Illinois
Fertilizer Conference Proceedings
January 24-26, 2005
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S. A. Ebelhar, E. C. Varsa, and C. D. Hart
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.
Soil pH affects Mn uptake because of the competition for uptake with Mg and Ca. Therefore, high soil pH may result in Mn deficiencies. Long-term no-tillage systems are known to cause nutrient stratification, including pH, where high pH levels are formed in the upper few inches of the soil profile. This could create an environment of limited Mn availability for uptake.
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:
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 1 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, FS 4516, or Pioneer 94B74) 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 ©) 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, 1× (22 oz/acre), 2× (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' × 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. 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 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.
Soil test Mn ranged from 15 to 43 ppm across the locations ( Table 1). This is still much higher than the critical soil test level of 1 ppm in the UI Agronomy Handbook. 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. There was a very high negative correlation between the soil Mn level and yield (Figure 1). This may only be a coincidence, but does indicate the possible toxic effect of Mn if high Mn levels are present in the soil. High soil test Mn levels led to high levels of Mn of the trifoliate leaf samples taken at early pod fill (after Mn and roundup treatments were applied, Figure 2).
Roundup rate significantly affected soybean yield at three of the locations ( 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 ( Table 3). 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.
The was also a significant roundup rate by Mn treatment effect at DS (Figure 3). Both the foliar and soil applied Mn treatments had higher soybean yields at the high roundup rates, compared to the check (no Mn) treatment.
Roundup induced “flash” ratings were significantly affected by roundup rate (Table 3). Ratings were scored on a 1-5 scale with 5 having no symptoms of yellowing and 1 having severe stunting and chlorosis (Figure 4). Surprisingly, the RW site showed the least visual symptoms of flash 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).
Mn treatments had no effect on soybean yield, trifoliate leaf Mn composition, or flash ratings (Table 4). Apparently 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 (Table 5). At DS, the FS 4516 yielded slightly higher that the other varieties, whereas at BR, the Pioneer variety 94B74 yielded slightly higher. The effect of lime treatment on soybean yields was not significant at either location. The lime treatment did significantly reduce trifoliate leaf Mn concentrations at both locations.
After the first year of study, it appears that although roundup rate had some effects on flash and soybean yields, there is no direct correlation between flash and yield (Figure 5). Mn treatments had little effect on preventing flash or in preventing a loss of soybean yield associated with high rates of roundup. Future plans include at least one additional treatment to determine if a rescue application of Mn 8-10 days after the roundup application might lead to better soybean responses, as shown by researchers at Purdue (Huber et al., 2004).
Table 1. Study site information, 2004.
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 Mn treatment on soybean yield, leaf Mn level, and flash rating, 2004.
Table 5. Variety effects on soybean yield and leaf Mn level, 2004.
1 S. A. Ebelhar is an agronomist, Dept. of Crop Sciences, University of Illinois, Dixon Springs. E. C. Varsa is a professor, Southern Illinois Univ., Carbondale. C. D. Hart is a senior research specialist, Dept. of Crop Sciences, University of Illinois, Dixon Springs
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.
