Forage nutritive value and morphology of three old world bluestems under a range of irrigation levels

Introduced forages offer alternatives to traditional cropping systems in the semiarid Texas High Plains, but effects of irrigation on nutritive value are not well known. Three old world bluestem (Bothriochloa) species [B. bladhii (Retz) S.T. Blake 'WW-B. Dahl', B. ischaemum (L.) Keng. var. ischaemum (Hack.) 'WW-Spar', and B. caucasica (Trin.) C.E. Hubbard 'Caucasian'] were surface drip-irrigated with zero (dryland) and low, medium, and high irrigation levels, to determine nutritive value and morphological development in 2001 to 2003 under a completely randomized block design, a split-plot treatment arrangement, and three replicates. Water applied in the high treatment was 100% replacement of potential evapotranspiration minus precipitation. Medium and low treatments were 66 and 33% of the high treatment. Dry matter digestibility (DMD) of all species was higher (P < 0.05) at low irrigation than at other levels (580, 560, 550, and 570 g kg(-1) for low, medium, high, and dryland, respectively; SE = 4.0). Dahl was higher (P < 0.05) in crude protein (87 g kg(-1)) than Spar (76 g kg(-1)) and Caucasian (75 g kg(-1); SE = 2.0) during the growing season. In all species, total nonstructural carbohydrates and DMD declined while neutral detergent fiber and acid detergent fiber increased with increasing irrigation. Before hay harvest, leaf blade:stem-plus-sheath ratio declined also, but after hay harvest, effects of irrigation were less consistent. Results suggested that irrigation likely affected nutritive value through effects on plant morphology and physiological age. Thus, irrigation strategies and species selection may aid in optimizing forage quality.