Efectos de la fecha de siembra sobre el rendimiento de soja en condiciones de riego y secano

In the 2021-2022 crop year, a trial was conducted to evaluate the effect of sowing date on a soybean cultivar (Glycine max, L.) with a V short maturity group, which presents a high yield potential under irrigated and rainfed conditions. The trial was done at the Experimental Station “Dr. Mario A. Cassinoni” (EEMAC), Faculty of Agronomy, in Paysandú, Uruguay. The evaluation consisted of three sowing dates, October 9, November 10 and December 11, each under irrigated and rainfed conditions. It was carried out in a design of complete random blocks in divided plots, where the three sowing dates were the major plots, while the water availability (irrigated and rainfed) were considered as minor plots, which were randomly arranged in the major plot. Irrigation management was carried out with a watering threshold of 60% of available water during the critical period and 40% of available water in the non-critical stages, thus maintaining the crop under water welfare conditions. Four replications were carried out with six treatments in each, distributed in 24 experimental units. Irrigated yields were higher than in rainfed treatments at each sowing date. Under irrigation the December sowing date yielded 5016 kg ha-1, followed by the November sowing date with 4701 kg ha-1 and finally the October sowing date with 4277 kg ha-1, there were differences between the December and October sowing date. There were no differences in the number of grains per square meter, but the grain weight did differ, with the December sowing date having the highest value, followed by November and finally October, part of which explains the differences in yield. The high average and maximum temperatures to which the early sowing date were subjected, added to the effect of solar radiation, which was lower during the critical period, meant that the early sowing date could not express their potential. Both the October and November sowing date transformed less efficiently the dry matter production to grain production, being December the most outstanding in this point. In rainfed conditions, the December sowing date stood out over the other sowing dates, this performance is mainly explained by water availability and by the escape to the high atmospheric demands in the critical period. In conclusion, the early sowing dates placed the critical period in more favorable situations to obtain higher yields; however, the year presented solar radiation levels and temperatures that negatively affect the achievement of yield.