There are over 3.5 million milk cows in the Western United States, making dairy one of the dominant sectors of western agriculture. Organic milk production is the fastest growing segment of U.S. organic agriculture and as a result there has been an increase in pasture-based milk production. To meet this increasing demand, improved grass-legume pastures that require fewer inputs, have high forage production and nutritive value, improve ruminant utilization of nitrogen, and have high dry matter intake are critical to the economic viability of pasture-based organic dairies. While grazing has many benefits, it may accelerate nutrient cycling and potentially increase nitrate leaching along with being a significant contributor of ammonia (NH3) and greenhouse gases (GHG). Dietary changes can impact emissions. This study examines the effect of condensed tannins (CT) on nutrient cycling in the grass-legume versus grass monoculture grazing systems. The nitrogen content in urine and feces of cattle grazing forages with, and without CT, was also examined and compared to a traditional TMR diet.
What did we do?
Four grasses, with and without the addition of a tannin-containing legume, birdsfoot trefoil (Lotus corniculatus), are examined in this study. The treatments include tall fescue (Lolium arundinaceum); meadow bromegrass (Bromus biebersteinii); orchardgrass (Dactylis glomerata); perennial ryegrass (Lolium perenne) planted as monocultures; and each of the four grasses planted with birdsfoot trefoil for a total of eight treatments. Treatments were grazed by Jersey heifers using a rotational grazing system(1 week intervals). All treatments were fertilized with Chilean nitrate in April of 2017 and 2018. Grass monocultures were fertilized with feather meal in June 2017 and March 2018, and Chilean nitrate in July 2017 and 2018.
Grab fecal and urine samples were collected at the beginning of the grazing season and additionally every five weeks at the end of grazing rotations. Fecal samples were analyzed for total nitrogen by combustion method. Leachate was collected weekly by means of suction cup lysimeters and bi-weekly by means of zero-tension lysimeters and analyzed for nitrate-nitrogen using method 10-107-04-1-R on a Lachat FIA analyzer.
What we have learned?
Urea in urine, and fecal nitrogen were highest in the feedlot system (TMR diet). Urea and fecal nitrogen in the grazing systems were higher in the grass-legume mixtures than the grass monocultures even though tannins have been shown to shift nitrogen from the urine to the feces. This is most likely due to the higher protein (nitrogen) content of the grass-legume mixtures compared to the grass monocultures (data not shown). Despite the higher protein content of the grass-legume mixtures, the treatments containing birdsfoot trefoil exhibited less nitrogen loss due to leaching than the grass monocultures. Grass-legume mixtures have the potential to greatly improve the economic viability of a grazing operation while reducing the environmental impacts.
This study will be repeated for another year using Holstein heifers instead of Jersey heifers to see if there is a difference in nitrogen utilization between breeds. Treatments that are not being grazed will be harvested and fed in a feedlot setting to see if the benefits of birdsfoot trefoil remain when it is fed as silage.
Jennifer Long, Agricultural Systems Technology and Education Dept.; Utah State University
Rhonda Miller, Ph.D.; Agricultural Systems Technology and Education Dept.; Utah State University
Blair Waldron, Ph.D.; USDA-ARS Forage and Ranger Research Lab
Clay Isom, Ph.D.; Animal, Dairy and Veterinary Sciences Dept.; Utah State University
Kara Thornton, Ph.D.; Animal, Dairy and Veterinary Sciences Dept.; Utah State University
Kerry Rood, Ph.D.; Animal, Dairy and Veterinary Sciences Dept.; Utah State University
Michael Peel, Ph.D.; USDA-ARS Forage and Range Research Lab
Earl Creech, Ph.D; Plants, Soils, and Climate Dept.; Utah State University
Jacob Hadfield; Animal, Dairy and Veterinary Sciences Dept.; Utah State University
Marcus Rose; Plant, Soils, and Climate Dept.; Utah State University
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