Urea placement and nitrification inhibitor effects on growth and nitrogen accumulation by no-till winter wheat

Reduced forage production, grain yield, and visual N deficiency symptoms in no-till winter wheat (Triticum aestivum L.) prompted this study. Our objec tives were to determine the effect of urea placement and two nitrification inhibitors-nitrapyrin [2-chloro-6 (trichloromethyl) pyridine] (NP) and dicy andiamide? (DCD)-on aerial dry matter and N accumulation. A 4-yr field stud y,vas conducted on a Renfrow silt loam soil (fine-silty, mixed, thermic Ude rtic Paleustoll) silt loam near El Reno, OK. The seven treatments included urea fertilizer (60 kg N ha(-1)) with and without the two inhibitors, surfa ce broadcast or bended 3 to 4 cm below the seed row, and an unfertilized co ntrol. Aerial dry matter accumulation during autumn was higher for banded u rea plus DCD (22%) and broadcast urea plus DCD (25%), than for broadcast ur ea without an inhibitor. Whole plant N content during autumn also was highe r For banded urea plus DCD (28%) than for broadcast urea without an inhibit or. Neither inhibitor nor placement affected total dry matter accumulation measured at tillering, but differences were noted at spike emergence, grain fill, and final harvest. At spike emergence in 1991, when climatic conditio ns were favorable for early season wheat growth, N content was greater for broadcast urea plus nitrapyrin than for broadcast urea or banded urea witho ut inhibitor. At final harvest, both DCD treatments exhibited greater N con tent than all other N treatments. In 1993, when climatic conditions were fa vorable for leaching, DCD was effective in slowing nitrification and increa sing N uptake, regardless of fertilizer placement. Our results suggest that fall application of DCD with urea can potentially increase autumn forage p roduction and N content, thus prolonging the sating season or accommodating increased stocking rates.