Ramsey County

 DON’T FERTILIZE YOUR WEEDS

The following was written by Dr. Chris Boerboom, Weed Scientist, U of WI.

        Considering the cost of nitrogen, we can ill afford to allow weeds to use this expensive input. We conducted an experiment for the past 2 years that provided results that truly surprised us in regard to weeds and nitrogen. Basically, we confirmed that weeds are highly effective consumers of nitrogen. As a consequence, the potential exists for weeds to capture some of the nitrogen that the corn needs for optimum yields. We just didn’t expect the effect to be so large. In term of competition, the biggest risk would seem to be when weeds are allowed to grow too large before being controlled postemergence. Therefore, our experiment was designed to determine how delayed weed control might affect optimum nitrogen rates in corn.

        In the experiment, we applied increasing rates of nitrogen over the range of 0 to 200 lb/a. Within these increasing nitrogen rates, we controlled weeds either preemergence, at a recommended 4-inch height, at a 12-inch height, and a weedy check. Weeds were controlled postemergence with glyphosate in the Roundup Ready corn. The 12-inch height was used to mimic a time delay that might occur because of rainy or windy weather or other delays. The time difference between the 4-inch height and the 12-inch height was 8 days in both years.

A few key findings:

~When weeds were sprayed at 4 inches, they had accumulated an average of 12 lb/a N. However, by 8 days later, they had accumulated an average of 25 lb/a N when they were 12 inches tall.

~The nitrogen uptake by the weeds reduced the nitrogen that the corn was able to accumulate. Corn had 15 lb/a less nitrogen (or 18%less) at tasseling when weeds were controlled at 12-inches tall as compared to when weeds were controlled preemergence.

~Surprisingly, corn yields were similar when 200 lb/a N was applied, regardless of whether the weeds were controlled preemergence or when they were 12-inches tall. How could that be? Apparently, we replaced the nitrogen that the weeds robbed from the corn early in the season to regain this yield. (When the weeds were controlled preemergence, the corn had excess nitrogen that it did not use efficiently.) The economic optimum nitrogen rate was at least 100 lb/a lower when weeds were controlled preemergence or by 4-inches tall compared to when weeds were controlled at 12-inches (Table 1).

Table 1. Economic optimum nitrogen rates in corn with four weed management treatments at a 0.15 nitrogen:corn price ratio.

~ Corn yield loss averaged 15 bu/a or 7% in 2006 and 25 bu/a or 11% in 2007 when weeds were not controlled until they were 12-inches tall compared to the preemergence timing. This yield loss would not be acceptable at $2/bu corn and certainly would be outrageous at $5/bu corn. The bulk of this yield loss must have been caused by competition for nitrogen.

~ At the end of the season, the best weed control (or fewest weeds) may not always provide the best corn yield. In 2006, the plots sprayed at the 12-inch timing had the fewest weeds at the end of the season, but not the highest yield. Remember, the goal of weed management is to protect crop yield.

 PHOSPHATE - TO BUILD OR NOT TO BUILD

        If Hamlet farmed today and the soil test recommended phosphate, no doubt he would ask that question. I took a break from my nitrogen calibration work last weekend to pour over reams of papers and data from NDSU, Canadian and corn-belt studies on various crops. NDSU work, along with that of the USDA in Culbertson, MT and Mandan, are quite extensive in wheat. Taking those calibration equations developed by Halvorson, Grant Jackson and Black, I inserted today’s costs of phosphate of about $1/lb P2O5, and $8 wheat into them. The following graph illustrates that building P at this time is not a wise decision.

 Unless the P test is less than 7 ppm, the economics do not support building P test levels even when wheat is $8/bu with P costs of $1/lb. If MAP costs drop to $600 (P cost about 50 cents/lb P2O5), then building low P tests to about 10 ppm makes “cents” again.

        I also reviewed the ND and Canadian literature on broadcast vs banded P for wheat, and clearly there is at least a 2-1 advantage in efficiency, with banded having the distinct edge. If you request University recommendations from your lab du jour this fall, you will be presented with a recommendation for P based on broadcast rates. With P costs at current levels, my recommendation is to band, either with seed, or near seed, and reduce these rates by one-half.

        For corn, there is also an efficiency to banding, but probably not one-half. The biggest mistake a grower may make is not requesting University recommendations. Differences between “industry” and “University” recommendations for corn can be huge, with little difference in resulting yield when using either one. Remember, we live in North Dakota, not the central corn-belt. Reducing these rates by one-third would be wise with current cost/price structure. Banding for corn will require either a strip-till-style system of about 2 inches below the projected seed zone, or a 2X2 band planter set-up.

        For soybeans, broadcast P with rates not cut appears profitable for $10 or higher soybeans and low P levels. Once P test hits about 10 ppm, there is no profitable response to P in soybeans.

Dave Franzen
NDSU Extension Soil Specialist

 TIME TO SCOUT FOR RED SUNFLOWER SEED WEEVIL

        Red sunflower seed weevil are emerging and can be found in R4 and early blooming sunflower fields in the major sunflower producing areas. The map below illustrates the “hot spots” of the red sunflower seed weevils from the 2007 Sunflower Survey in North Dakota (sponsored by the National Sunflower Association). Last year, populations were lower than 2006 and sunflower fields in the south central, central, east central and north central regions had high levels.

Identification:  Adults are small, about c inch and reddish-brown in color. Newly emerged adults feed on the bracts, sunflower buds, and pollen. Peak emergence is usually in late July and early August. The female seed weevil must feed on pollen for fertile egg development.

 Scouting:  Sunflower normally reaches the bud stage in late July at which time only about 30 percent of the weevils in the soil have pupated and emerged. Most weevils emerge from the soil by the first week of August. Field scouting for adults should begin when plants are showing yellow ray petals (R5.0) to 30% of the head shedding pollen (R5.3), and should continue until most of the plants have reached 70% pollen shed (R5.7). A plant that has reached R5.7 has few seeds still suitable for red seed weevil egg laying and should no longer be susceptible to further significant damage.

  Seed Weevil Damage:  Research reveals that most seeds are only partially consumed or destroyed by the seed weevil larvae (see photograph) and that damaged seeds have lower oil content than undamaged seeds. The economic loss caused by larval feeding includes the loss of both seed weight and oil content.

 Economic Threshold:

Oilseed Sunflower . . . The threshold can be calculated using the following formula:

Threshold (Weevils per head) = Cost of Insecticide Treatment divided by (Market Price x 21.5) (0.000022 x Plant Population + 0.18)

Confection sunflowers economic threshold is only one weevil per head

NDSU Extension Service, North Dakota State University of Agriculture and  Applied Science, and U.S. Department of Agriculture cooperating.  Duane Hauck, Director, Fargo, North Dakota.  Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914.  We offer our programs and facilities to all persons regardless of race, color, national origin, religion, sex, handicap, age.  Vietnam era veterans status, or sexual orientation; and are an equal opportunity employer.

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Devils Lake  ND  58301
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