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Soybean Diseases in Arkansas
Asian Soybean Rust - Questions and Answers - County Agents
Early Planting, Early Maturing Soybean Cultivars, Widers Rows and soybean Rust

1. What are the PROS and CONS of planting MG 3 and MG 4 soybean cultivars at the earliest possible planting date?

Pros:

The basis for the early planted soybean production system is a productive way for many producers on marginal land without irrigation capabilities to perhaps avoid the hot dry periods of summer and still achieve higher yields. This is achieved by planting mainly MG IV soybeans during the latter part of April and early-May. Lately some have experimented with late MG III lines and the same principle applies. What we are essentially doing is shifting the reproductive period to a time in the year where we are most likely to receive a rainfall event and possibly milder nighttime temperatures. Seed companies have responded to this production system with as many as 150 to 175 different late MG III and MG IV lines being tested each year. The success of this program has spread to other states and Mississippi is probably the state to adopt this production system quicker and over a large percentage of their acres. Each year, Arkansas typically plants 25-30% (750,000 to 900,000 acres) of the soybean crop by May 15. Here are some data from this year that show the influence of planting date on yield. The MG IV's (Early 4 and Late 4) yields are higher the earlier we plant (April and somewhat in May).

These data support our recommended planting dates and the maturity groups used for those planting periods. Historically, the earlier you plant, yields are higher when compared to later planting dates. There can be consequences to early planting as well.

Cons:

As with any agricultural system, too much of a good thing can, at times, be bad. This was the case last year and could possibly be this year. Despite what some say in the popular press, you can plant MG III and IV's too early. This was the case this year in Phillips, Lee, and other counties in Arkansas . The way I explained it to producers is like this. Let's say a maturity group 4.4 ranges from 125 to 135 days (from planting to physiological maturity). By planting earlier than the first week of April, we increase our chance of early-season stress (meaning we have a plant that is subject to harmful environmental conditions). This can be from cool, wet soil conditions leading to seedling disease, or possibly delayed emergence and/or worst delayed vegetative growth due to less than optimum temperatures. We still only have 125 to 135 days and if we stress this crop too early and for too long, we lose days from our total. These conditions lead not only to reduced yields, but also shorter plants that can lead to harvest problems (late-season weed interference at harvest and shorter plants are harder to harvest with combines).

The reason we have our planting date recommendations is because we feel this is the earliest our growers should plant based on the historical risk of cool conditions while still taking advantage of the chance to receive “milder weather conditions” during the reproductive stages.

However, I don't let the calendar totally rule out recommending a grower plant before April 15. This past spring is a good example. We were experiencing soil temperatures in the 55-65 degree range the last of March and first of April. Based on this, we planted a number of acres (approx. 250,000) by April 9. But some planted as early as March 7 or 10. These fields resulted in shorter stunted plants and yields were off as much as 15-20% compared to the same variety planted 2 weeks later. This is not only for MG III's and IV's, we see this same trend also with the V's. This is some data from this year:

There are exceptions to every rule. I know some operations must start this early in order to get the entire crop planted. Field conditions may also influence this as well. “ This may be the only chance we can get a field planted” is a reason I here sometimes as well. Regardless of the reason, some have made this system work for their management style and will continue in the future, with or without the threat of soybean rust.

What is the likelihood that this would help growers escape some soybean rust, assuming in moves up from a southern overwintering site in late spring or summer?

Even if they did not completely escape infection, what is the likelihood with the shorter maturity - that these cultivars could make it to R6 with only one fungicide application?

Does it not make sense to plant mostly 135 day or less soybean varieties rather than 150 day plus varieties given the potential of soybean rust? The shorter growing period reduces the time for damage, in theory?

This has been a rather interesting question and has provided its share of opinions. Right now, avoidance and fungicides have been identified as the only source of dealing with this disease. Advising producers on ways to avoid this disease and still plant soybeans will remain a topic of discussion and we should rely on sound science to help with this decision.

First, let's consider what the literature can provide us regarding soybean growth stages and time required to reach these stages. The following table can be found in a number of references including the Arkansas Soybean Handbook.

Days

On average, soybeans remain most susceptible to rust for approximately 46 days (R1 to R6 growth stage). However, this may range from 25 to 85 days depending on maturity group, planting date, environmental conditions, etc. Based on this, can we accurately use planting date and maturity group as a rust avoidance mechanism? Not sure, what we can do is look at how maturity groups and planting date influence the length of reproductive growth. We have been successful in the past with predicting days to maturity and have found how planting date influences this. The earlier soybean is planted, the longer season (emergence to physiological maturity) we experience. From this data we see that the shortest season is from late planted (July) plots. However, you will also remember these planting dates also have the lowest yields. There is a direct correlation to number of days to yield.

Let's focus more on reproductive stages (since this has been show as most damaging from rust). The following chart displays the influence of planting date and maturity group on days of reproductive growth (R1 to R8).

