A.W. Becker, M. Ruffo, and F.E. Below 1
Several reports of reduced weed control from tank mixes of manganese (Mn) and Roundup herbicide indicate a problem of Roundup interacting with Mn to reduce the effectiveness of the Roundup. There is also increasing evidence that the Roundup is rendering the Mn less metabolically useful within the plant. This has led to more frequent observations of Mn deficiency symptoms appearing in soybean fields, even though tissue test levels for Mn are found to be adequate. This study should provide information as to whether Mn levels in Roundup Ready soybeans can be manipulated by foliar or soil applied Mn, and whether there is an antagonism between Roundup and Mn which could lead to yield loses.
A recent review by Huber et al. (2004) indicated a trend of reduced Mn uptake and physiological efficiency by glyphosate resistant soybeans. In their studies, glyphosate immobilized Mn applied before, concurrent with, or within 6-8 days after the application of glyphosate. Applying Mn more than eight days after glyphosate application reduced the antagonism.
The objectives of this study are to 1) evaluate the effects of foliar applied Mn and soil applied Mn on yields of Roundup Ready soybeans, 2) determine the interaction of Roundup rates on Mn levels in soybean leaves and effects on soybean yields, 3) evaluate whether differences in soil pH levels affect Roundup x Mn interactions. The goal of this project is to determine if Roundup application to Roundup Ready soybeans reduces the bioavailability of Mn in soybean leaves leading to reduced grain yields.
Field sites were identified at two locations, the UI Dixon Springs Ag. Center (DS) and the UI Brownstown Agronomy Res. Center (BR). Lime treatment blocks (0 versus 4 ton/acre) were established in the spring of 2004 to create pH environments of 6.0-6.2 which would encourage higher Mn availability and pH 6.8-7.0 which would encourage lower Mn availability. Treatments identified in Table 2 were applied to soybeans grown under these two pH environments. The first group (A) of treatments (1-9) was used to determine if Mn interacts with Roundup and whether variety (Asgrow 4702 [in 2004, Asgrow 4502 in 2005], FS 4516, or Pioneer 94B74 [in 2004, Pioneer 94B54 in 2005, Pioneer 94M79 in 2006]) had any impact on this interaction. The second group (B) of treatments (10-18) was used to test to see if soybeans respond to Mn without the presence of Roundup. Hand weeding occurred on a weekly basis or as needed to insure no weed pressure. The third group (C) of treatments (19-24) when combined with treatments 7-9 was used to determine if rate of Roundup affects the interaction with Mn. Roundup rates consisted of none, 1x (22 oz/acre), 2x (44 oz/acre) and 4x (88 oz/acre) of Roundup WeatherMax®.
A split plot design was implemented with pH level as main block and treatments as subplots with three replications per location. Plot size was 10' by 30' with the center 5' x 30' harvested for grain yield. The foliar Mn was supplied as a 5% mannitol chelate (Brandt Consolidated) and was applied 3-5 days prior to application of Roundup (Pre RU). In 2005 and 2006, an additional treatment of foliar Mn applied 10 days after Roundup (Post RU) was included (on farmer fields) or replaced the Pre RU at DS and BR. The soil applied Mn was supplied as Manganese Sulfate (32% Mn, United Suppliers, Inc.) and was applied surface broadcast immediately after soybean planting.
In addition to the two locations above, three other sites were identified on farmer fields where Mn problems were thought to exist. The sites were located near the towns of Ridgway (RW) and Wayne City (WC) [in 2004, New Haven (NH) in 2005] in southern IL and Monmouth (MN) in northern IL. On these sites the four Roundup rates and three Mn treatments were used with a single soybean variety (farmer own) and there were four replications in a randomized complete block design. Further study details are presented in Table 1, below. An additional treatment of foliar Mn applied 10 days after Roundup (Post RU) was included on these farmer fields.
Soil test Mn ranged from 15 to 43 ppm across the locations (Table 1). However, it is interesting to note that the soil Mn level was significantly reduced with the application of lime at both DS and BR even though there was not a lot of reaction time for the lime since it was applied in the spring of 2004. Lime and pH changes may be associated with this effect. In general, liming of the acid soils at DS and BR reduced plant uptake of Mn (Figure 1) which occasionally led to a yield increase (Figure 2).
Roundup rate significantly affected soybean yield at three of the locations in 2004 (Table 3). At RW, the 88 oz/acre rates had the lowest yield, and at WC and DS (limed) the 44 and 88 oz rates both significantly reduced yields. The 88 oz/acre rate is twice the recommended highest labeled rate and represents a possible overlap effect, so is not likely to occupy much of the area within a field. In cases where the zero roundup rate yielded less than the 22 oz rate, it is likely that weed control was not as timely and that significant weed competition existed before handweeding. At DS, there was a significant lime x roundup rate interaction. The limed plots had significantly higher soybean yields, even with the negative effects of the high roundup rates, than the unlimed plots. This was due to the lower Mn availability in the limed plots as indicated by the lower levels of Mn in the trifoliate leaf samples. This same pH effect on leaf Mn was seen at BR, but to a lesser extent. Roundup rate had no effect on leaf Mn level.
