Terry Greqoire. Area Extension Agronomist

Devils Lake, North Dakota

Herbicide carryover from this drought season is going to present potential problems for next season crop choices and on herbicide applications this fall.

Persistence of herbicides are influenced by cultivation practices. soil temperature. moisture conditions. soil type and amount of herbicide applied.

Degradation or breakdown of Dinitroaniline herbicides proceeds more rapidly under flooded. anerobic conditio ns (no oxygen) than under aerobic conditions (oxygen available) .

Pendimethalin, Profluralin, Trifluralin, (examples of Dinitroaniline herbicides). were equal in persistence when applied under no-tillage conditions and were found to be most persistent in a silty clay and least persistent in a sandy loam.

Persistance of Dinitroanilines in a sandy loam at 75% field capacity (FC) is longer at cooler soil temperatures. The half-life was 98 days at 86o Fahrenheit and 409 days at 50 degrees Fahrenheit. At 77 degrees Fahrenheit the half-life also increased when soil moisture declined. In seven different soils there was a trend toward a slower rate of loss as soil organic matter content increased.

In a study with Prowl herbicide, (Figure 1), it was shown that under dry warm soil conditions (25'~ of field capacity). about 604. of the amount applied was still in the soil 180 days after application. In contrast only 15-20% remain when the soil was moist (75% of field capacity)

A study with Trifluralin, (Figure 2). in moist and air dry soils showed that once the soil was air dry very little breakdown occurred which was very similar to the Prowl study. In moist soil Prowl degraded more rapidly than Trifluralin which is similar to our normal field experiences. It took about 80 days with moist soil to breakdown 80% of applied Prowl and 110 days to breakdown 85% of applied Trifluralin. It would appear that one could estimate the amount of Prowl or Trifluralin remaining this fall by a estimating a 1% loss for each day of moist soil above a soil temperature of 50 degrees Fahrenheit .

In North Dakota this year many soil moistures were at 25% or less field capacity for the whole season. The above data and previous experience in North Dakota tells us that Dinitroanilines will most likely carryover into next spring in higher than normal amounts.

Several questions arise.

• What about additional Trifluralin this fail?
• What about Fargo applications this fall?
• What about spring applications of Far-go and Trifluralin?
• What about Buckle applications?

A bioassay should be performed this fall on soils that could have a carryover problem. Sample soil from several areas in the treated field and a non-treated field. Plant seeds of intended crop in each soil. Moisten the soil and keep outdoors unless freezing temperatures occur. Observe after two weeks for growth. If injury occurs use caution in applying additional herbicide this fall. Question if additional herbicide is necessary.