Dennis D. Warnes
Agronomist, University of Minnesota
West Central Experiment Station, Morris
Robert N. Andersen
Research Agronomist,
Agr. Res. Service
U. S. Dept. of Agri., St. Paul
Robert E. Stucker
Professor, Department
of Agronomy & Plant Genetics
University of Minnesota, St. Paul

Metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] is used for weed control in soybeans and potatoes. Sugarbeets, which are very susceptible to metribuzin, are grown in the same cropping area as are soybeans and potatoes in Minnesota. Type of fall tillage affects persistence of some herbicides into the year following application (1, 2, 4). High soil pH levels increase the chance of soybean injury from metribuzin (3). Overlapping with herbicide application equipment results in doubling of herbicide rates.

We studied the effect of applying high rates of metribuzin to soybeans. We also determined the effects of these applications to succeeding crops, particularly sugarbeets, as influenced by level of soil pH and kind of fall tillage.

Materials and Methods

1975. A field on the West Central Experiment Station at Morris, Minnesota, in which the soil pH varied from 7 to 8 was selected. The soil graded from a Hammerly clay loam, to a McIntosh silt loam, to a Winger silty clay loam. A total of 65 plots (each 15' by 30') was established. Soil samples were collected May 24, 1975, from each plot and later tested for pH. Five treatments (metribuzin at 0, 1.0, 1.5, 2.0, and 2.5 lb/A) were applied May 27, 1975,with a small tractor sprayer and a 15-ft boom at a volume of 22 gal/A. Treatments were assigned to plots at random. After these treatments were applied,1 lb/A of trifluralin (a,a, a-tri=fluoro-2, 6-dinitro-N,N-dipropyl-p-toluidine)was applied to the entire area and incorporated by tandem discing twice (in two directions). Final seedbed preparation was with a spike-toothed harrow. 'Evans' soybeans were planted in rows 30 inches apart on May 27, 1975. Soybean injury index ratings were taken in early July of 1975. Because of differences in rate of maturity, soybeans were harvested September 22 from check plots, October 6 from plots treated with 1 lb/A of metribuzin, and October 13 from all remaining plots (following a killing frost on October 6). The entire area was disced on October 16, 1975, and on October 22, half of each plot was plowed with a moldboard plow.

1976. Several days before planting, the seedbed was prepared on the entire area with a field cultivator and a spike-toothed harrow. On May 28, 1976, the following assay crops were planted across all plots: 'Dal' oats, 16 rows, 6 inches apart; 'Evans' soybeans, four rows, 12 inches apart; and 'American Crystal Hybrid B' sugarbeets, four rows, 12 inches apart. Injury index ratings for all crops were made July 13, 1976. The entire area was plowed with a moldboard plow on October 15, 1976.

1977. Several days before planting, a seedbed was prepared in the same manner as in 1976. On May 18, 1977, the entire area was planted to sugarbeets. On May 30, 3.62 inches of rain and hail in one hour caused a hard crust on the soil, which prevented complete emergence. Hence, on June 20, 1977, the entire area was replanted to sugarbeets. Many times during the summer each plot was examined for herbicide injury.

Injury Index Calculation. In all cases, injury index was calculated from visual observations using the following formula:

Injury Index = % stand reduction + (% stand remaining x % injury of plants remaining) x 100.

In all years crop injury from metribuzin was based on comparisons with the trifluralin treated check plots; rainfall in 1975 was sufficient to eliminate any carry-over effects of trifluralin on the assay crops grown in 1976.

Statistical Treatment. If the soil pH categorization of plots is ignored, the statistical design for this experiment was completely random with unequal replication. Since the pH of each plot was considered as a treatment variable, the experimental units were logically suited for regression analysis within each metribuzin rate, with the pH measurement as the independent variable and yield or injury indices as dependent variables. Differences among metribuzin treatments were evaluated using pH observations as a form of replication. The unbalanced number of observations prohibited calculation of a pH level by metribuzin rate interaction. The number of plots in each pH-metribuzin combination is shown in Table 1; soil pH values were grouped into three categories to facilitate presentation of data. Because of extreme responses to metribuzin, heterogeneity of errors occurred for the metribuzin rate treatments. Means are presented with their respective standard errors.

Results and Discussion
The rates of metribuzin (0, 1.0, 1.5, 2.0, and 2.5 lb/A) in this experiment are realistic in that metribuzin may be applied at rates of 0.25 to 0.75 lb/A for soybeans and 0.25 to 1.0 lb/A for potatoes. The highest rate used on potatoes is the lowest rate in this experiment. A doubled rate from overlapping by application equipment for either soybeans or potatoes could result in rates within the range of those used in this experiment.

