A study of water use efficiency in field crops of Maize and beans

Crop water use efficiency nay be defined aa the ratio of total dyy matter produced to total evaporation from the orop and the aoil. In Bast Africa, the areas of high photosynthetic potential (highest number of sunshine hours) are also the driest* Success in growing annual food orops like maise and beans in these areas is therefore heavily dependent on the date of planting and the ability of the crop to complete all its stages of growth within the short rainfall seasons* While breeding of short term and high yielding varieties has been emphasised, little has been done in finding out the water use patterns of the varieties. Experiments were therefore designed to provide information on the water use patterns of one hybrid of maize (H 511) with a medium maturity period (4 months) and one popular variety of field bean (Phaseolue vulgaris var. Canadian Wonder), in all stages of growth, and to observe the effect of reduced soil moisture on the water use and rate of growth of these two crops* Ratios of orop water use (St) to Penman estimate of open water evaporation (Bo) gave values as high as (m i l ) 1.4 - 1.5 for uiit and 1.3 - 1.4 for beans under vet conditions. It is shown that the excess water use* at least in the maize crop may be due to the combination of large net interception of rain and low aerodynamio resistance. There was a reduction of 20% in water use and 40% in yield in the dry treatment of the maise experiment. Measurement of soil moisture in situ by the neutron scattering technique was studied with the intention of using the method for routine determination of crop water use in the field. Although reliable calibrations were obtained for two makes of neutron moisture meters* B.A.L. and B.I.V.-I the method was shown to be successful only in the absence of drainage. Because of errors in calibration and spatial variations in moisture contents* the precision of soil moisture determination by the neutron moieture meter le not adequate for small differences and the interval between meaeuremente should be at leaet 7-10 days. In irrigated fielde* the inherent poor die- trlbution of irrigation water ie a major limitation. Attempts to derive drainage correction data from tensiometer readings were not successful. Theoretical estimates of gross photosynthesis hare been successfully correlated with measured dry natter production in anise and beans The correlations suggest that in the local environment respiration loss for the two crops is a constant proportion of gross photosynthesis in all stages of growth. This result enables the prediction of the maximum yields of these crops from meteorological data, mainly solar radiation. Studies of partition of energy in field crops of maize and beans have shown that in the local environment, when the crops are supplied with adequate water, all net radiation may be converted into latent heat, and for periods of 1-2 hours during the day, latent heat may greatly exceed net radiation, the extra energy being derived from the air. Finally the implications of the above findings on plant breeding and agronomic techniques for water use efficiency are discussed.