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A comparison of daughters of Artificial Insemination (AI) Holstein bulls demonstrated that the highest ranked bulls for production, health, or reproduction in confinement herds also ranked highest in grazing herds for the same trait. There is some genotype by environment interaction for production traits between confinement and grazing herds in the United States. Little re-ranking of bulls based on their daughter’s performance in the different systems was detected; and genetic progress can be achieved in grazing herds by selecting current active AI bulls for particular traits. However, the selection emphasis that different traits deserve economically in each system may be quite different.
Grazing as a form of low input dairy production to maximize profit is increasing in popularity in the United States. Herds where cows consume mostly grass forage produce, on average, less milk than their confinement. However, it has also been documented that the lower costs of production associated with grazing are more than enough to offset this decrease, and maintain or improve farm. A major concern for graziers is the choice of genetics for optimal performance in pasture-based systems. The primary question is whether a genotype by environment interaction (G×E) exists between the two distinct environments, confinement and grazing. In other words, does one expect those sires whose daughters are producing in both environments, to rank the same genetically in both environments?
Theoretically, such GxE differences exist and result from interactions between genotypes and environments. Under confinement systems, ideal cows produce the most milk with maximal energy inputs. Under sustainable pasture based systems, cows with optimal genotypes produce moderately high levels of milk given somewhat limited inputs. Even cows with the same genetic make-up may produce at different levels when exposed to different environments. Such differences in genetic expression are termed genotype-by-environment interactions.
Reproductive efficiency is important on any dairy, but especially to allow seasonal calving to match cyclic milk production and forage needs with seasonal availability of grass. Reproduction is influenced by level of production. Some graziers are turning to use of foreign genetics or natural service sires, due to a lack of satisfaction or confidence that U.S. genetics are best for their situation. This lack of confidence stems from genetic evaluations being dominated by records from confinement systems, in which production and reproduction breeding objectives may be somewhat different.
Accurate genetic evaluations of animals on a national basis require consistent sire rankings across all environments. Differences in the ranking of sires across different herds, regions, or management systems indicate the presence of interactions between genotype and environment. Understanding these interactions is vital for selecting the best sires for different management systems.
Compelling evidence supports the existence of genotype-by-environment interactions for cattle managed in conventional versus pasture-based systems. First are the genetic correlations between countries that participate in the Interbull international sire evaluation (Banos, 1997). The magnitudes of these genetic correlations indicate the similarity between the sire rankings across countries. Correlations near 1.0 indicate that little or no interactions between genotype and environment exist. However, as correlations decrease, the potential of sire re-ranking increases. The correlation between the United States and Canada is 0.96, indicating that daughters of the same sire will generally perform similarly in each country. Not coincidentally, the production systems in both nations are very similar. Most producers rely on confining cows and providing primarily stored feeds. In fact, dairy producers in most countries around the world also feed their cattle primarily with stored feed.
For most countries, the genetic correlations with the United States and Canada are about 0.90 or greater for milk yield. The obvious exceptions are for New Zealand and Australia. For these countries, the genetic correlations with the US are only 0.76 and 0.81, respectively (Wickham and Banos, 1998). Not surprisingly, dairy production in these areas is primarily pasture-based and widely different from the norm in North America. Moreover, there has been widespread usage of North American dairy bull semen for genetic improvement in Australia and New Zealand, such that low genetic correlations are not likely to arise strictly from divergent gene pools.
Within Australia, evidence of genotype-by-environment interaction has also been reported. Fulkerson (1997) compared the production of cows from genetic lines selected for high and average production when fed only pasture, pasture and a low level of stored feeds, and pasture and a high level of stored feeds. He found that the high genetic line produced more milk than the average line did under all three diets. However, the difference between lines decreased as less stored feed was fed. Cows from the high line also tended to lose more weight during lactation than did the cows from the average line when fed the pasture-based diet. A similar study in Ireland by Dillon et al. (1998) found no significant effects of genotype-by-environment interaction, but the differences among treatment groups in amount of concentrates fed were much less than in the Australian study. Another Irish study by Cromie et al. (1998) compared the genetic correlations among the top and bottom Irish herds for amount of feed concentrate fed per cow. They reported correlations of 0.89 for fat yield, 0.91 for protein yield, and 0.92 for milk yield. Correlations in this range are high, but can result in substantial re-ranking of sire genetic evaluations.
Recently, Cienfuegos-Rivas et al. (1998) estimated the genetic correlation of milk yield between the Northeastern United States and Mexico, searching for evidence of genotype-by-environment interaction. Although management intensive grazing is not a defining feature of the dairy industry in Mexico, one can generally describe the management of dairy herds in Mexico to have lower inputs and to be less intensively managed than are conventional United States herds. The genetic correlation between the U.S. and Mexico was only 0.63, indicating a significant level of interaction among genotypes in the two different environments. Stanton et al. (1991) found a higher genetic correlation between US and Mexican milk yield (0.90), but reported a low correlation (0.78) between U.S. and Colombian milk yield.
