Balancing Act - Research Shows We are Sacrificing Fertility for Production Traits

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We've always believed that the difference between good and poor reproduction could be attributed to management. We're now starting to see evidence that dairy cow fertility is declining genetically. Several studies have found reproduction is negatively related to production level, body condition score, angularity and body size. The current belief is that selection especially single trait selection for production has reduced cow fertility.

A recent paper summarizing reproduction trends in Ohio Dairy Herd Improvement (DHI) herds, for example, concludes the state's overall reproductive performance declined during 1990s. Calving interval in creased by 15 days during that time. The days from calving to conception were similar, the number of services per conception increased and first service conception rate declined from 50 per cent to 47 per cent.

There were, however, some contradictions in the Ohio data. Calving interval (CI) was seven days longer in low herds versus high production herds. There were also some differences according to size of herd; CI was slightly longer in small and mid sized herds versus large herds. Large herds had slightly poorer conception but tended to breed their cows earlier.

Calving intervals reported by Ontario DHI in Ontario and Quebec's milk recording organization, Programme d'Analyse des Troupeaux Laitiers du Quebec (PATLQ), reveal similar trends over the past 10 years. CI has increased about two days per year (see table).

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At the University of Guelph, Dr. Larry Schaeffer and co-workers have started a three year study to look at the genetic aspects of reproductive performance in Canadian dairy cattle. This study means Canadian dairy farmers will start to reap the benefits of integrating the national milk recording system, Vision 2000 and the data exchange system. Researchers like Schaeffer can now connect information needed to analyse cow reproductive performance, although only data on breedings from 1998 onward are available for analysis.

Schaeffer's preliminary findings include the age at heifer's first service improving to 504 days (16.5 months) in 2002 from 509 days (16.7 months) in 1998. Heifers have better reproductive performance than lactating cows. First service conception rate (CR) and CR after 56 days are higher for heifers as a group. Number of services per conception and interval from first service to conception are lower. Measures of fertility tend to get poorer according to lactation number with slight decreases from first to second to third lactation.

Similar to the Ohio study, preliminary Guelph data suggest a slight decrease per year in first service conception rate and conception rate after 56 days. Number of services to conception increased, as did interval from first service to conception from 1998 to 2000. The interval from calving to first service, and interval from calving to conception and days open show no obvious direction.

The voluntary waiting period makes up a great part of these measures. This is a result of owners' decisions and may reflect changes in herd management priorities rather than changes in cow fertility.

A study by Brian Van Doormaal, Canadian Dairy Network (CDN), of Canadian 56-day non-return rates (NRR) the measure used to evaluate artificial insemination bull fertility looked at cow factors such as age, lactation number, season and the year. The average rate of successful first inseminations for virgin heifers was 79.3 per cent (56-day NRR). That's 12 per cent higher than the rate for milking two-year-olds, which averaged 67.2 per cent. Fertility declined slightly with age every year from two to nine years of age. There was a small drop in NNRs over time to 67 per cent from 68 per cent.

Canadian data seem to support the belief that dairy cow fertility is declining, although maybe not as severely as in the U.S. It's obvious enough, however, that we should be concerned and should support further analysis to better identify genetic factors and relationships.

Reproduction is a complex function. Many factors such as environment, nutrition, disease status, timing, fertility of the mate and genetics affect success. Modern dairy herd management is leading to higher production and, in many herds, a longer voluntary waiting period after calving before cows are bred back. Basically longer lactations fit the management scheme with high production cows. This gives the appearance of poorer fertility. We need to sort out the difference between intentional management strategies and costly losses due to health, nutrition or genetic factors.

Reproductive traits tend to have a low heritability. This factor and our inability to measure reproductive traits very well means we've focused on nutrition and management to improve or at least maintain reproductive performance. Results of the Guelph and CDN studies show heifer fertility to be much higher than that of lactating cows. This could lead you to conclude there's a lot to be gained from managing lactating cows better and compensating for the demands of high production. That's not to say that we can ignore genetic factors and compensate with optimum nutrition and management forever as DHI record analysis is now starting to show.

Several U.S. and United Kingdom studies show production has a negative genetic correlation with reproductive performance. As we've made continued genetic gain in production traits, fertility has declined. It's suggested that for every 1,000 kilogram increase in milk production, average days open go up by eight days. The goal of genetic selection should be a balance of optimum gains in production without losing fertility.

The level of inbreeding needs to be considered too. As it increases, there's a negative or downward effect on performance especially on reproduction and fitness traits. Some breeders see crossbreeding as the solution to this problem and it probably could benefit individual herds. However, if fertility is negatively correlated with production, and production is the single most important income factor in dairy herds, this conflict needs to be resolved. As the production level of crossbreds inevitably rises, reduced fertility will become a factor there as well.

The decline of dairy cow fertility may not be as catastrophic as some suggest. Management factors in today's modern dairy operations may contribute to longer calving intervals. Nevertheless, with studies strongly suggesting a decline in dairy cow fertility, our industry needs to address this issue.

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Selected references:

Abdallilh, JM. and B.7: McDaniel. 2000. Genetic parameters and trends of milk, fat, days open, and body weight after calving in North Carolina experimental herds. J Dairy Science 83: 1364-1370.

Berry, D.P., R Buckley, P. Dillon, R.D. Evans, M. Rath, and RF Veerkamp. 2003. Genetic relationships among body condition score, body weight milk yield and fertility in dairy cows. J Dairy Science 86:2193-2204.

Rajaln-Schulz, PJ and G.S. Frazer: 2002. Reproductive performance in Ohio dairy herds in the 1990s. Animal Prod Science 76:127142. Van Doormaal B. 2002. Effect of age and month of insemination on fertility. Canadian Dairy Network.

How We Can Improve Reproductive Performance

As an industry, we can take several steps to address the reproductive performance conundrum. For instance, we should look more seriously at routine body condition scoring (BCS) and including it in selection indexes.

BCS has been shown to be correlated (0.29 to 0.42) with improved fertility. It's a possible factor to be included in a selection index to maintain or improve reproduction. It's easily measured, has herd management benefits and has a medium correlation with fertility. The U.S. is starting to measure BCS routinely and the UK has already included it in a selection index.

We also need to explore other innovative measures. Milk progesterone tests, for example, would provide a measurable factor we can use. Some preliminary results from Dr. Scheaffer's project at the University of Guelph suggest promise that his project using a multi-trait analysis will help define the reproductive traits we can work with to improve reproductive performance through genetic selection. If we understand some of the parameters, we can consider breeding strategies to maintain fertility levels.

Having a complete and accurate dataset of breedings and reproductive events for every herd is crucial to getting meaningful results and a genetic strategy in which we can have confidence. Because reproductive traits have low heritabilities, we need more information than with production traits. Once the information is entered in the DHI system, then and only then is it useful to researchers such as Scheaffer's group.

Individual herd owners can get involved too. Here are some suggestions:

• record all breedings and reproductive events. Analyse your records. Differentiate between management strategy and costly problem areas;

• keep good reproductive records on a herd basis so researchers can obtain information on genetic values for fertility traits;

• monitor the inbreeding coefficient of potential matings in your herd. As the relationship of the parents increases, the fitness and survival traits tend to decrease; use health and nutrition programs to optimize your herd's reproductive performance.

I maintain that Canadian dairy breeders voluntarily keep the world's best records. This may be more important than ever before as our industry tries to unlock the puzzle of inheritance of fertility.

This article appeared in the August 2003 ruminations column of the Ontario Milk Producer magazine.

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