In 2005, similar observations were made at several of the locations (Table 4). In general, the highest rate of Roundup significantly reduced yields by an average of about 3 bu/acre. Again, Roundup rate had no effect on leaf Mn levels. In 2006, only the on-farm locations had a significant yield decrease associated with only the highest Roundup rate (Table 5).
Roundup induced “flash” ratings were significantly affected by roundup rate (Tables 3-5). Ratings were scored on a 1-5 scale with 5 having no symptoms of yellowing and 1 having severe stunting and chlorosis (Figure 3). Surprisingly, the RW site showed the least visual symptoms of flash in 2004 but had the larger yield decrease associated with the 88 oz/acre roundup rate. This site also had the lowest soil test Mn level and lowest tissue Mn levels (approaching deficiency). In 2005 and 2006 only MN was rated for “flash”, however the same injury symptoms appeared at the other locations in 2005 and 2006 that were observed in 2004. At MN there was a very good relationship between the effects of Roundup rate on yield and the effects of Roundup rate on flash ratings (Figure 4).
In an effort to get a better evaluation of leaf injury (“Flash”) associated with Roundup rates, a crop canopy active sensor was used to determine NDVI (normalized difference vegetative index) on each plot. In general terms, NDVI is rated on a 0-1 scale with numbers approaching one having a higher yield potential (less damage) than lower numbers. NDVI was significantly decreased with increasing rates of Roundup at two of the locations in 2006, RW and BR. NDVI seemed to share a relationship to yield and Roundup rate similar to that of flash ratings (Figure 5).
In general, Mn treatments had no effect on soybean yield, trifoliate leaf Mn composition, or flash ratings (Tables 6-8) except for very high leaf Mn levels at several locations associated with the post Roundup foliar application of Mn. Apparently the application of Mn just prior to tissue sampling elevated the leaf levels, perhaps due to low rainfall after application not washing materials off the leaf surface. In the other situations it appears that the soil test levels of Mn were high enough that the addition of fertilizer Mn did not significantly increase plant uptake above the check treatments.
Three variety comparisons were made at DS and BR with mixed results (Tables 9-11). At DS in 2004, the FS 4516 yielded slightly higher that the other varieties, whereas at BR, the Pioneer variety 94B74 yielded slightly higher. In 2005, at DS the A. 4502 and FS 4516 yielded about equally, whereas at BR, the FS 4516 had the higher yield. In 2006, the FS 4516 had slightly lower yields at BR. There were no interactions between varieties and either Roundup treatments or Mn treatments. There also were no differences among varieties for leaf Mn levels in 2005 and only slight differences in 2004 and 2006.
The effects of Roundup rate on soybean flash and grain yields do not appear to be related to Mn level in the plant or to Mn application (Figure 6). Mn treatments had little effect on preventing flash or in preventing a loss of soybean yield associated with high rates of roundup. The additional treatment of a “rescue” application of Mn 8-10 days after the roundup application did not lead to better soybean responses, contrary to results shown by researchers at Purdue (Huber et al., 2004).
Table 1a. Study site information, 2004.
Table 1b. Study site information, 2005.
Table 1c. Study site information, 2006.
Table 2. The following treatments are applied for each rep and pH level (6.0-6.2 vs 6.8-7.0).
Table 3. Effects of roundup rate on soybean yield, leaf Mn level, and flash rating, 2004.
Table 4. Effects of roundup rate on soybean yield, leaf Mn level, and flash rating, 2005.
Table 5. Effects of roundup rate on soybean yield, leaf Mn level, and flash rating, 2006.
Table 6. Effects of Mn treatment on soybean yield, leaf Mn level, and flash rating, 2004.
Table 7. Effects of Mn treatment on soybean yield, leaf Mn level, and flash rating, 2005.
Table 8. Effects of Mn treatment on soybean yield, leaf Mn level, and flash rating, 2006.
Table 9.Variety effects on soybean yield and leaf Mn level, 2004.
Table 10. Variety effects on soybean yield and leaf Mn level, 2005.
Table 11. Variety effects on soybean yield and leaf Mn level, 2006.
Figure 1. Limestone Effects on Trifoliate Leaf Mn, 2004-06
Figure 2. Limestone Effects on Yield, 2004-06
Figure 4. Mn Study, Monmouth, 2004-06
Figure 5. Mn Study, Ridgway, 2004-06
Figure 6. Mn Study, On-Farm Locations, 2004-06
1 S. A. Ebelhar is an Agronomist at Dixon Springs Ag. Center, E.A. Adee is Principal Research Specialist at Monmouth, and C.D. Hart is a Research Specialist at Dixon Springs Ag. Center, all in the Dept. of Crop Sciences, University of Illinois.
Huber, D. M., J. D. Leuck, W. C. Smith, and E. P. Christmas. 2004. Induced manganese deficiency in GM soybeans. In R. G. Hoeft (ed.) Proc. Thirty-fourth North Central Ext.-Ind. Soil Fert. Conf. Vol. 20:80-83.