Different responses of African rice cultivars to alternate wetting and drying irrigation and anaerobic digestate application

Context

Water and nutrient limitations often restrict African rice production. Scarce fertiliser supplies require suitable alternatives such as anaerobic digestate residues, but few trials have tested rice cultivation with this nutrient source. Applying alternate wetting and drying (AWD) irrigation, that decreases crop water use and maintains yield, can enhance climate resilience of irrigation schemes, but whether African rice (Oryza glaberrima) cultivars are better adapted to AWD is uncertain.

Objective

We aimed to develop farmer recommendations for optimal water and nutrient use in West African rice cropping systems. We hypothesised that anaerobic digestate was a viable alternative nitrogen source to chemical fertilisers, and that a local African rice cultivar tolerated AWD better than an improved Asian cultivar.

Methods

Factorial experiments at Fumesua, Ghana during the 2021 and 2022 dry seasons varied irrigation type (alternate wetting and drying, AWD versus continuous flooding, CF), nutrient source (synthetic fertiliser versus anaerobic digestate at the same nitrogen application rate) and cultivars (improved O. sativa cv. CRI-Agrarice versus local O. glaberrima cv. Viowornor short) in a split-plot design. Water- (leaf relative water content and stomatal conductance) and nutrient- (chlorophyll content, leaf nitrogen content) related traits were measured during the irrigation season, with yield and its components (number of effective tillers, grain number per panicle, thousand grain weight) measured at harvest.

Results

AWD decreased water use by 28 % but maintained grain yield, except synthetic fertiliser-treated local O. glaberrima in which grain yield decreased by 31 %. Harvest index better explained treatment differences in yield than variation in water-related physiological traits. Nutrient source affected the cultivars differently. Synthetic fertiliser increased improved O. sativa grain yield by 18 % compared to digestate coincident with higher leaf chlorophyll content, whereas digestate increased local O. glaberrima grain yield by 66 % despite 20 % lower chlorophyll content. Digestate increased the number of effective tillers of both cultivars by 24 %, but diminished thousand grain weight by 4 %.

Conclusions

AWD requires cultivars adapted to the prevailing nutrient management to avoid yield losses, while applying digestate boosted grain yields of the local cultivar while maintaining comparable yields to the improved cultivar receiving synthetic fertiliser.

Significance

Bespoke irrigation × nutrient source × cultivar combinations can boost African rice production while decreasing fertiliser costs and water use.