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Field Evaluation of Two Urease Inhibitors with Transplanted Lowland Rice

Int. Fertilizer Dev. Ctr. (IFDC)/Int. Rice Res. Inst. (IRRI) Coop. Project, P.O. Box 933, Manila, Philippines

S. K. De Datta, J. L. Padilla and M. I. Samson

Dep. of Agronomy, IRRI, P.O. Box 933, Manila, Philippines

^* Corresponding author.

The amendment of urea with a urease inhibitor has been proposed as a strategy to increase yield of lowland rice (Oryza sativa L.) through the reduction of N loss by ammonia volatilization. Field studies were conducted at two locations in the Philippines to determine the effects of two urease inhibitors, phenyl phosphorodiamidate (PPD) and N-(n-butyl) thiophosphoric triamide (NBPT), on N uptake and yield of rice. Urea was split applied at 16 to 18 d after transplanting (DT) and at 5 to 10 d after panicle initiation (DAPI). On a silty clay soil (Vertic Tropaquept) at Muñoz, the amendment of urea with PPD or NBFT significantly increased grain yield relative to conventional urea (P = 0.05). Agronomic efficiency for 35 kg N ha-' increased from 43 kg grain kg^–1 fertilizer-N for urea to 51 and 52 kg grain kg^–1 fertilizer-N for urea + 20 g PPD kg^–1 and urea + 9 g NBPT kg^–1, respectively. Both PPD and NBPT increased grain N, averaged for four non-zero N rates, by 5.7 and 7.8 kg ha^–1, respectively. The increased uptake of N with the inhibitors occurred prior to 35 DT; subsequent N uptake was not affected. On a clay soil (Andaqueptic Haplaquoll) at Pila, the addition of inhibitors failed to give significantly greater grain yields than unamended urea in each of the three experiments. However, the quadratic response models for urea and urea + PPD obtained from combined results for the three Pila experiments were significantly different (P = 0.01). Amendment of urea with PPD increased agronomic efficiency by 4 kg grain kg^–1 urea-N. Physiological efficiency (kg grain kg^–1 N uptake) was not significantly affected by the inhibitors, which suggests that an increase in N uptake with the inhibitors resulted in a corresponding yield increase. This study indicates that urease inhibitors can increase grain yield of lowland rice, but the yield increases with inhibitors might be small.

Received for publication September 17, 1987.