Combinations of in-field moisture conservation and soil fertility management reduce effect of intra-seasonal dry spells on maize under semi-arid conditions

Samenvatting

The high frequency of prolonged intra-season dry spells since the turn of the 21st century continues to heighten risk of crop failure in rainfed cropping systems of Southern Africa including Zimbabwe. This study explored the effects of combining in-field moisture conservation techniques and soil fertility management on maize (Zea mays L.) productivity under rainfed conditions in semi-arid eastern Zimbabwe. Treatment combinations were co-designed with farmers through participatory approaches, and tested on-farm on sandy and clayey soils over three consecutive seasons (2015/16–2017/18). Two tillage practices namely conventional (CT) and reduced tillage (RT), with 30 % mulch cover of dried thatching grass (Hyparrhenia filipendula (L.) Stapf) applied either at planting or tasseling or at both stages, were combined with low (35 kg N ha⁻¹, 14 kg P ha⁻¹and 3 t ha⁻¹ of manure) and high (90 kg N ha⁻¹, 26 kg P ha⁻¹ and 7 t ha⁻¹ of manure) fertilizer application rates in a split-split plot design. Intra-seasonal dry spells were more frequent during the first two seasons (i.e. 2015/16 and 2016/17), while the 2017/18 season was rather wet with well-distributed rains. Soil water content varied significantly (p < 0.05) across treatments when a dry spell length was between 3 and 15 days, and 3 and 20 days on sandy and clayey soil, respectively. Conventional tillage retained 9–27% more soil water compared to RT, particularly when combined with mulching. During 2015/16 and 2016/17, seedling emergence occurred earlier by 2 days when mulching was applied at planting, while anthesis-silking interval (ASI) was shorter by 4 days following mulching at tasseling. On the sandy soil, the combination of CT + mulching (at both stages) + high fertilizer rate achieved the highest maize grain yield of 2.6 and 2.8 t ha^-1 during the 2015/16 and 2016/17, respectively. However, during the wet 2017/18 season, the combination of RT + mulching at planting + high fertilizer rate yielded the best (3.5 t ha^-1). On clayey soil, CT + mulching (at both stages) + high fertilizer rate gave the highest yields of 2.4, 2.9 and 3.4 t ha^-1 in 2015/16, 2016/17 and 2017/18, respectively. Water use efficiency (WUE) was greatest under CT + mulching (at both stages) + high fertilizer rate especially during seasons that had high occurrence of intra-seasonal dry spells. Overall, the combination CT + mulching (at both stages) + high fertilizer rate increased maize yield by over 200 % and 300 % compared to the non-mulched treatments on sandy and clay soil, respectively, particularly during seasons with high incidences of intra-seasonal dry spells. Consequently, the treatment combination achieved the best economic returns during the drier seasons although income returns were reduced during the wetter season because of considerable yield loss due to waterlogging, particularly on the sandy soil. We thus conclude that mulching at strategic crop growth stages in combination with tillage and judicious addition of organic and inorganic fertilizers is a promising agronomic technique for reducing effects of intra-seasonal dry spells on maize productivity in rainfed smallholder cropping systems. Increasing farmer access to organic and inorganic fertilizers is, however, key to accelerated adoption of such agronomic techniques.