librarian said:
I have been told that a 3 way cross yields maximum heterosis. I'm not convinced because of the volatility.
What are the "it depends" that factor in?
The question concerns F1 cross cows and the optimal way to breed them.
It's somewhat of a loaded question. There are a few considerations and with those considerations come tradeoffs- The most important consideration is how can we, or at what cross can we, maximize heterosis while simultaneously maximizing consistency. We know these pursuits are antagonistic so any attempt to maximize both is going to be unsuccessful. I think where the 3 way cross comes in is that its been deemed the point at which these two pursuits intersect. That is, the point at which heterosis and consistency are OPTIMIZED.
Here we have to break heterosis down into 2 categories. Maternal heterosis and expressed heterosis. Maternal heterosis is maximized with the F1 female. This is a different phenomenon altogether than experienced with expressed heterosis in a calf. With maternal heterosis, this synergistic phenomenon diminishes/becomes diluted when additional (more than one) crosses are added to the makeup of the cow. "THERE IS NO SUBSTITUE FOR THE F1 FEMALE". With expressed heterosis, this is not the case.
With expressed heterosis, as observed in a resulting calf, the more crosses = the higher the degree of heterosis. The resulting progeny of an F1 Hereford x Angus cow bred to a Charolais bull, for example, will have less expressed heterosis than the resulting progeny of an F1 Hereford x Angus cow bred to a F1 Charolais x Brahman bull. The tradeoff being that, because double cross hybrids (F1 x F1) are so inconsistent,
the value of the added heterosis is more than offset by the reduction in value from lack of consistency.
In general, as long as we're outcrossing to
different breeds, the level of heterosis will continue to increase... but always at the expense of consistency and predictability.
The flip side to heterosis, is breed regression. Breed regression is the inevitable result of backcrossing. An example of backcrossing is breeding those Angus x Hereford cows back to Angus or Herefords bulls. As opposed to the synergistic effect of heterosis, which increases all measurables, breed regression results in depression which will reduce performance across the board.
