Abstract

The role of harvest management on the N nutrition of perennial forage legume-grass mixtures requires further investigation. Closer examination would provide a better understanding of the roles of defoliation, N₂ fixation, and N-transfer in the management of red clover (Trifolium pratense L.)-orchardgrass (Dactylis glomeruta L.) mixtures. Periodic harvesting of shoots of forage legumes removes a primary source of energy for maintaining N₂ fixation. Nodule structure and function and the initiation of new nodules is affected, also. The ability of nodules to remain functional after removal of the vegetative shoots may depend upon a number of factors (e.g., plant species, microsymbiont, nodule morphology, rate of shoot regrowth, nutrient availability, and competition between plant parts for energy). Field research was undertaken to determine how harvest management (three-cut vs. six-cut systems) affected amounts of N, fixation and N transfer in four red clover-orchardgrass mixtures. Dinitrogen fixation and N transfer were estimated by ¹⁵N isotope dilution using pure stands of orchardgrass as a reference. Over the 2-yr study, percentage legume N derived from fixation (%Ndfa) ranged from 91 to 96% and from 91 to 97% under the three- and sixcut systems, respectively. Total-season yields of fixed-N in red clover herbage ranged from 70 to 242 and from 59 to 174 lb/acre under the three- and six-cut systems, respectively. Differences in %Ndfa and fixed-N yield between the two harvest managements generally were not significant (P 5 0.05). Percentage N in orchardgrass herbage derived from fixation (%Ngdfa) ranged from 18 to 44 and from 33 to 50% under the three- and six-cut systems, respectively. Total-season yields of transferred-N in orchardgrass herbage ranged from 9 to 27 and from 17 to 29 lb/acre under the three-and six-cut systems, respectively. Differences in %Ngdfa and transferred-N yield between the two harvest managements generally were not significant (P ≤ 0.05). It is concluded that, under the conditions of this study, the intensive harvest management system did slightly increase N, fixation by red clover but not N transfer from red clover to associated orchardgrass.

Research Question

The role of harvest management on the N₂ fixation and N-transfer capacities of red clover-orchardgrass mixtures requires further investigation in order to better understand how defoliation affects these processes in mixed swards. Field research was undertaken to determine how three-cut vs. six-cut harvest management systems affected amounts of N₂ fixation and N transfer.

Literature Summary

It has been demonstrated in field and greenhouse studies that N₂ fixation is depressed when forage legumes are defoliated severely. Likewise, previous studies have shown that factors which depress the legume component (e.g., severe defoliation) also increase the rate at which legume fixed-N is released to the soil and made available for grass growth. The release of N to the soil may inhibit either additional initiation of legume nodules or the nitrogenase activity of nodules already present. Certain physical characteristics of red clover (e.g., degeneration of taproot) may severely limit production and N₂ fixation capacity beyond the second year of growth.

Study Description

This field study was conducted at the Iowa State University Agronomy and Agricultural Engineering Research Center on a fine-loamy, mixed, mesic Typic Hapludoll soil. An isotope dilution study was conducted to estimate the amounts of N₂ fixed and transferred to associated orchard-grass. A small amount of a stable isotope of N (¹⁵N) was applied to each plot. This served as a label to distinguish between soil-derived N and symbiotically fixed-N in both legume and grass components. Red clover cultivars Arlington, Mammoth, Redland 11, and experimental line APR-8701 (AgriPro Biosciences, Napier, IA) were seeded in binary mixtures with ‘Dawn’ orchardgrass. A pure stand of orchardgrass was used as the nonfixing control (reference crop) for the isotope dilution study. An intensive management (six harvests taken at 3-wk intervals) and a moderate management (three harvests taken at 6-wk intervals) were imposed on each of the red clover mixtures and the reference grass.

Applied Question

What effect did harvest management have on N₂ fixation and N transfer in red clover-orchardgrass mixtures?

Red clover and orchardgrass plants subjected to the three-cut system visibly had a higher percentage of stem growth that contributed to increased dry matter yields. Three-cut system dry matter yields, however, were not significantly greater than the six-cut system for Arlington, Mammoth, and Redland 11 in 1991 (Fig. 1). Dinitrogen fixation in 1990 was slightly higher under the intensive (six-cut) management system for all cultivars except Arlington (Fig. 2). Legume proportion in the sward has been shown to affect N₂ fixation by forage legumes. Maximum N₂ fixation usually is observed when legume proportion is in the range of 50 to 70%, depending on legume species. In this study, legume proportion of the three-cut system in 1990 averaged 79% among the four cultivars while legume proportion of the six-cut system averaged 66%. This may explain the differences in N₂ fixation between the two harvest management systems in 1990. The biological significance of these small differences may be questionable, however. No significant differences in N₂ fixation were shown between harvest management systems for 1991. Fixed-N yields reflected dry matter yields of red clover and usually did not differ among cultivars for either management system in either year. Yields of fixed-N were greater in 1990 than in 1991 for both systems, largely the result of greater yield of legume in the mixture during 1990. Differences in N transfer and transferred-N yield among harvest management systems were shown for Redland II in 1990 only. Based on the results of this study, we conclude that harvest management did have a small effect on dry matter yield and N₂ fixation capacities of red clover-orchardgrass mixtures. Harvest management, however, did not affect N transfer from red clover to associated orchardgrass.

Fig. 1

Total-season herbage dry matter yield of four red clover-orchardgrass mixtures managed under two harvest systems for 2 yr.

 

Fig. 2

Total-season percentage N in red clover derived from the atmosphere (Ndfa) from four red clover-orchardgrass mixtures managed under two harvest systems for 2 yr.

 

Copyright © 1994. . Copyright © 1994 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, 5585 Guilford Rd., Madison, WI 53711 USA