Sunday, June 14, 2015

Is crossbreeding truly a breeding “method” or just a breeding “option” ?

Crossbreeding has gained momentum in three major areas of dairy decision making:
(1)     use of Jersey sires on virgin Holstein heifers, to insure calving ease at earlier calving ages.
(2)     use in conversion of dairy herds to intensive rotation grazing to improve fertility and vigor.
(3)     use in large confinement herds to improve health, fertility, and feed conversion qualities.

On the last genetic base update by AIPL (USDA), it was noted that “crossbreed” has become the second largest breed on DHIA test, behind Holsteins, ahead of Jerseys and all other pure breeds. This has led to the inclusion of crossbred progeny in their sires’ individual genetic evaluations.

What is different between “pure” breeding and “cross” breeding?

A dumb question, right? A purebred is descended from a single breed sire line. A crossbred has a multiple of breeds in her recent sire stack. But on a molecular (DNA) basis, this is not as big a deal as it sounds—because 85+% of all bovine gene patterns are shared in common among all dairy breeds.

The entire theory of crossbreeding is based on the assumption that less than 15% of the gene total will be different enough to cause extraordinary “heterosis” [hybrid vigor] in such a mating. In a biological sense, the difference between “outcross” (different bloodlines within a breed) and “crossbreeding” (use of mates from another bred) is only a matter of degree.

The interest in crossbreeding stems from dairymen’s observation that over time AI sire lines become more closely interrelated. Fears of “inbreeding” combined with loss of calf and cow vigor and slower, more costly reproduction, adding up to shorter average herd life, led to crossbreeding experiments. 

What measurable effects are seen?

To me, three key research studies tell us how much to rely on crossbreeding:

(Netherlands)  “inbreeding depression” is caused by “single trait selection” (not pedigrees with ancestors in common)
(New Zealand)  “hybrid vigor” is worth 6% on a first cross, 3% on a second cross, 1.5% on a third cross—ie, the more crossbred generations, the level of heterosis declines to an undetectable level.
(Iowa State)  “crossbreed” vigor is good in a first cross, gains a bit on second cross, peaks by the third cross, and then declines—after eight crosses, you have lost all predictability of results.

To state it simply—“crossbreeding” is a short-term fix, but not a substitute for genetic selection.

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