The response of field bean (Vicia faba L.) to irrigation and sowing date

The response of autumn and spring sown crops of Vicia faba cv. 'Maris Bead' to irrigation applied during the vegetative, flowering and pod-filling phases was investigated during the 1981/82 and 1982/83 seasons on a Templeton silt-loam soil in Canterbury. During any phase trickle irrigation was applied weekly in amounts equal to the difference between the estimated potential evapotranspiration, adjusted for crop cover and soil evaporation, and the rainfall of the previous week. Measurements of crop physiological and environmental variables were made to help establish the causes underlying the variations in yield associated with irrigation, sowing date and season. Averaged over two seasons, yields of the fully irrigated autumn and spring sown crops were 5.2 and 3.3 t ha⁻¹ respectively, about 40% greater than yields obtained from the unirrigated crops. Autumn sowings out yielded spring sowings by about 55% due to both a greater dry matter production and a larger harvest index. Both total dry matter and grain yield were linearly dependent upon the sum of the rainfall and irrigation received by the crops during the main growth period. Penman's model described the response of crop yields to irrigation reasonably well suggesting a limiting potential soil moisture deficit (D1) of about 65 mm beyond which yield declined linearly with maximum deficit reached during growth. A soil type having a similar available water content in the rooting zone (top 1.0 m) may be expected to have a D1 similar to that reported here, equivalent to about 40% of the available water content in the effective rooting zone. The response of grain yield to irrigation for a well husbanded crop of field beans in Canterbury is expected to be about 6 and 4 kg ha⁻¹ mm⁻¹ for the autumn and spring sown crops respectively. Within the precision of these experiments there was little evidence supporting the existence of 'moisture sensitive periods' during flowering or any other phase of development in field beans. Harvest index was stable within either autumn (0.48) or spring (0.35) sowings. Irrigation therefore increased grain yield by increasing total dry matter (TDM) production. During the early stage of development faster crop growth resulting from irrigation was almost entirely associated with increased radiation absorption. The utilisation coefficient was stable at about 1.7 g MJ⁻¹. Later in the season irrigation increased TDM and yield by increasing both the amount of radiation absorbed and the utilisation coefficient. Autumn sowing increased TDM by increasing the absorption of radiation due largely to a larger duration of green crop cover. This more than compensated a slightly smaller utilisation coefficient which may have been caused by a more horizontal leaf orientation and perhaps a greater partitioning of DM to roots early in the season. Seasonal differences in TDM and yield were entirely associated with radiation absorption, the utilisation coefficient being stable. Early sowing and irrigation were both important in ensuring maximum radiation absorption. Variation in yield with irrigation was closely correlated with variations in the number of beans m⁻², which arose mainly due to differences in the number of pods plant⁻¹ and, to a lesser extent the number of beans pod⁻¹. The number of pods plant⁻¹ - the most important yield component - was closely correlated with both green area and crop growth rate during the 3 weeks after pod-set. The rate of crop development was strongly temperature and to a lesser extent photoperiod. After time of about 2000 degree C d above a base temperature must be accumulated for a crop to reach maturity. Calculation of photothermal time above a base photoperiod of 6 hours enabled the maturity of field beans to be predicted with greater accuracy (within 6%) than by using thermal time.