ABSTRACT Late sowings anticipate flowering and decrease the soybean vegetative plasticity. These changes may limit the ability of plants to tolerate leaf area losses. The objective of this research was to evaluate the sowing date effect on soybean tolerance to defoliation at the beginning of pod formation. The experiment was set in Lages (Santa Catarina State, Brazil) during the 2016/2017 growing season. Two sowing dates were tested: November 2, 2016 (preferential) and December 15, 2016 (late). Five levels of defoliation (0%, 17%, 33%, 50% and 67%) of cultivar NA 5909 RG were imposed at the R3 growth stage. Grain yield showed a quadratic response to defoliation, ranging from 4,313 to 6,478 kg ha-1 in the preferential sowing date and from 3,374 to 4,443 kg ha-1 in the late sowing date. The plants tolerated up to 45.6% of defoliation in early sowing and 55.8% in late sowing, without yield losses, in comparison to the control. The highest level of defoliation reduced grain yield by 26.9% and 13.4% in early and late sowings, respectively, compared to the control. The delay of sowing date did not increase the sensitivity of cultivar NA 5909 RG to defoliation at the beginning of pod formation.

ABSTRACT Soybean tolerance to defoliation may be affected by population density, as the plant population interferes with the crop leaf area index . This study aimed to evaluate the effect of defoliation at the beginning of pod formation on the agronomic performance of soybean at different plant densities. The experiment was conducted under field conditions in Campos Novos, SC, Brazil, during 2016/2017 and 2017/2018 growing season. The experimental design was a randomized block in a split-plot arrangement. The main plot consisted of three densities (100,000, 300,000, and 500,000 plants ha−1), whereas the subplots consisted of the cultivar NA 5909 RG submitted to five defoliation levels (0, 16.6, 33.3, 50, and 66.6%). Grain yield ranged from 4,219 to 5,356 kg ha−1 in the 2016/2017 growing season and 3,732 to 5,186 kg ha−1 in the 2017/2018 growing season. Plant density did not interfere with the grain yield response to defoliation performed at the beginning of pod formation. The 16.6% defoliation increase grain yield by 7.5 and 5.6% relative to the control in 2016/2017 and 2017/2018 growing seasons, respectively. The soybean tolerate defoliation of up to 33.3% at R3 with no significant decrease in grain yield, regardless of the plant density.

ABSTRACT. Water deficiency reduces leaf expansion and photosynthetic efficiency, potentially reducing the ability to withstand leaf area (LA) loss. This study aimed to evaluate the effects of water restriction on soybean tolerance to defoliation in the vegetative and reproductive periods of development. Two experiments were conducted in a greenhouse located in Lages, Santa Catarina State Brazil, during the 2017/2018 growing season. Cultivar NA 5909 RG was subjected to three WR levels (none, moderate, and severe) and five defoliation levels (0, 17, 33, 50, and 67%). Defoliation occurred at the V6 stage in the first experiment and R3 in the second. WR occurred for 7 days after defoliation. LA at R2 and R5 after defoliation at V6 and R3 decreased by 27.5 and 64.6%, respectively, regardless of WR. LA between V6 and R2 was not influenced by WR or defoliation. Moderate or severe WR reduced plant ability to recover LA between R3 and R5. Severe WR decreased grain production by 22.2% in the vegetative period and 21.2% per plant in the reproductive period compared to that of the control, regardless of defoliation. The highest defoliation level reduced grain production per plant by 24.7 and 24.3% relative to the control at stages V6 and R3, respectively, regardless of WR. WR imposed at the vegetative and reproductive stages did not increase defoliation sensitivity.

RESUMEN El fraccionamiento de la fertilización nitrogenada en la superficie, con la aplicación de una parte de nitrógeno en el panojamiento, puede aumentar el rendimiento de los granos de maíz, principalmente cuando el cultivo se siembra en la fecha más favorable para obtener una alta productividad. El objetivo de esta investigación fue evaluar los efectos de la fecha de siembra sobre la respuesta del maíz al fraccionamiento de la fertilización nitrogenada en la superficie y a los estados fenológicos. Se evaluaron dos fechas de siembra: preferencial (15/10/2015 y 20/09/2016) y tardía (5/12/2015 y 5/12/2016). En cada fecha de siembra fueron analizados seis tratamientos de aplicación de nitrógeno en la superficie: testigo sin N; aplicación de dosis completa de N en V5, V10 y VT; ½ aplicación de N en V5, + ½ aplicación de N en V10; y ⅓ de la aplicación de N en V5 + ⅓ de la aplicación de N en V10 + ⅓ de la aplicación de N en VT. La dosis de nitrógeno utilizada fue de 300 kg ha-1. El rendimiento de granos varió entre 8.9 a 15.3 t ha-1 en Lages y 7.4 a 16.4 t ha-1 en Atalanta. No hubo diferencias significativas en el rendimiento de granos ni en la eficiencia agronómica del uso de nitrógeno (EUN) entre los tratamientos con aplicación de la dosis completa de N en los estadios V5 y V10 y aquellos en los que el nutriente se dividió dos o tres veces, independiente de la fecha de siembra. La EUN fue mayor en la siembra realizada al en la fecha preferencial que en la fecha tardía, independientemente del manejo de la aplicación nitrogenada en la superficie. El fraccionamiento de la fertilización nitrogenada en la superficie hasta VT no incrementó el rendimiento de granos ni la EUN del maíz.

ABSTRACT The splitting of nitrogen side-dressing fertilization with the application of a nitrogen dose at maize tasseling may increase grain yield, mainly when the crop is sown at the most favorable date to achieve high productivity. This research was carried out to evaluate the effect of sowing date on the response of maize to the splitting of nitrogen side-dressing fertilization at different growth stages. Two sowing dates were tested: preferential (10/15/2015 and 09/20/2016) and late (12/5/2015 and 12/5/2016). Six nitrogen side-dressing management treatments were evaluated at each sowing date: control without N; complete nitrogen rate applied at phenological stages V5, V10 and VT; ½ N rate at V5 + ½ N rate at V10; and ⅓ N rate at V5 + ⅓ N rate at V10 + ⅓ N rate at VT. The N rate was equivalent to 300 kg ha-1. Grain yield ranged from 8.9 to 15.3 t ha-1 in Lages and from 7.4 to 16.4 t ha-1 in Atalanta. There were no significant differences in grain yield and agronomic efficiency of nitrogen use (AE) among the treatments with the entire N side-dressing fertilization at V5 and V10 and those when the nutrient was split into two or three times, regardless of sowing date. AE was higher when the crop was sown at the preferential sowing date than in the late sowing date, regardless of N side-dressing application. The splitting of nitrogen fertilization up to the crop tasseling did not increase maize grain yield or AE.