As before, the later planting date will decrease our period or susceptibility and also our yields, regardless of maturity group. So what is the best answer? I believe the MG IV's are probably the safest maturity group, since their period of susceptibility is shorter (60-90 days compared to 78-115 for late MG V's). Now, when should they be planted? We must consider the shortest period of susceptibility (R1-R6), yield, and the chances of rust spores being introduced. Since we can't accurately predict spore introduction, we must look at the agronomic factors. I would suggest MG IV's (more accurately 4.7-4.9 range) planted in mid-April to mid-May. We still achieve high yields and these have a relatively shorter period of susceptibility. Will planting MG III's help more? In my opinion, NO. These lines typically do not mature much quicker than early MG IV's (usually within 4-5 days) and these have historically had significantly lower yields than the MG IV's, regardless of planting date.

Would wider rows help with the rust problem?

Based on agronomics, wide rows will only influence plant stature (plant height). Our tests have shown if proper plant populations are used, wide rows usually produce similar yields as narrow rows (less than 20”). There have been instances where we have seen 5-10 bushel increases with narrow rows, usually when planting date is either premature or delayed (in order to produce enough biomass to be able to harvest). We do recommend narrow rows to help with canopy coverage in order to restrict weed emergence/growth.

Another consideration with regard to planting date and soybean rust is geography and climate. Arkansas is located about 35 deg N of the equator, with a temperate climate influenced by the Mississippi River Delta in the east and the Ozark and Ouachita mountains in the west. In South America, the central and south-central parts of Argentina are about 35 deg S of the equator with a temperate climate but opposite seasons from us. The soybean growing regions of Brazil are closer to the equator with a more stable, milder and wetter climate than Arkansas. Argentina soybean growing areas tend to be in the North part of the country with milder and more stable temperatures and rainfall during the growing season than Arkansas . In Arkansas, we start to plant in late March in the southernmost parts of the state and continue into July, but the optimum planting window tends to be late April to mid-May. In comparison to South America, our most favorable months for soybean rust development appear to be April, May and June with mild temperatures and frequent rainfall while July, August and September often may be too hot and dry for maximum rust development. This is somewhat of a guess, but clearly, our temperatures tend to be hotter at times and rainfall more variable during the growing season than the soybean regions of South America. The following images and charts illustrate location, climate and growing regions for soybeans in the U.S. and parts of South America.

Latitude of the United States

http://worldatlas.com/webimage/countrys/usanewd.htm

Latitude of South American Countries.

http://worldatlas.com/webimage/countrys/sa.htm

Soybean Production Areas of the United States.

http://www.usda.gov/nass/graphics/county03/sbhar.htm

Soybean Production Areas of Brazil (still growing).

http://www.usda.gov/oce/waob/jawf/profiles/
documents/Brazil/BrazilSoybean.pdf

Soybean Production Areas of Argentina.

  http://www.usda.gov/agency/oce/waob/jawf/profiles/
html/Argentina/ArgentinaSoybean.html

The following charts are arranged starting with earliest planting month, which would be March for Arkansas and Mid to Late October for Brazil and Argentina . Data from http://www.usda.gov/oce/waob/jawf/profiles/updates.htm#SAM for Brazil and Argentina weather stations and from http://www.srcc.lsu.edu/southernClimate/atlas/AR for Arkansas weather station data.

Again, temperatures in Brazil and Argentina soybean growing regions appear to be milder and more stable during the growing season than in Arkansas . Our spring and early summer appears to be most favorable for soybean rust according to temperature so soybeans entering the reproductives stages from late May to late June could be at maximum risk, if spores of the pathogen are in the area. During July and August, temperatures in Arkansas often exceed 90 deg F and this could be less favorable for soybean rust development although it can also be detrimental to the soybean plant in other ways. Rainfall in Brazil is more stable and higher than in Argentina or Arkansas, while Argentina is more stable than Arkansas during the soybean growing season. Typically, July – September in Arkansas will likely be hot and dry, conditions unfavorable for maximum soybean rust development. Given these data, it appears parts of the eastern Midwest of the U.S. could be more favorable for soybean rust development than the South during mid to late summer so it would seem important that spore production in the southern states be minimized in spring and early summer.

The above image shows airflows in China versus the United States from the Weather Based Assessment report on soybean rust by Isard et al at the above website. The point is that should the soybean rust pathogen overwinter along the coast of the U.S. this winter, normal wind patterns will move it northward primarily through the Delta soybean regions of Louisiana, Mississippi and Arkansas (largest acreage) and on into the Midwest. Overwintering in South Texas could mean direct northward movement into the Plains and upper Mid-West if hosts are available along the way and the weather is favorable for infection – a given in the Mississippi River Valley delta region.

Fungicides will be the REAL control option during the first few years of soybean rust in the U.S., just like they have been elsewhere. Fungicides available will be dealt with in Question 22.

Back to Asian Soybean Rust

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