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Select cows based on natural fertility and adjusted inter-calving period.

Alan Newport, Editor, Beef Producer

November 11, 2020

8 Min Read
Masculine bull with heifers
Selecting the right heifers helps select the right bulls, and vice versa.Alan Newport

Editor’s note: This article first ran in Beef Producer’s print edition in 2015 but was lost in cyberspace.

Johann Zietsman from Zimbabwe redefined bull selection in the article Selection Made Simple: Pick The 8-in-5 Package! for Beef Producer. Now, in this story he describes his system for selecting cows.

It's a method that typically moves cows forward into the best calving window – as the cows themselves define it in your environment – and then scores them for fertility as second-calf heifers, based upon which ones return to heat cycling most quickly after calving.

Although conventional wisdom says reproductive traits are lowly heritable, Zietsman effectively makes the case that fertility, as he defines it, is highly heritable and that is well proven by nature. (See section on fertility inheritance.)

He says it is the type of cattle we have chosen which cause our problems. They are, by all selection pressures we put on them, late maturing and resource-demanding. Although standard practices argue that increasing supplementation, decreasing stocking rate and improving parasite control will improve the body condition of almost any herd, Zietsman says body condition attained through feeding is neither heritable nor profitable.

Zietsman, who helped pioneer ultra-high-density grazing methods, always advocates cattle selected for range conditions with minimal supplementation.

Although Zietsman advocates using a 12-month maturity ranking in bulls as a percentage of their mature size, he says in heifers that is not an essential selection criterion because maturity rate will be reflected in 14-15 month conception.

He says in order to select effectively we need to be able to identify the individual fertility of each cow. In turn, that will let us choose which cows from which we'll keep bulls and replacement heifers.

First, you will demand all your first-calf heifers breed at 14-15 months. Ideally this would be done under planned grazing, including some high-stock-density grazing.

Then you will re-breed them to calve as second-calf heifers in that favored timeframe and rank them for fertility. Again, any which don't breed will have their genetics removed from the gene pool.

Fundamentally, we're looking for the young cows that return to estrus most quickly within their contemporary group, in a given year. We need rankings so we can choose the most fertile heifer calves under our conditions.

The tool Zietsman recommends is an adjusted calving interval, which he calls adjusted inter-calving period (ICP). Without adjustment, calving interval really measures the effect of environment as much or more than it measures genetic material. But when second-calf heifers are ranked among their peers under similar conditions in the same year, you have a much more useful tool.

Zietsman says, "Corrected ICP in a contemporary group of these heifers will largely reflect genetic differences in body condition and hormonal balance and as such will be a fair indicator of genetically determined fertility."

To create this index, Zietsman says to group the 3-year-olds into three or four groups to adjust for seasonal variations. He divides the individual ICP for each animal into the average for the time-group she is in and assigns her a fertility score based upon that. (See section on fertility scoring.)

Eliminate environment

Even better is to move all calving into the ideal timeframe for your area, Zietsman says. This virtually eliminates variations caused by nutritional issues and/or feeding. Then you are choosing almost entirely upon genetic reproductivity.

In his home country of Zimbabwe the calendar is opposite of our northern hemisphere conditions. There, Zietsman quotes multiple research studies favoring the rainy season and/or growing season of summer, or December-January, when the day length is longest and the forage quality is highest. In North America this would be around the summer solstice of June 21.

In the U.S. the late nutritionist Dick Diven taught the same things about day length, forage quality and reproductivity. Hence, calving should be in summer to make equal the nutrition for all cows and give them the shortest biological/solar-influenced return to estrus.

If you are calving this much in synch with nature there is a more effective tool than adjusted ICP, Zietsman says. He says when cows calve in very good condition in the ideal nutritional environment some can start cycling 20 days after calving. Some could cycle several times before the bull is turned in so this invalidates calving interval as a measuring tool.

Instead, you should use corrected post-partum anestrous period (corrected PPAP), which is an adjusted ranking of days from calving until first estrous cycle.

It is calculated very similarly to Zietsman's "corrected ICP," except the cows are checked for heat after calving in order to record individual anoestrous period (PPAP). Then this number is corrected for influence of date of calving.

Zietsman says in well-managed herds PPAP will become more important than ICP.

He adds if it is not feasible for you to use heat checking to record PPAP, an alternative would be to extend the second breeding season by introducing bulls immediately after calving commences until the end of the breeding season and calculate fertility from those pregnancies.

