Interaction between nitrogen, soil water condition and herbicides in Urochloa plantaginea control in irrigated rice crop

ABSTRACT The present study sought to verify the existence of interactions between the soil water conditions, nitrogen and herbicides commonly used to control alexandergrass in irrigated rice crop. The experimental design was completely randomized, arranged in a 2 x 3 x 4 factorial scheme, corresponding to two soil water conditions (5 cm water depth and 100% soil water holding capacity [WHC]), three nitrogen doses (0, 80 and 160 kg of N ha-1) and four doses of herbicides (0, 0.5, 1 and 2 times the registration dose). The herbicides evaluated were cyhalofop-butyl, imazapyr + imazapic and imazethapyr. The treatments were repeated four times. Phytotoxicity and dry mass were evaluated 28 days after application. For cyhalofop-butyl and imazapyr + imazapic, the phytotoxicity at 100% WHC and 5 cm water depth conditions was higher in treatments with nitrogen fertilization. However, for imazethapyr under the 100% WHC condition of soil increased phytotoxicity in the absence of nitrogen fertilization. For the flooded condition, from the dose of 35 g of a.i. ha-1, the result in the control for the herbicide imazethapyr was optimized under the conditions of fertilization with 80 and 160 kg of N ha-1. Cyhalofop-butyl and imazapyr + imazapic undergo synergistic interaction with nitrogen fertilization and water status in the control efficiency, yet with imazethapyr, the synergistic interaction only occurs under 100% of soil WHC. RESUMO Buscou-se constatar a existência de interações entre condições hídricas do solo, nitrogênio e herbicidas comumente utilizados para controle de papuã no cultivo do arroz irrigado. O delineamento experimental utilizado foi inteiramente casualizado, arranjado em um esquema fatorial 2 x 3 x 4, sendo duas condições hídricas do solo (lâmina d’água de 5 cm e 100% de capacidade de retenção de água do solo), três doses de nitrogênio (0, 80 e 160 kg de N ha-1) e doses de herbicidas (0, 0,5, 1 e 2 vezes a dose de registro). Os herbicidas avaliados foram cyhalofop-butyl, imazapyr + imazapic e imazethapyr. Os tratamentos foram repetidos quatro vezes. Avaliou-se a fitotoxicidade e a massa seca aos 28 dias após a aplicação. Para cyhalofop-butyl e imazapyr + imazapic, a fitotoxicidade nas condições de 100% da capacidade de retenção de água do solo e lâmina d’água foi superior nos tratamentos com adubação nitrogenada. Entretanto, para imazethapyr na condição de 100% da capacidade de retenção de água do solo houve aumento de 45% da fitotoxicidade na ausência