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 objectives were to determine the effect of urea placement and two nitrification inhibitors—nitrapyrin [2-chloro-6 (trichloromethyl) pyridine] (NP) and dicyandiamide (DCD)—on aerial dry matter and N accumulation. A 4-yr field study was conducted on a Renfrow silt loam soil (fine-silty, mixed, thermic Udertic Paleustoli) silt loam near El Reno, OK. The seven treatments included urea fertilizer (60 kg N ha⁻¹) with and without the two inhibitors, surface broadcast or banded 3 to 4 cm below the seed row, and an unfertilized control. Aerial dry matter accumulation during autumn was higher for banded urea plus DCD (22%) and broadcast urea plus DCD (25%), than for broadcast urea without an inhibitor. Whole plant N content during autumn also was higher for banded urea plus DCD (28%) than for broadcast urea without an inhibitor. Neither inhibitor nor placement affected total dry matter accumulation measured at tillering, but differences were noted at spike emergence, gainful, and final harvest. At spike emergence in 1991, when climatic conditions were favorable for early season wheat growth, N content was greater for broadcast urea plus nitrapyrin than for broadcast urea or banded urea without inhibitor. At final harvest, both DCD treatments exhibited greater N content than all other N treatments. In 1993, when dimatic conditions were favorable for leaching, DCD was effective in slowing nitrification and increasing N uptake, regardless of fertilizer placement. Our results suggest that fall application of DCD with urea can potentially increase autumn forage production and N content, thus prolonging the grazing season or accommodating increased stocking rates.