags i. e. red vs. blue q Red sired by A, bred to B q Blue sired by B, bred to A

3 -Breed Rotation n 86% retained heterosis n 3 breeding pastures q Min. 25 cows in each pasture (75 cows) n Must ID breed of sire to ensure proper matings

2 -Breed Roto-terminal n 67% maternal heterosis, 90% calf heterosis n 3 breeding pastures q Min. 100 cows n Must ID breed of sire in rotational part n ½ herd: A, B maternal traits n ½ herd: C Terminal, curvebender n More management-intensive, more heterosis

Terminal Cross on F 1 Females n 100% maternal heterosis, 100% calf heterosis n Simple! n Highest usage of heterosis n All calves marketed have same breed composition and are more uniform n Purchasing vs. raising q Rule of thumb: 50 head

2 Breed Terminal Sire 2 -breed terminal sire A n 0% maternal heterosis n 100% calf heterosis n Females must be purchased n Terminal only

Rotate Bull n Retained heterosis dependent on how many replacements from which year q Averages ~50% retained heterosis q 3 breeds: 67% n Useful in small herds with minimal management and labor n Replacements only kept during first 2 years q Will accumulate inbreeding unless get new bull after 2 yrs. q Less efficient use of bull as capital investment If starting with straightbred cows, first breed of sire should be used for 5 calf crops

Composites n Simple! q Manage as straightbreds q No minimum size (unless forming own composite) n Can use 2, 3 or 4 breeds (50%, 67%, and 75% retained heterosis) n Increasingly common and availability of a variety of genetic options is available q Sim. Angus, Balancer, Lim. Flex, Maine. Tainer q MARCI, MARCIII, Stabilizer, + others

Rotating F 1 Bulls n Much like composites, but greater opportunity to incorporate superior genetics q Populations are bigger q Less inbreeding n Strategy 1: Same breeds in cows and bulls q 50% Retained Heterosis

Rotating F 1 Bulls n Strategy 2: 1 breed in common between bulls and females q 67% retained heterosis

Rotating F 1 Bulls n Strategy 3: No breeds in common between bulls and females q 83% retained heterosis q Nearly equivalent to 3 breed rotational, but much easier to manage and implement

Tips for Crossbreeding n Variability can be increased in the beginning q Use breeds with similar coloring (and maybe same characteristics) q Make sure animals fit your environment and market targets n May need to have terminal system to hit market targets and maintain cows that fit environment q Takes time to establish retained heterosis-does not happen overnight n Pick a plan and stick with it!!!

Questions?