Source: North Dakota State University Extension
Low available mineral nitrogen limits grass production more than low water on grassland pastures.
Low rainfall during the growing season is the most obvious factor causing reduced grass production. However, low available mineral nitrogen at less than 100 pounds per acre is responsible for greater than half of the reduction in herbage production.
"Most grassland pastures managed with traditional practices have mineral nitrogen available at 60 to 75 pounds per acre," says Lee Manske, North Dakota State University Dickinson Research Extension Center range scientist. "Long-term use of a deferred grazing practice reduces mineral nitrogen to 31 pounds per acre. Because of these chronic nitrogen deficiencies, these pastures have reduced grass production, even during growing seasons with no water deficiencies."
Growing seasons with water deficiencies are normal conditions on the northern Plains. In western North Dakota, 79 percent of the past 118 growing seasons have had an average of two months with water deficiencies. The water deficiency frequency has been 17 percent in April, 14 percent in May, 10 percent in June, 38 percent in July, 53 percent in August, 50 percent in September and 47 percent in October.
Growing seasons with moderate drought conditions have occurred during 12 percent of the years. Severe drought conditions, such as in 1919, 1934, 1936 and 1988, have occurred during 3 percent of the years, or about one in every 29 years.
Growing seasons without water deficiencies are an abnormal phenomenon and have occurred during 6 percent of the growing seasons, or about one in every 17 years. Some level of water deficiency has occurred during 94 percent of the past growing seasons.
Grass production is reduced during growing seasons with water deficiencies because of low water and low mineral nitrogen.
"However, grassland pastures are not low in nitrogen," Manske says. "Most of the nitrogen in grassland soils is in organic form, but plants cannot use organic nitrogen. The organic nitrogen must be mineralized by soil microorganisms. Soil organism biomass is limited by access to a simple carbon chain energy."
Soil organism biomass can be increased greatly with biologically effective management strategies that have grazing rotation dates coordinated with grass growth stages. Partial defoliation by livestock during vegetative growth stages causes grass carbohydrates to be released into the soil. These exudates increase the quantity of energy available to the microbes.
The resulting increased population of soil organisms mineralize nitrogen at quantities greater than 100 pounds per acre. As a result of the increased quantities of available mineral nitrogen, greater herbage biomass is produced during growing seasons with water deficiencies.