Assessment of the effect of tie-ridging on smallholder maize yields in Malawi

Tie-ridging is being promoted in Malawi as an on-field rainwater harvesting technique to ensure a maize (Zea mais L.) crop during a dry or drought yea r. Resource-poor smallholder farmers are likely to take up tie-ridging if i t increases and not decreases maize yield in most years. A numerical study was conducted to calculate the expected maize yield gain due to tie-ridging taking into account the probability of occurrence of drought, dry, normal and wet years (climatic uncertainty). Mean yields due to tie-ridging in dro ught, dry, normal and wet years at different N levels were derived from obs erved smallholder maize yield data using a linear nitrogen response model a nd field-observed retained rainwater amounts in tie-ridged fields. Simulati on results indicate that tie-ridging will result in hybrid maize yield gain in a drought year (1050 kg ha(-1)) and dry year (560 kg ha(-1)). There wil l be a hybrid maize yield loss in a normal year (350 kg ha(-1)) and wet yea r (700 kg ha(-1)). For local maize, there will be a yield gain in a drought year (500 kg ha(-1)), dry year (220 kg ha(-1)) and normal year (120 kg ha( -1)). There will be a slight yield loss in a wet year (60 kg ha(-1)). Consi dering observed probability of the occurrence of drought, dry, normal and w et years in Malawi, the study reveals that there will be no hybrid maize yi eld gain in any coming year with tie-ridging. For local maize, the expected yield gain in any coming year was positive (133.3 kg ha(-1)) but this gain is less than the minimum gain required considering the opportunity cost of labour (142.5 kg ha(-1)). Thus under the smallholder conditions and climat e of Malawi, the expected yield gain in any coming year due to tie-ridging is likely to be minimal and uneconomic. (C) 1999 Elsevier Science B.V. All rights reserved.