Precipitation and temperatures for the period of the experiment are presented in Table 2. Annual temperatures were near average throughout the experiment. However, the spring months of 1976 and 1977 were warmer than the 90-year average. After the application of metribuzin in late May of 1975, there were more than 15 inches of precipitation during the 1975 growing season. This was more than the long-time average and would likely have favored metribuzin dissipation. A severe drouth started in the fall of 1975 and continued through 1976. Only 9.39 inches of precipitation were received in the entire year of 1976, a condition that would likely have been very unfavorable for dissipation of metribuzin.

In 1975, soybean injury (Table 3) significantly increased with rate of metribuzin. With the exception of the 0 and the 2.5 lb/A rates of metribuzin, injury increased with increase in soil pH when the entire range of pH was considered; for the 0 rate, there was no injury and, for the 2.5 lb/A rate, injury was so severe that any effect of pH would have been impossible to see. Soybean yields (Table 3) were drastically reduced at all rates of metribuzin compared to the check. The loss in yield based on the check, 1.0, and 1.5 lb/A of metribuzin was remarkable linear (r = 0.99), and yields approached zero but at the higher rates. Excluding the check and high rate of metribuzin, the reduction in yield with increasing pH was significant within metribuzin rates. For example, at 1.0 lb/A soybean yield was 30 bu/A at pH 7.0 to 7.3, but only 5 bu/A at pH 7.8 to 8.0.

In 1976, injury index data were taken on three assay crops (Table 4). Metribuzin injury to soybeans and oats tended to increase with increasing rates of metribuzin applied in 1975, but injury was not severe in any case. No consistent effect of soil pH on metribuzin injury was observed for soybeans and oats, and the difference between 1975 fall plowing and fall discing was negligible. Sugarbeets, however, suffered considerable injury in 1976 from metribuzin applied in 1975. Injury increased with increasing rates of metribuzin. Fall discing in 1975 resulted in significantly greater sugarbeet injury in 1976 than did fall plowing in 1975. This result is similar to that with trifluralin where plowing has reduced carry-over problems when compared to discing (2). Increasing soil pH tended to increase sugarbeet injury in most cases (Table 4). However, regression computations on individual plot values failed to detect a significant effect of pH.

In 1977, only sugarbeets were planted to assay the metribuzin treatments applied in 1975. Even though sugarbeets are highly susceptible to metribuzin, no injury was detected.

Summary and Conclusions
In 1975, metribuzin was applied as a preplanting soil-incorporated treatment at 0, 1.0, 1.5, 2.0, and 2.5 lb/A in combination with trifluralin at a constant rate of 1 lb/A. Soil pH in the experimental area ranged from 7 to 8. All rates of metribuzin injured soybeans and reduced yields. Injury and yield reductions increased as soil pH increased; for example, at 1 lb/A of metribuzin, soybean yield was 30 bu/A at pH 7.0 to 7.3, but only 5 bu/A at pH 7.8 to 8.0.

In the fall of 1975, half of each plot was plowed with a moldboard plow and half was disced. In the spring of 1976, soybeans, oats and sugarbeets were planted to assay metribuzin persistence. Sugarbeets were injured much more than soybeans or oats. Sugarbeet injury tended to increase as soil pH increased. Sugarbeet injury in 1976 was significantly greater following fall discing in 1975 than following fall plowing in 1975.

The entire area was plowed with a moldboard plow in the fall of 1976. In the spring of 1977, the entire area was planted to sugarbeets. No sugarbeet injury was detected in 1977 from any metribuzin treatment applied in 1975.

(Cooperative investigations of the Minnesota Agricultural Experiment Station and the Agricultural Research Service, U. S. Department of Agriculture. Published with the approval of the Agricultural Experiment Station, University of Minnesota, St. Paul, as Scientific Journal Series 10, 112 in the North Central Weed Control Proceedings.)

Literature Cited

Burnside, 0. C. 1974. Trifluralin dissipation in soil following repeated annual applications. Weed Sci. 22:374-377.

Fink, R. J. 1972. Effects of tillage method and incorporation on trifluralin carry-over injury. Agron. J. 64:75-77.

Ladlie, J. S., W. F. Meggitt, and D. Penner. 1976. Effect of pH on metribuzin activity in the soil. Weed Sci. 24:505-507.

Menges, R. M. and S. Tamez. 1974. Movement and persistence of bensulide and trifluralin in irrigated soil. 1974. Weed Sci. 22:67-71.

1977 Sugarbeet Research and Extension Reports. Volume 8, pages 64 - 69.