Water use efficiency of dryland cowpea, sorghum and sunflower under reduced tillage

Drought-adapted, early maturing crops combined with reduced tillage systems have the potential to stabilize and increase dryland crop yields in semiarid regions of the world. The objectives of this research were to evaluate soil water use and dryland grain yield response of cowpea [Vigna unguiculata (L.) Walp], sorghum [Sorghum bicolor (L.) Moench], and sunflower [Annus helianthus (L.)] under no tillage (NT) and stubble mulch tillage (SMT) systems. The study was carried out on a Torrertic Paleustoll in the high plains of Texas, USA from 2000 to 2001 when growing season precipitation totaled 47 and 62%, respectively, of the historical mean (317 mm). The experimental design was a split plot, with tillage as the main plot and crop type as the subplot. Soil water contents were measured weekly throughout two growing seasons, from 0.10 to 2.30 m depth at 0.20-m intervals using a neutron moisture meter. Crop water use was calculated as the change in stored soil water plus measured precipitation and assuming negligible runoff and drainage beyond 2.3 m. In 2000, after a 10-month fallow following winter wheat, soil water content at planting was 53 mm greater (P = 0.012) under NT compared with SMT. At harvest, soil water content was greater under cowpea compared with sorghum and sunflower in both tillage systems (P < 0.001). However, greater soil water content at harvest under cowpea did not always result in significantly greater plant available water content the following year compared with sorghum. Differences in water use were dominated by a crop effect (P < 0.028) largely as a result of lower water use by cowpea compared with sorghum and sunflower. Water use of sorghum and sunflower averaged 97 and 29 mm greater than that of cowpea across tillage treatments, respectively. Mean grain water use efficiency (WUE) was greatest for sorghum (1.03 kg m−3), followed by cowpea (0.52 kg m−3) and sunflower (0.32 kg m−3). Sorghum under NT had similar or slightly greater water use, yet consistently greater grain yields compared with SMT. This suggests that a larger fraction of evapotranspiration was diverted to transpiration under NT compared with SMT. Greater residual soil water under cowpea at harvest may increase plant available water the following year provided that the soil has a high soil water storage capacity and slow drainage rates. Tillage effect on soil water use depended on the crop and year, however only sorghum exhibited a significant yield response to increased soil water storage afforded by NT.

Research highlights▶ No tillage conserved more soil water compared with stubble mulch tillage. ▶ Only sorghum had a yield response to greater soil water storage afforded by no-till. ▶ Sorghum and sunflower extracted more soil water at deeper soil depths than cowpea. ▶ Cowpea did not always increase plant available water content in the following year.