Cultivated forage crops are grown on almost 12 million ha on the northern Great Plains. This paper reviews the benefits of diversifying annual crop rotations with forage crops and highlights innovations in forage systems. Agronomic benefits of rotating forage crops with annual grain crops include higher grain crop yields following forages (up to 13 yr in one study), shifts in the weed population away from arable crop weeds, and improved soil quality. Perennial legumes in rotation also reduce energy requirements by adding significant amounts of N to the soil. Soil water availability may limit the extent to which forages benefit following crops. Under semiarid conditions, forages can actually reduce yields of the following crops, and as such, tillage practices that conserve soil water have been developed to partially address this problem. Forages in rotation provide environmental benefits, such as C sequestration, critical habitat for wildlife, and reduced NO3 leaching. A wider range of annual plant species are now used in forage systems in an effort to extend the grazing season and to maximize use of water resources. Intensive pasture management using cultivated forages is on the increase as is the use of alfalfa (Medicago sativa L.) in grazing systems; in some cases, bloat‐reduced alfalfa cultivars are used. Pasture‐based systems appear to provide benefits for both animal and human health and arguably the health of the environment. Pasture systems are less nutrient exhausting than hay systems. As a result, nutrient management strategies will differ in the following crop. Additional research is required to optimize the role of cultivated pastures in grain‐based cropping systems.
Cereal crop yields frequently are greater when grown after soybean [Glycine max (L.) Merr.] than after continuous cereal cropping. Information on other grain legumes is limited in eastern areas of the northern Great Plains. The objectives of this field study were to determine (i) soil nitrate‐N status in the spring following grain legumes, and (ii) grain legume effects on grain yield, grain yield components, and N nutrition of the subsequent hard red spring wheat (Triticum aestivum L. emend. Thell.) crop fertilized with 0, 75, and 150 kg ha−1. Two‐year crop sequences were evaluated on a Fargo silty clay (fine, montmorillonitic, frigid Vertic Haplaquoll) soil at Fargo, ND, and on a Perella‐Bearden silty clay loam (fine‐silty, mixed, frigid Typic Haplaquoll, fine‐silty, frigid Vertic Calciaquoll) soil at Prosper. Six grain legume species were harvested for grain, and aboveground residues were removed in 1984 and were uniformly spread and incorporated into the soil in 1985. Spring soil nitrate‐N level following legumes was 28% greater than that following N‐fertilized wheat across three environments but 43% lower than that following fallow. Unfertilized wheat grain yields following grain legumes were equivalent to or greater than that following a wheat crop fertilized with 75 kg N ha−1 and similar to fallow at the same fertility level. Total N accumulation by wheat following grain legumes was 9% greater than that following wheat but 13% lower than that following fallow. Nitrogen‐use efficiency for wheat following legumes, however, was up to 32% greater than that for wheat following fallow and up to 21% greater than that for continuous wheat. These studies indicate that grain legumes should be considered in cropping systems in higher moisture areas of the northern Great Plains to help maintain subsequent crop productivity.
ing soybean cultivars in Canada (Saliba et al., 1982). Hume and Jackson (1981) evaluated 30 soybean geno-Spring frost in cooler regions periodically kills seedling legumes types of different sources and maturity groups at Ϫ2, and makes replanting necessary. Experiments were conducted in the growth chamber to determine freezing tolerance of 10 legume species Ϫ2.5, and Ϫ3ЊC at the cotyledon, unifoliolate, and first at four growth stages and to determine the freezing temperature that trifoliolate leaf stages. Prefreezing growth temperatures kills 50% of seedlings (LT 50 ) for each species under temperatures in the greenhouse were 15/9, 20/14, and 25/19ЊC (day/ more commonly found in the field. Four temperatures (Ϫ2, Ϫ4, Ϫ6, night). They found that the greatest soybean mortality and Ϫ8؇C), four seedling ages (1, 2, 3, and 4 wk after planting), and occurred at the unifoliolate stage at Ϫ3ЊC when growth 10 legume species [alfalfa (Medicago sativa L.), red clover (Trifolium temperature in the greenhouse was 25/19ЊC compared pratense L.), sweetclover (Melilotus officinalis Lam.), alsike clover with the lower growth temperatures. With exceptions, (T. hybridium L.), white clover (T. repens L.), sainfoin (Onobrychis temperature drop in the spring does not occur abruptly viciifolia Scop.), pinto bean (Phaseolus vulgaris L.), navy bean in the northern Great Plains but over a 2-or 3-d period. (Phaseolus spp.), soybean [Glycine max (L.) Merr.], and field peaThe seedlings were not hardened in Hume and Jackson's (Pisum sativum L.)] were included. Hardened (vernalized) seedlings were placed in a programmable freezing chamber at 3؇C and the (1981) experiments, temperature was dropped sequentemperature decreased or increased 1؇C h Ϫ1 to or from a minimum-tially from prefreezing to freezing over a 9.5-h period. freezing temperature. Pinto and navy beans were the least tolerant, But dropping the temperature from 9, 14, or 19ЊC at soybean and field pea were moderate, and forage legumes were the night to a freezing temperature within 9.5-h generally most tolerant to freezing temperature. The LT 50 was Ϫ3.25 to Ϫ3.5؇C does not happen in the northern Great Plains. A for dry beans, Ϫ4.5؇C for soybean and field pea, and Ϫ6.3 to Ϫ7.4؇C weather front could drop the air temperature abruptly, for forage legumes. One-week-old seedlings had the highest tolerance but rarely does a freezing temperature occur the first to freezing temperature when close to the LT 50 , compared with older night, which allows some seedling acclimation prior to seedlings. However, this tolerance at 1 wk of age disappeared when freezing. temperature was lower than the LT 50 of the species. The prediction Abbreviations: LT 50 , temperature that kills 50% of seedlings.
Knowledge of legume N production and legumeffects on subsequent crop yield and quality is necessary to encourage legume use instead of the traditional fallow on set‐aside land. Objectives of these studies were to: (i) compare seeding‐year herbage and N yields five forage legume species, (ii) determine soil NO3‐N status in the spring following green‐manure legume crops, and (iii) evaluate effects of green‐manure legumes on grain yield, grain yield components, and N nutrition of the subsequent wheat (Triticum aestivum L.) crop when fertilized with 0, 75, and 150 kg N ha−1. Field experiments were conducted on a Fargo silty clay (fine, montmorilloritic, frigid Vertic Haplaquoll) at Fargo and on a Perella‐Bearden silty clay loam (fine‐silty, mixed, frigid Typic Haplaquoll, fine‐silty, frigid Aeric Caiciaquoll) near Prosper, ND, during 1984 to 1986. All legume species had equal herbage and N yields across the four environments and were greater than the wheat‐straw check. Accumulated legume herbage and fall regrowth were incorporated into the soil in late fall. Spring soil NO3‐N following Terra Verde alfalfa (Medicago sativa L.) and hairy vetch (Vicia villosa Roth.) was equal to the fallow check and greater than the soil NO3‐N following the wheat check or other legume species. Grain yield, grain N, and N uptake of unfertilized wheat following the legume treatments generally were similar to those following fallow and wheat fertilized with 150 kg N ha−1. Increases in all grain yield components following legumes contributed to this yield advantage. Efficiency and utilization of N generally were greater following a green‐manure crop than following either fallow or wheat checks. This study suggests that green‐manure legumes should be considered as an alternative to fallow on set‐aside land in higher moisture areas.
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