Finding Forage Legumes Adapted to West Texas for Reduction of Water and Energy Use and Improvement of Nutritive value for Livestock

Inclusion of legumes in pastures is becoming more important as N prices escalate; however, legumes require more water for growth than warm-season grasses in water-limited semi-arid environments. Our hypothesis was that legumes could be identified for use in mixtures with warm-season grasses to increase yield, nutritive value, and N cycling without increasing irrigation appropriate to the grass-alone. Thus, in research conducted between 2007 and 2010 on the Texas Tech University Forage/Livestock Research Farm in the Texas High Plains, alfalfa (Medicago sativa L.), yellow sweetclover (Melilotus officinalis Lam.) and sainfoin (Onobrychis viciifolia Scop.) were each interseeded into three old world bluestems including ‘WW-B.Dahl’ [Bothriochloa bladhii (Retz) S.T. Blake], ‘Sparr’ [B. ischaemum (L.) Keng. var. ischaemum], and ‘Caucasian’ [B. caucasica (Trin.) C.E. Hubbard]. Additionally, no N (control) and 60 kg N ha-1 applied to the grasses alone were included as treatments. Each of the five treatments was replicated three times in a randomized complete block design with a split-plot treatment arrangement. Total irrigation water applied through subsurface drip was limited to a maximum of 250 mm, annually. Forage mass (kg ha-1) and concentrations of N and C in both the grass and the legume components were determined in late spring, mid-summer, and autumn. At the same times, forage nutritive value including calculated dry matter digestibility (DMD), and concentrations of crude protein (CP), cell wall, hemicellulose, cellulose, and lignin were determined. Soil particulate organic matter (POM) N and C concentrations were measured in composites of 10 soil cores representing the surface 5 cm of soil in each replicate at the end of 2009 and 2010. Inclusion of legumes increased total forage yield in both 2008 and 2010 (P < 0.05). Response of forage mass to N fertilization depended on precipitation distribution. Grass-legume mixtures had lower cell wall and cellulose concentrations, as well as greater DMD and CP concentrations than grass monocultures regardless of sampling date (P < 0.05). Grass N concentration was increased by inclusion of legumes in the entire growing season (P < 0.05). Additionally, inclusion of yellow sweetclover increased grass N concentration more than the other two legume species in late spring and mid-summer but not in autumn of each year. Similar effects were detected in grass C concentrations, but differences were smaller. By yr 4 (2010), legumes increased N concentration in associated grasses compared with grasses grown alone at each sampling date. Most grass species × treatment interactions were observed in autumn (P < 0.05) and were largely a result of continued effects of legume inclusions on N concentrations in associated grasses, while N fertilization effects decreased by autumn. Higher soil POM-N concentrations under grass-legume mixtures than grass monocultures were detected by 2009 and 2010 (P < 0.05). Bothriochloa ischaemum monoculture plots generally had greater soil POM-N than either B. caucasica or B. bladhii (P < 0.05). Soil POM-C was less influenced by treatments but was more influenced by grass species (P < 0.05). B. bladhii plots had higher POM-C than the other two grass species. However, B. ischaemum plots were more responsive to inclusion of legumes. Our data suggest that inclusion of legumes improves forage nutritive value of both the mixed sward and the associated grass components. Legumes also increased C sequestration more than either N fertilization or the 0-N control. When yellow sweetclover was included, these effects became measurable by July following seeding in April of yr 1. Both alfalfa and yellow sweetclover seem to have potential for increasing yield and forage quality in old world bluestems in the Texas High Plains with limited irrigation. However, performance of biennial yellow sweetclover requires more management to ensure stand longevity than perennial alfalfa. Future studies are needed to understand transfer mechanisms for rhizobia/legume symbiotic N2 fixation and to define optimal management practices for achieving high yields and nutritive value with optimal C capture under limited irrigation.