Recent studies in the US have examined the effects of genotype-by-environment interactions on milk production (Dimov et al., 1995 and 1996) and somatic cell score (Dimov et al., 1995). These studies examined the effects of the interactions by estimating the proportion of phenotypic variance in each trait accounted for by interaction effects of sires across different herds. Results indicated that less than 5% of the phenotypic variance in the respective traits was accounted for by interactions of sire and herd. However, grazing and confinement herds were pooled in their study and gross effects of such divergent environments may be important.
For the national genetic evaluation, the USDA currently assumes that 14% of the variance in production traits is associated with interaction of sires and herds (Wiggans and VanRaden, 1989). The use of such an inflated value is intentional, to help guard against the bias associated with preferential treatment for sires with many daughters in a single herd. Dimov et al. (1995) concluded that the differences in accuracy of sire estimated breeding values associated with using a realistic rather than inflated value for the variance of interaction effects were minor.
Weigel and Pohlmann (1998) identified 27 herds in Wisconsin in which at least 50% of annual feed intake was obtained from pastures. They used DHI records to compare the performance in these herds of daughters of commercially available sires to the performance of cows in conventionally managed herds. They reported that phenotypically, milk yield was similar in both environments, but that milk fat percentage was lower in the pastured herds. They attributed this to the difficulties faced by graziers in maintaining consistent quality of pasture throughout the growing season. When examining genetic effects, they found that sire PTA's predicted daughter performance in pasture and confinement systems equally well for milk and protein, but not for fat. Because of the limited scope of their study, they recommended that similar work be repeated using more herds, and thus allowing comparison of daughters of more sires with offspring in both management systems (1997, personal communication).
Researchers at the Scottish Agricultural College (Veerkamp, et al., 1995) found similar results when comparing the performance of selected and control lines of dairy cattle when fed two different total mixed rations (neither including pasture) with either high or low levels of concentrate. Phenotypically, no significant effects of interaction were found. However, the regression of actual performance on pedigree index was slightly lower for the cows on the low concentrate diet than on the high concentrate diet. As was the case in the Wisconsin study, fat production was the trait most strongly affected. The genetic correlation between fat yields on the low versus high concentrate diets was only 0.64. That study only considered the effects of different feeds but not overall management system.
These results might seem to suggest that effects of changes in sire rankings between conventional and pasture-based systems are rather small. However, these studies were not all designed to estimate the differences in ranking of sires across conventional and pasture-based systems. When data was taken randomly from national databases with no concern for the specific production system employed for each herd, it is likely that records from pasture-based operations comprised a relatively small proportion of the total records. Also, herds were not grouped according to production system, so any differences in effects common to specific types of herds would have been difficult, if not impossible, to detect. A study such as the one proposed here is needed to accurately determine if effects of genotype-by-environment interaction exist for conventional versus pasture based systems.
Furthermore, all of the studies cited have focused on the traits associated with milk production. Although sales of milk are the primary source of income for dairy producers and milk production is, therefore, the most important genetic trait for all dairy producers; other traits have considerable genetic and economic value. Reproductive traits are among these other traits. Average conception rates have decreased from 66% to less than 50% in the US since the 1950s (Butler and Smith, 1989) despite advances in veterinary technologies. This decrease in conception rate has coincided with substantial genetic gains in average production per cow and, therefore, increased selection for milk production has been implicated in part in this decrease in conception rate. The genetic correlation between production and reproduction is generally in the range of about -0.20. In particular, recent studies have demonstrated that high producing cows tend to have delayed first observed estrus (Harrison et al., 1990, and Senatore et al., 1996). The precise physiological reasons for these effects are not well understood. However, the period of negative energy balance that high producing cows endure in early lactation when their energy intake does not match the energy produced in their milk could evoke a number of consequences that have a detrimental effect on reproductive function (Webb et al., 1998), thus delaying estrus.
Little is known about the effects of genotype-by-environment interaction on reproduction. Although, reproductive traits are clearly of considerable importance to all dairy producers, they are of particular concern to graziers and their interactions with production may be more important. First, many graziers practice seasonal calving to match times of greatest milk production to seasons of greatest pasture yield. In such herds, cows must conceive within a window of 60 days or less, because otherwise the subsequent calving will occur too late in the next growing season. Zartman and Shoemaker (1994) pointed out additional steps necessary on an intensive grazing dairy system to ensure cows successfully conceived in a short window of opportunity. These measures included rather expensive use of weekly veterinary support, milk progesterone assays, and hormonal drug therapy. Routine practices of heat detection and chalking of rumps was intensified. Dairy producers in conventional management systems are not faced with this restriction of a narrow window of opportunity to successfully breed cows.
