Wheat yield, water productivity and double-cropping feasibility in irrigated drylands of Uzbekistan

Irrigated drylands of Central Asia need to raise wheat productivity under constrained water supplies while making better use of the short summer fallow between wheat harvest and autumn/winter sowing. We combined a two-season field experiment at Karshi, comprising 13 treatments and an embedded 2 × 2 nitrogen (N) × irrigation factorial, with on-farm evaluation of five summer crops at six sites in 2023 and 2024 and APSIM simulations over 39 seasons. Identically managed controls differed by 0.244 t ha⁻¹; treatment interpretation therefore focused on responses that were reproducible across seasons. ‘Turon’ gave the largest and most consistent response, yielding 36–37% more than its matched control in both seasons. Increasing seed rate to 250 kg ha⁻¹ also increased yield by about 1.0 t ha⁻¹ across seasons. Nitrogen and irrigation interacted (P = 0.008): increasing N from 126 to 234 kg ha⁻¹ raised yield by 0.92 t ha⁻¹ under reduced irrigation but had no statistically detectable effect under increased irrigation. Combining 30% more N with 30% less irrigation increased total water productivity from 0.86 to 1.31 kg m⁻³ relative to farmers’ practice. Within the tested treatments, random forest analysis ranked variety highest (20.0% of total importance), followed by season (17.6%), seed rate (16.7%), irrigation amount (13.2%), N rate (12.4%), sowing date (12.0%), and establishment method (8.1%). Long-term simulations indicated that residue retention reduced soil evaporation by approximately 60 mm, but increased grain yield only when nitrogen supply was adequate. The feasibility of a second crop depended mainly on wheat harvest date and the duration of the summer crop. On-farm, crop durations remained similar between 2023 and 2024 for four crops; a shorter-duration pearl millet type was grown in 2024. Cereal yields were 44–64% lower in the hotter 2024 season, whereas mungbean yield remained similar. Mungbean matured in about two-thirds of seasons, whereas 118-day maize matured in fewer than 10%; early-maturing wheat extended the available window by 16 days compared with late-maturing wheat. However, adding a second crop approximately doubled or tripled annual irrigation and produced less additional grain per unit of water than wheat alone. Overall, the results show that improving productivity in irrigated drylands requires suitable varieties, balanced nitrogen and irrigation, appropriate plant density and residue management, and careful matching of wheat maturity and summer-crop duration with the available water and growing period.