Nitrogen use efficiency of legume-grass and grass pastures

West, Charles
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In the subhumid to humid, temperate regions of the world, nitrogen (N) availability is very often the factor most limiting to pasture productivity, both in terms of dry matter (DM) yield and animal performance. This study was carried out to characterize the contribution of N fixation to the N economy of an alfalfa (Medicago sativa L.)-orchardgrass (Dactylis glomerata L.)-smooth bromegrass (Bromus inermis Leyss.) pasture and to compare the animal-use efficiency of N of two N-forage systems: the above-mentioned alfalfa-grass mixture receiving no N fertilizer vs. reed canarygrass receiving N fertilizer at 180 kg/ha(.)yr. Two levels of grazing pressure were imposed on each system;The pastures, located in south-central Iowa, were grazed by steers with initial weights of ca. 250 kg for 6-month grazing seasons in 1979 and 1980;Dilution of ('15)N in the alfalfa compared to the accompanying grass indicated that the percentage legume N fixed declined from 97 to 80% (aver. 91%) as legume DM yield and percentage legume increased. The amount of legume shoot N from fixation (aver. 70 kg/ha(.)yr) and the proportion of total herbage (legume + grass) N from fixation (aver. 36%) were positively correlated with legume DM yield and percentage legume. Maximum levels of N fixation in this mixture can be obtained at a legume percentage above 50% while maintaining a high degree of percentage N fixed (80 to 90%);In comparison to the reed canarygrass pastures, the alfalfa-grass system supported a higher level of DM and N yield per unit of N input from N fertilizer or N fixation, respectively. The alfalfa-based system also supported a higher level of beef liveweight gain per unit of DM and N intake, greater N retention in animal tissue, and less N excretion per unit of N intake than the N-fertilized grass system.

Agronomy, Crop production and physiology