A second reason reproduction may be more important for pasture-based systems is that even the highest quality pastures may lack the concentration of energy needed to meet the requirements of a high producing dairy cow, increasing the potential for negative energy balance. The relatively high ratio of protein to energy in fresh pasture may exacerbate the problem by increasing the concentrations of nitrogen in the blood, further compromising reproductive efficiency. These effects would effectively increase the magnitude of the antagonistic genetic correlation between production and reproduction in pasture-based herds, requiring an increase in the relative weight on reproduction versus production in the selection program.
The overall aim of this proposed project is to determine genotype-by-environment interactions that may be important when selecting among US dairy sires for the ability of their daughters to produce milk and reproduce efficiently in sustainable pasture-based versus confinement systems. Different genetic profiles may be optimal under each situation. Genotype-by-environment interactions fall into two categories, changes of scaling and changes of ranking. Changes in scaling, often simply reflecting different production levels, can be handled quite easily in genetic evaluation procedures. While changes of scaling affect the magnitude of breeding values (predictors of the genetic transmitting ability of a cow or bull based on its own records and those of its relatives), they do not usually affect the rank order of genetic evaluations. Thus the impact on which animals are chosen in separate environments is minimal. Changes in ranking cause the rank order of breeding values to differ depending upon the environment in which the cows milk weights or other records were produced.
Specific objectives and their rationale were:
1. To test for genotype-by-environment interactions among dairy cows managed under pasture-based versus confinement systems.
Genetic selection is an inexpensive means for dairy producers to make permanent improvements in the average productivity and efficiency of their herds. The Animal Improvement Programs Laboratory (AIPL) of USDA systematically evaluates the dairy cattle population and predicts the breeding values of production traits for all bulls and cows on approved milk recording programs. Rapid genetic improvement for milk, fat, and protein yield has been achieved with the current system, but yield records of daughters producing milk in confinement systems dominate results.
Dairy cattle that perform best while being fed stored feed with high concentrate to forage ratios in confinement may not be best suited for a pasture based system. Indeed, milk production may be slightly different traits genetically under each system. Different genes may affect milk production in each system or the same genes may have different sized effects across production systems.
In the US, little information is available to assist producers in making selection decisions based on cows' genetic ability to reproduce. The genetic ability of cows to reproduce is especially important for seasonal calving, which is a common practice in sustainable grass-based dairy production. Thus, current systems may not identify sires of cattle that perform most efficiently in pasture-based systems. On the other hand, if GxE interactions are not important, then all producers can use the results of the USDA evaluation with confidence. If major effects of interaction exist among environments or traits, then changes to the current evaluation system may be warranted to ensure that cattle, especially bulls used for artificial insemination, can be ranked properly for both conventional and pasture-based systems.
2. To identify differences in the management factors of conventional and pasture-based systems that contribute to genotype-by-environmental interactions.
Confinement dairies usually attempt to maximize profit by maximizing milk production per cow, whereas pasture-based producers are more likely to maximize profit by reducing input costs, while maintaining adequate production. Management factors are likely to differ between the types of operations as certain input and production parameters receive varying levels of emphasis. Confinement and pasture-based herds may differ in factors such as average milk yield, breeding practices, lactation length, age at calving, and cull ratio, to name a few. Degree of variability in these traits may also differ. Accounting for these differences in the statistical models now used for genetic evaluation may help to eliminate the effects of genotype-by-environmental interactions and provide accurate ranking of genotypes under all management systems.
Alternatively, animals may rank the same under both management systems, but the relative economic importance of certain genetic traits may differ under each system. For example, reproductive efficiency of cows and fertility of bulls may be especially important when seasonal calving is practiced. This study will help identify which traits are most important for pasture-based systems of production and provide valuable information to help determine selection goals that are specific to such systems.
3. To educate pasture-based dairy producers about the implications of genotype-by-environment interactions on their sire selection goals and decisions.
Managing cattle and maintaining profitability on a truly low input and sustainable dairy grazing system is complex. The complexity is compounded when the grazier attempts seasonal calving to match grass production to milk production cycles. The only genetic information currently available for US dairy sires is primarily from data originating from confinement dairy production systems. Graziers need to know whether those genetic evaluations of dairy bulls serve as accurate predictors of how the offspring of those bulls will perform under grazing conditions. Hence, they will be the primary target audience for results of this study.
If we find no evidence for genotype-by-environment interactions, graziers will be able to select the best available US genetics, regardless of source, and be confident the resulting heifers will do well. On the other hand, if genotype-by-environment interactions exist, graziers will want to know which bulls perform best under grazing conditions and where to find optimal genetics, either semen or natural service bulls. Providing information to graziers through popular press articles and grazing conferences and by packaging and providing educational materials resulting from this research to dairy professionals who work with graziers will allow them to make wise genetic selection decisions.