Optimum Genetic Selection of Cattle for Pasture-Based Dairies

Optimum Genetic Selection of Cattle for Pasture-Based Dairies

Summary

Pasture-based dairying, especially when utilizing rotational or management intensive grazing, is an option for low-input sustainable dairy production in the U.S. There has been little effort to define optimum genetic selection criteria for cattle in pasture-based dairy production systems. The overall aim of this project is to determine genetic differences among U.S. dairy sires as measured by the ability of their daughters to produce milk and reproduce efficiently in sustainable pasture-based versus confinement systems. Theoretically, such differences result from interactions between genotypes and environments.

The specific objectives of this proposed project are to:

1) Test for the existence of genotype-by-environment interactions among production and reproduction characteristics of dairy cows managed under confinement systems versus pasture-based systems;

2) Identify differences in management factors of conventional and pasture based systems that contribute to genotype-by-environment interactions; and

3) Educate pasture-based dairy producers about the implications of genotype-by-environment interactions on their sire selection goals and decisions.

Under confinement systems, ideal cows produce the most milk with maximal energy inputs. Under sustainable, pasture-based systems, profitable cows produce moderate levels of milk given somewhat limited energy inputs. While records of cows from confinement systems dominate current genetic evaluations, different genetic profiles may be optimal under each situation. For example, reproductive efficiency of dairy cows is especially important to allow seasonal calving to match seasonal availability of grass on pasture-based dairy farms with cyclic milk production and forage needs. Understanding genotype-by-environment interactions is vital to the accurate identification of genetically superior bulls and cows in each environment.

Differences in certain management parameters, such as herd size, age and month at which cows calve, and geographic location, will be identified. The effects of any differences will be accounted for in genetic evaluations, and the impact on the relative ranking of individual bulls will be examined. Detailing management differences between confinement and pasture-based dairy production systems will provide insight into the mechanisms from which genotype-by-environmental interactions arise. Ultimately, this work will determine whether the genetic evaluations as currently published are adequate predictors of genetic ability of cows to perform in pasture-based systems in the U.S. The same cattle may not be best suited to both confined and pasture-based systems.

Little data exists to allow graziers to select the best dairy artificial insemination sires for a specific environment. Confirmation that genotype-by-environment interactions exist and influence the genetic selection of daughters to be raised in confinement versus pasture-based dairy operations will allow calculation of more accurate and specific genetic evaluations of dairy cattle targeted for sustainable dairy systems. Isolation of management factors that manifest themselves in genotype-by-environment interaction would allow calculation of more accurate genetic evaluations after accounting for such management factors. Increased ability to select animals best suited under grazing or confinement will increase dairy farm profitability and improve the longevity and well being of cows through better adaptability to environmental conditions.

On the other hand, determination that genotype-by-environment effects do not exist between dairy production systems allows pasture-based producers to broaden their sire selection choices. They could then select the best dairy bull semen that is already available. This will reduce semen costs, which may be especially important under low input dairy systems.

Identification of grazing dairy herds that are enrolled in national testing schemes has been difficult. Often, testing programs are seen as too expensive under the goals of low-input dairy production. Nevertheless, Dairy Herd Improvement Association records of 353 grazing herds in 15 U.S. states and 2 Canadian provinces have been assembled. Initial analysis has identified that genetic correlations were lower when comparing production traits across environments that when comparing within environments. This is evidence that suggests bulls’ daughters may indeed have differing genetic ability to produce milk under confinement or grazing conditions.

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