Wednesday, October 1, 2014

Do we really grasp what “calving ease” data is telling us?

Dairymen are split somewhat equally on what “calving ease” data means to them.    Some will only use “calving ease” sires, in hopes of avoiding heifer or cow paralysis from oversize bull calves.   Others will avoid using sires with really low “calving ease” %s in a belief that such bulls make small, harder calving cows, perpetuating the problems into the future.

Countries do not agree on how to collect or present the existing data.   In Canada, calving ease is called “percent of unassisted births”—an easy-calving bull is above 87% unassisted births, a hard calving bull is below 84% unassisted.      In the USA, calving ease is called “percent of difficult births”—an easier calving bull is 8% or below, average is 9%, a harder calving bull is 10% or above.     Researchers have found that length of gestation affects calving difficulty, with bulls that sire a shorter gestation length (eg, 6H999 Cole RC, whose c/e is 5% currently) being considered more desirable.

Should you be concerned with “calving ease”?

First—this is more of a Holstein/Brown Swiss issue than a concern of Jersey, Guernsey, Ayrshire, or Shorthorn breeders, where calving ability is a noted breed advantage.    Brown Swiss have the longest gestation length of the major dairy breeds, with Holstein next—so when a calf runs over term in those breeds, calving assistance is expected.      But the Holstein cow tends to produce larger calves than any other cow, regardless of breed of service sire—roughly 7% of mature body weight (Jerseys run about 5% of mature body weight).      So breed differences are a genetic influence on calving ease.

But within breeds, more recent research notes that “percentage of stillborn calves” has also been found to be heritable—a situation where cow fertility genetics overlaps with calving ability genetics.   So the calving ease figures you now read for bulls are 70% “calving difficulty” (a subjective measure) and 30% “stillbirth frequency” (an objective measure) rather than a pure “percent of difficulty”.    Geneticists are assuming that the goal of calving is not just a surviving cow but also a live calf—a better assumption.

Heifers being fed “hot” (high energy) rations in the last trimester of gestation will produce bigger calves given the nutrient diversion to the calf to complete its development.    Much calving difficulty is induced by feeding heifers like steers, prompting weight gain rather than frame development— so what sires you breed them to will have less success in avoiding difficult calvings under “steer feed” nutrition (that pack fat deposits into the pelvic cavity, the udder, and around internal body organs).

Sometimes type selection preferences interfere with calving ease.    The wide, flat “boxcar rump” that is a traditional Holstein standard, finds less favor in most other breeds, who recognize their calving ability comes in part from a “diamond shape” to the birthing channel.    You need thurl and pin width to get the hips out, but first you need pelvic height to get the head and shoulders started.    Both dimensions need to remain in proportion for minimal pelvic interference in birthing.     [This is one of the key areas of the cow physique improved by using the “aAa” breeding guide, instead of linear mating systems.]

Management pressure to breed heifers at earlier ages (to compete with the faster maturity of smaller frame breeds like Jerseys) has produced more heifer calving difficulty.   Heifers are not ready to breed until they are 55% of their expected mature weight.   Thus, if you like 1600 pound cows, do not breed heifers until they weigh 880 pounds—the blanket Holstein recommendation of 700 pounds is assuming you still milk the 1350 pound short-legged smoothies University dairies had in the 1960s.

Genetic   tradeoffs  in  calving ease

Most sire proof lists only show direct “calving ease”—the bull’s ranking as a service sire for birthing.
But in fact, two levels of data are calculated—“direct” and “maternal” calving ease.    The “maternal” rating is an indication of how his daughters are doing for birthing ability.

Exceptions to the assumed rules may prove the rule is defective

What you see across the broad data is a pattern—the lower the “calving ease” direct rating, the higher the “calving ease” maternal rating.    The old saw, “calving ease bulls produce small hard calving cows” is questionable, on the average of the data.    But who wants to milk “average” cows??    The secret is to find “calving ease” bulls whose daughters also calve easily, and produce live calves, for a full lifetime.        

The scoop on calving ease

Calving difficulties have led people to various strategies (breed heifers to Jersey bulls, breed heifers to sexed semen, use only calving ease rated sires) that seem to either raise our AI costs or lower our herd equity value.     Why did this happen?      It relates to the delayed recognition that calving ease was as “genetic” a trait as production or type, and is linked to broader health and fertility issues—such as the rate of still born calves—currently averaging 8% of all Holstein births-- as well as to structural mating issues in the width, length, and proportions of the pelvic rump shape.

One thing for sure—the data does not support some old-timer attitudes like “select for calving ease and you end up with small, narrow, frail cows”.     Increasingly there is recognition that “frailty” is a result of broader selection issues, such as the importance of balancing “sharp” and “round” body qualities in the overall [bull x cow] mating design.     [Programs like aAa breeding guide produce heifers that are more capable of calving unassisted with a living calf—selecting sires to avoid the genetic sources of a lack of vigor in calving, such as using maternal stillbirth rates, add to success in this area.]

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