Zietsman surmises: "A cow that has calved at 2 and 3 years on veld (pasture), with limited supplementation and with a top fertility score for corrected ICP and/or PPAP must have high inherent body condition and a desirable hormonal balance.

"If mated to the appropriate bull all her progeny, including males, will have a high inherent fertility. The first step in bull selection is to identify such cows and select young bulls from them."

This fertility is heritable

Johann Zietsman agrees that body condition is the primary determinant in reproduction.

However, he says that body condition and the key factors that improve it are highly heritable. That’s because its key components are efficient grass conversion (high relative intake), good body condition, a desirable hormonal balance, optimum milk, a high meat-to-bone ratio and easy-care characteristics. All these things effect a high rate of reproduction.

These traits are highly heritable, Zietsman says, therefore reproduction is highly heritable.

It's important to note, Zietsman says, that current measures of fertility such as culling non-pregnant cows, scrotal circumference, calving interval and calving tempo are poor measures of the genetic variation of the components of practical fertility. Instead, it is determined by hormonal balance and inherent body condition.

He says creating grass-efficient and highly reproductive animals is one of two important factors which help reach the goal of maximum sustainable profit per acre. The other is high-density planned grazing and a result of a sustainable, high stocking rate.

Fertility score 3-year-olds

This chart shows an example of how Johann Zietsman calculates cow fertility scores.


He says the line on the graph is essentially a regression curve representing the statistical decrease in anestrous period and possibly in calving interval as cows calve closer to the middle of the rainy season and therefore to good nutrition. Zietsman says over a conventional 90-day calving season this equates to a 60-70 day shorter inter-calving period for a cow calving at the end of the calving season as opposed to the beginning.

Conversely, the farther a cow's calving time is from the ideal time in any environment, he says, the longer will be her calving interval, or her inter-calving period (ICP).

In this example, the cows labeled A, B and C are calving the furthest into the dry season and so have the longest average ICP of 407 days. Cow D is in a mid-point group with an average ICP of 335. Cow E is in the best timeframe to calve and therefore in the shortest average ICP group of 325 days. Cow F is in the group with the longest ICP of 371 days.

Zietsman then divides each 3-year-old cow's individual ICP by the average for the timeframe she calves in, relative to the regression curve, to get a fertility index. That number is in the third column. Mathematically this is not unlike adjusting weaning weights for age.

This is where it gets interesting. By correcting ICP we now see that cow A, even though she had the longest individual ICP, is a top performer in the environment in which she is calving. Cow E, which was calving in the best time of year and had the shortest individual ICP, was only at a midpoint in her reproductive index. Cow F, which calved with nearly the shortest ICP of the bunch, was actually a poor performer when indexed against the regression curve for the ideal time in which she calved.

Zietsman then assigns a fertility score of 1 to 10 for each of these indexes to make sorting and selection a little easier.

This same ranking system can be used with corrected post-partum anestrous period (PPAP) for 3-year-olds calving in the ideal time of year. Zietsman describes these processes and reasoning at greater length in his book, Man, Cattle and Veld.

Maintaining herd while breeding up

Zietsman says his goal of breeding heifers as yearlings in order to calve at two years of age under range conditions is a tough one, but there are ways to keep cash flow while putting pressure on cattle to do this.

"A starting point would be to at least allow individual heifers the chance to prove their ability to conceive at a young age by breeding all heifers at 14 to 15 months," he says. "Those that do calve at the age of two years and conceive again for their second calf at three years should become part of a nucleus bull-producing herd. Those which do not you can bred at a later stage in order to maintain herd size."

Newer is better

Once Johann Zietsman's system for selection is instituted in a herd, genetic turnover should be rapid to capture the improved reproductive and true performance traits.

Zietsman says, "As soon as a bull produces breeding-age sons (15 months) he is obsolete. Either he is being used extensively in the herd via A.I. or he is at the point of being replaced by the younger generation. Either way his services are no longer required. To a lesser extent the same applies to the older-generation cows waiting to be replaced by the younger generation. The quicker one generation can be replaced by the next generation, the better -- assuming appropriate selection."

About the Author(s)

Alan Newport

Editor, Beef Producer

Alan Newport is editor of Beef Producer, a national magazine with editorial content specifically targeted at beef production for Farm Progress’s 17 state and regional farm publications. Beef Producer appears as an insert in these magazines for readers with 50 head or more of beef cattle. Newport lives in north-central Oklahoma and travels the U.S. to meet producers and to chase down the latest and best information about the beef industry.

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