Effect of sowing date on water use efficiency of sunflower crop

Results of a trial carried out on sunflower in order to evaluate the most appropriate sowing date and irrigation regime for a more efficient water use are reported. Sunflower was sown in 1995-1996 at the usual date (end of April) and in advance (end of March). It was subjected to three irrigation regimes: full restoration of consumptive water use, supplementary irrigation at the bud stage and flowering and unirrigated control. During the growing cycle, the following parameters were measured: water use, the main climatic data that can affect growth and evapotranspiration, yield and its components. Despite the lower vapor pressure deficit of the air during the cropping cycle of the first sowing date caused a reduction in the average daily evapotranspiration, the colder temperature regime of this period, by making longer the growing cycle, caused almost the same total water use respect to the usual sowing dates. However, with early sowing, the crop could benefit from the spring rainfall at the initial stages of its cycle that reduced the seasonal irrigation volume, in the case of full irrigation and made available a greater amount of water in the case of unirrigated treatment or with supplementary irrigation. The greater water availability in the stressed treatments also produced higher grain yield in early sowing, so that an interesting interaction between the sowing date and the irrigation regime in terms of water use efficiency was observed. In fact, a significant higher irrigation yield water use efficiency and an interesting yield response was measured in the treatment with supplementary irrigation of the first sowing date. No effect of sowing date, both in terms of yield that of water use efficiency was measured in the treatment irrigated with the full restoration of evapotranspiration.