Influence of varying replacement of potential evapotranspiration on water use efficiency and nutritive value of three Old World bluestems (Bothriochloa spp.)
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Forage systems offer alternatives to traditional cropping systems in the Texas High Plains, but information on water management is lacking. This research investigated whether differences in water use efficiency (WUE; kg total seasonal dry matter [DM] yield ha"' mm"' water [precipitation, irrigation, and soil water depletion]) existed among species of old world bluestems (Bothriochloa spp.). Additionally, effects of irrigation amounts on forage nutritive value and plant morphology were explored. Established stands of three Bothriochloa species ('Dahl' [bladhii]; 'Caucasian' [caucasica]; and 'Spar' [ischaemum]) were surface drip-irrigated weekly during the growing season to replace: 1) 0% (dryland); 2) 33% (low); 3) 66% (medium); and 4) 100% (high) of potential evapotranspiration minus precipitation during 2001,2002, and 2003. Soil organic carbon as influenced by irrigation level and optimum metabolic temperature for growth were investigated in 2003. Each species and water treatment was replicated three times in a complete randomized block design with a split plot treatment arrangement. In 2001 and 2002, no differences in water use efficiencies among species were found. Averaged across these 2 yr, WUE of 19.0, 19.3, and 15.3 kg ha"' mm"' were observed for Caucasian, Dahl, and Spar, respectively. In 2003, Caucasian produced 16.9 kg ha" mm', differing (P < 0.05) in slope fi-om Spar (7.4 kg ha"' mm"') but Dahl (12.4 kg ha"' mm"') was not different in slope from either Caucasian or Spar. Maximum seasonal DM yield was obtained with Caucasian under high irrigation (18.0 Mg ha"') vs. Dahl (15.2 Mg ha") and Spar (12.55 Mg ha"') averaged over all years. Percentage dry matter digestibility (DMD) was higher (P < 0.05) in all forage species irrigated at a low level (58% DMD) than for other water treatments (57, 56, and 55% DMD for dryland, medium, and high irrigation, respectively). Dahl generally averaged higher (P < 0.05) percentage crude protein (CP) than other species during the growing season. More differences in percentage CP were observed between dryland and irrigated forages than within the irrigated treatments but CP would have met nutritional needs of most livestock only in May. In all species, percentages total nonstructural carbohydrates (TNC) and DMD and leaf: stem ratio declined while percentage cell wall increased with increased irrigation amount in the first half of the growing season. Effects of irrigation on cell wall, TNC, and DMD appeared related to changes in plant morphology (leaf:stem) and to an increase in physiological age as indicated by growth stage (mean stage count and mean stage weight). Leaf:stem ratio following hay harvest in July showed fewer effects of irrigation treatments but ratios of live:dead plant material generally increased during the growing season in response to increased irrigation (linear effects; P < 0.05). Soil organic carbon was higher (P < 0.05) in all irrigated soils (5.5 g 100 g"') compared with dryland (4.6 g 100 g"') but did not differ among irrigation treatments. Optimum metabolic temperatures for growth of Caucasian, Spar, and Dahl old world bluestems were 24, 24, and 28°C, respectively. Our data suggest that differences in WUE among Bothriochloa species can be identified, and that yield, chemical composition, and morphology can be manipulated through irrigation management to optimize total nutrient yield and nutritive value.