By Amber Moore, Don Morishita & Oliver Neher*
Specifically, growers are concerned that the areas with little residue will be droughty and more susceptible to weed growth, while areas with heavy residue coverage may have more fertilizer and herbicide binding in the residue — and more soil-borne disease pressure under a cooler, more-moist and higher-carbon soil environment.
Surface-applying nitrogen fertilizers increases the potential for volatilization losses (conversion of fertilizer ammonium to ammonia gas) and binding with surface residues. Shanking is effective for avoiding these issues, but it may be costly for the grower to outfit tillage equipment for fertilizer applications. Using recommended fertilizer rates may also be problematic for shanking, as the concentrated band of nitrogen can potentially burn roots.
To address these concerns, we developed a study with varying amounts of barley residue cover, nitrogen application methods and applied nitrogen rates for sugarbeet production.
Crop residue levels in this study ranged from 0.8 to 7.7 tons/ac. Nitrogen application methods used were: (1) broadcast granular urea fertilizer without incorporation; (2) shanked liquid urea-ammonium-nitrate (UAN) to a depth of 4.0 inches using strip-tillage equipment; and (3) no applied nitrogen.
A four-row Strip Cat implement by Twin Diamond Industries was used for fertilizer application and seedbed preparation. Nitrogen application rates of 71 and 142 lbs N/ac were based on an application goal of 4.0 and 6.0 lbs of nitrogen per ton of beets.
Averaged across residue levels, beet yield did not increase significantly with added broadcast urea up to 142 lbs N/ac (Table 1). However, emergence and beet count decreased significantly while beet weight trended higher with increasing broadcast urea rates — suggesting that seed burn caused by contact with granular urea prevented seed germination. The surviving plants in the broadcast urea plots increased in beet weight with higher fertilizer rates, suggesting that if the seeds can make it past germination, they do very well.
Plants receiving 71 lbs N/ac as shanked UAN increased in yield from 25.8 to 32.9 tons/ac. However, yields decreased to 27.2 tons when rates were increased to 142 lbs N/ac (Table 1). It’s likely the concentrated UAN in the soil is burning the roots of the plant, thus lowering beet counts and beet weights.
Excluding controls, stunting rates also were highest for the high-shank N treatment (data not shown).
Sugar content was not affected by application method, but it did drop significantly as nitrogen rates increased (Table 1).
At a depth of 0-12 inches, soil bulk density did not appear to be affected by residue level. This was expected, as tillage effects often take several years before impacting soil density.
Gravimetric and volumetric soil moisture content trended 1% higher in mid-April for high-residue plots (3.9-7.7 tons/ac) compared to low-residue plots (0.77-1.71 tons/ac), but showed no effect in mid-October. This suggests that greater residue cover prevents some evaporation of soil water earlier in the season, likely prior to row closure.
At the 3.0-inch depth, maximum soil temperatures appeared to decrease from 83 to 81 degrees and minimum temperatures increased from 44 to 46 with increasing residue levels (0.8-7.7 tons/ac), indicating that greater residue coverage has a buffering effect on soil temperature. Average soil temp did not appear to change with residue level.
No stand reduction or root diseases caused by soil-borne pathogens were observed during the growing season. The absence of soil-borne pathogens was probably related to water management. The plots were irrigated at optimum levels and not over-irrigated, which would have increased disease potential.
Also, no effects of residue levels and weed densities in regard to increased pathogen pressure or disease occurrence were observed. Seedling emergence was uneven and stretched out over multiple weeks, and was probably more related to the shank treatment than to soil-borne pathogens or residue levels. As noted, the plots were strip tilled when the soil was wetter than it should have been. This sometimes left a visible crack in the soil three to four inches deep, where some of the seeds were deposited during planting.
Weed control was not significantly affected by crop residue level, nitrogen rate or application method (broadcast versus shanked). Evaluated weed species included common lambsquarters, redroot pigweed, kochia, common mallow, annual sowthistle, green foxtail and barnyardgrass.
The study area was overseeded in late fall with Russian thistle, hairy nightshade and all of the species previously mentioned (with the exception of annual sowthistle and common mallow). Though the study site was sprayed with glyphosate in the fall to control volunteer wheat, common mallow that had survived through the summer was not controlled, and we anticipated problems controlling this weed the following spring.
Glyphosate was applied three times at 22 fl oz/ac (0.75 lb ae/A) with ammonium sulfate at 17 lbs/100 gallons of spray mixture (3.5 lbs/ac). The first glyphosate application was made on May 23 when the crop was in the two-leaf stage. Subsequent applications were made June 3 and June 30.
Weed seedling emergence was counted on June 23 to see if any differences in weed populations by species and density could be seen among the crop residue levels and nitrogen application rates and methods. The only differences observed in weed seedling emergence was with redroot pigweed, i.e., redroot pigweed densities were higher in the low-straw-residue treatments compared to the high-residue treatments.
Plans are underway to repeat this study again in 2010 to further evaluate the interactions between residue cover and nitrogen application in a strip-till system.
* Amber Moore is extension soils specialist, Don Morishita is extension weed scientist, and Oliver Neher is extension
sugarbeet specialist/plant pathologist at the University of Idaho-Twin Falls.