Winter Annual Weeds Effect On Nitrogen Dynamics in No-till Corn.

Management of winter annual weeds (WAWs) is a concern to many no-till producers.  The objectives of this study were to examine how various dates to control WAWs may affect soil N, water, and corn response under no-till conditions.  Field research was conducted in 2010 at seven sites in Kansas on rainfed no-till corn-soybean systems. Timing of WAW control had four treatments; fall, early pre-plant (2-4 weeks prior to planting), pre-emergences (within 1 week of planting), and post-emergences (V2). Corn planting dates were 12 to 20 April 2010.  The second factor, nitrogen, had five treatments: 0, 13, 27, 53, 107 kg N ha^-1.  Locations were selected based on previous history of a high density of WAWs.  Two sites were excluded from this analysis due to loss of treatment timings of WAWs in relation to planting dates.  N uptake by WAWs ranged 9 to 25 kg N ha^-1 with a C:N ratio of 16 to 30 if left until maturity in early to mid May in 2010.  Soil nitrate-N in early June was not significantly affected by timing of control.  However, early corn growth (V5-V8) was significantly increased by early pre-plant control over pre-emergence control of WAWs.  Early growth was significantly increased with fall and early pre-plant control over post-emergence control.  Early growth of corn was significantly increased by added nitrogen, but there was no significant increase with the addition of more than 13 kg N ha^-1 over the control.  Gravimetric water content averaged from 0-90 cm, taken in early June when corn was V5-V8, was slightly lower with the fall and early pre-plant compared to post-emergence control. This suggests any water use by WAWs can be alleviated when early season rainfall is above average, as was in 2010.  Chlorophyll meter readings at R1-R2 with post-emergence control were significantly lower than with early pre-plant control suggesting N uptake by WAWs could significantly impact reproductive growth stages if controlled after corn emergence. In 2010, early pre-plant burndown of WAWs appears to maximize early growth and N uptake in no-till corn.  Additional responses and variables are still being assessed.