Some countries have routine or required insemination records. Denmark, Finland, Norway and Sweden measure health and fertility traits, as well as provide genetic evaluations and indexes on female fertility. France, Germany, Israel and the Netherlands routinely provide genetic evaluations for sires based on daughter fertility. In Canada, the U.S. and the U.K., there's no requirement to report breedings, disease incidence or other fitness traits. In these countries, the data from which we have to evaluate or calculate genetic parameters aren't very reliable.

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The Genetics of Fertility

The heritability of most cow fertility measures is low-around one to three per cent. This tells us that most reproductive traits are affected by management, environment and non-additive genetics more than by additive genetic variation.

Another possibility is that we haven't defined these traits correctly. Additive genetic variation allows us to make progress through selection. If we are to make any progress genetically, we need either to improve trait definitions or find new traits. Otherwise, we must be content to make very slow progress.

Sweden, for example, has selected against cystic ovaries in its AI population for many years. Even though heritability of cystic ovaries is low, breeders have reduced the incidence of this condition in the Swedish cattle population.

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Inbreeding Hinders Fertility

Inbreeding, or mating related individuals, is known to impair fertility and fitness traits. The closer the relationship between the parents of an individual the greater the inbreeding.

The current level of inbreeding in the dairy population is in the range of two to three per cent. Increased inbreeding will harm all economically important traits. It appears, however, that for every one per cent change in inbreeding, calving interval is expected to change by only 0.25 days-not enough to explain apparent fertility declines of 0.5 per cent to 1.0 per cent per year.

There is, however, good reason for herd owners to be aware of and avoid possible problems with mating closely related individuals in their herds. The heavy use of a few popular sires in the population can lead to mating very close relatives. In most cases, breeders should avoid a mating producing more than 6.25 per cent inbreeding.

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Fertility Linked Negatively to Production

Canada' s rate of genetic gain for production, including milk volume and components, is among the highest in the world. Gains in fat and protein yields directly benefit income.

Recent research in the U .K. and Holland has defined a negative genetic relationship between fertility and milk production.

Jennie Pryce and her colleagues at the Scottish Agricultural College found that genetic correlations between calving interval and milk, fat and protein yields were moderately high: between 0.56 and 0.61. That means higher production has a strong positive genetic relationship to increased calving interval.

In the U.K., Body Condition Score (BCS) is recorded at one month post-calving as part of type classification. BCS has been found to be 28 per cent heritable. Genetic correlations of BCS and the type trait angularity to calving interval were -0.40 and 0.47 respectively. In other words, thinner, more angular cows had longer calving intervals. After adjusting for milk yield, the relation between BCS and calving interval was still -0.22. This suggests the relationship is indeed genetic and not just from high production.

In the Netherlands, researchers looked at the relation between fertility, production, intake and live weight change. They used a relatively new measure of fertility: days from calving to first luteal activity (CLA)-the first cycle after calving-as measured by milk progesterone levels. They found CLA to be moderately heritable at 16 per cent. This is much higher than most other measures of reproductive performance. CLA was found to be negatively related to milk, fat and protein yields. High yields were associated with delayed CLA.

Energy balance, live weight during lactation and weight gain all had a positive relation to a reduced CLA. Feed intake, adjusted for milk yield, had a large relation to CLA: high intake was genetically related to reduced CLA.

Researchers concluded it would be possible to combine information on protein yield and energy balance into a selection index that would allow up to 80 per cent of possible gains in milk protein yield without losing fertility.

The interesting aspect of the Dutch research and the work done in the U .K. is that the scientists looked at more than the relationship between production and fertility. They also considered the factors that might affect reproduction -cows' ability to support high production such as energy balance, feed intake and BCS -and how the industry might use these traits in selection.

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Selection Indexes Show Promise

The negative genetic relationships between milk, fat and protein yields and fertility tell us that continued selection for production will reduce fertility over time. The low heritability of reproductive traits dictates that we wouldn't want to select for fertility at the expense of productive traits. However, including a measure of fertility in a selection index such as the LPI or TEV could prevent losses in fertility without seriously detracting from genetic progress for production.

The U.K has developed an index using BCS, calving interval and angularity. Other countries have used an index with a weighting on calving interval or non-returns to service. These measures depend on accurate on-farm reporting of all breeding dates, pregnancy diagnosis, and so on.

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Heifers are Different

Heifers have high fertility and tend to express fertility traits differently from lactating cows. There's a difference in the reproductive function of a non-lactating heifer versus the physiological response of cows in lactation to body weight loss and the demands of high production. The trends in fertility related to production and energy balance probably don't apply to heifers.

New measures, such as first luteal activity (CLA) as reported in Holland, hold some promise for a more accurate fertility measure.

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Tips For Your Farm

You can apply this information on your farm:

• Short-term solutions to improve reproduction within your herd involve managing the status of lactating cows with respect to body condition and energy balance in early lactation.

• There's a genetic component and interaction among production traits and weight loss in early lactation, feed intake, BCS and poorer reproduction.

• You can avoid high levels of inbreeding on individual matings by checking relationships of family members or using a mating program with inbreeding constraints.

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Ideas for the Industry

This research also raises some ideas for the Canadian genetic improvement industry to consider:

• We need to look at the data collected in our milk recording system to know how many herds already record complete breeding, pregnancy checks and calving data. This information, if sufficient in volume, could be useful in identifying genetic fertility values.

• We should consider collecting BCS information in early lactation as is done in the U,K

• The use of a selection index for the dairy industry with a fertility component may prevent further decline in fertility without sacrificing a lot in production.

• We need to work on practical measures of fertility in the Canadian system that will give values we can use in selection. .

This article appeared in the March 2001 Ruminations column of the Ontario Milk Producer.

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