Crop rotation benefits: Grain sorghum

Over the next couple of weeks, we will be looking through several crops grown in Oklahoma, discussing their benefits within a rotation and some management issues/considerations growers may be interested. This will by no means be an extensive list, but an introduction to the discussion.

Over the last several years, Oklahoma producers have begun integrating several crops into their production systems. There are several reasons to adopted new crops into production systems, including commodity price, new crops being introduced into the region, adoption of new technology (i.e. herbicide resistance, adoption of irrigation), or changes of practices. Beyond these, integrating a crop rotation has been shown to increase crop yields, decrease insect and disease pressure, improve weed management, improve nutrient cycling, and provide better water and nutrient use efficiency.  It has also been noted that there is no perfect or right crop rotation but to use crop rotation to improve the flexibility, intensity, and diversity of their production system.

Grain sorghum:

Grain sorghum has been heavily integrated into the western portion of the state. This is because sorghum is considered one of the most drought tolerant crops grown in Oklahoma. Grain sorghum only needs 6.5 inches of water for the plant to make grain. Once this level is met, the plant will gain 350 to 425 pounds of grain for each additional inch of water received. This not only includes received water but also stored water prior to planting.

The yield benefits of crops following sorghum have been well-documented. Two of the best documented benefits of grain sorghum have been with cotton and corn. This is primarily due to sorghum commonly being grown in regions that sustain a large number of corn and cotton acres. The most beneficial crop rotations frequently involve both a grass and broadleaf crop, due to the lack of insect, diseases, and weedy plants that will commonly affect both crop types. However, data from Kansas State University had demonstrated an 8.4 percent increase in corn yields following sorghum compared to continuous corn. These results have also been seen in Oklahoma. A study at the OPREC station in Goodwell, Oklahoma showed that a corn-sorghum rotation yielded similarly to a corn-soybean rotation, with both yielding nearly 20% more, compared to a continuous corn rotation.  Part of these documented benefits could be attributed to pest cycles, such as Goss’ wilt, root worms, and corn borers.

Cotton and sorghum is a common rotation throughout more traditional cotton production regions in Oklahoma as well as throughout Texas. This is primarily due to both crops being considered low water use and providing a number of rotational benefits to one another. Data from Texas A&M has shown that cotton following sorghum produced 22% higher yields than continuous cotton (Table 1).  One of the greatest values that grain sorghum can provide for cotton systems is residue. As very little residue is left following cotton harvest, most continuous cotton growers have to integrate cover crops or other high residue crops to decrease wind erosion, increase moisture storage, and minimize the negative impacts of wind on cotton seedlings.

Sorghum_Cotton
Credit: Trostle 2000, Texas A&M

While there are several agronomic benefits between cotton and sorghum, growers have to be mindful of herbicide restrictions.  For sorghum, several cotton herbicides pyrithiobac sodium (Staple LX/Pyrimax) and Trifloxysulfuron-Sodium (Envoke) possess long rotation restrictions and typically will require a minimum of a season between planting cotton and sorghum.  Therefore, growers should understand what herbicides they have used in their cotton system and how these might influence the next sorghum crop.

Less is known regarding the rotational effects of grain sorghum on soybean. This is partly because these two crops are typically grown in very different regions of the state. However, with an increase in soybean acres in the western portions of the state, there is an increased potential for more sorghum acres to be rotated with soybean. Previous studies have suggested that growing soybean following sorghum resulted in similar yield benefits compared to corn (up to 17% yield increase). Sorghum can provide several benefits to soybean, including improved fertility and soil physical properties. However, similar to cotton, sorghum residue can greatly improve the soil moisture storage and retention, which is critical for soybean grown in western Oklahoma.

Managing a grain sorghum crop:

With all of the benefits of sorghum discussed above, optimum management is critical to ensure growers are optimizing productivity and profitability of the sorghum portion of crop rotations. Several production challenges have arisen in recent years that should be addressed.

How late is too late for planting (outside of the panhandle):

As with most production issues like this, the realistic answer is “it depends”. There is no firm calendar date that will always tell producers when the optimum time for planting will occur. This is because growers should delay planting until soil temperatures are adequate and enough soil moisture is present to allow for rapid germination and emergence. In most years, this will result in planting beginning in early to mid-April for southern regions and mid- to late-April for northern regions. Planting can continue past these points as long as adequate conditions persist. However, planting too late into May (typically past May 10th to 15th) could risk the sorghum plant reaching critical growth stages, i.e. flowering and grain fill, during what is typically the hottest and driest portions of the year. In years that these conditions are pronounced, significant yield loss can occur with sorghum planted in the 2nd and 3rd weeks of May. However, years with more mild and wetter July and August, these later plantings could yield equivalent to or higher than earlier plantings. For 2019, with cooler and wet conditions persisting through early May, sorghum may need to be planted later. Only time will tell the impacts that these later planting dates will have on yields.

Managing weeds in grain sorghum production:

Grassy weeds are one of the most difficult production challenges for sorghum production.   There are virtually no methods for controlling grasses in-season. Therefore, growers must successfully manage these weeds prior to or at planting.  Most growers integrate some form of S-Metolachlor in order to help control these grasses. Unfortunately, this product only has “partial control” rating for johnsongrass (one of the more challenging grassy weeds in Oklahoma). This requires growers to manage these weeds outside the sorghum crop within their rotation. Prior to planting grain sorghum, growers need to ensure that grassy weeds have been previously managed and pressure is low.

N management in sorghum

Based on recent studies across Oklahoma, N still seems to be one of the most limiting yield factors in sorghum production. Sorghum needs between 1.1-1.2 pounds of N per bushel of sorghum grain. Therefore, if growers are applying 75 pounds N per acre, the top end yield potential will be approximately 68 bushels per acre.  As grain sorghum is considered a “low-input” crop, N is typically one of the first inputs growers reduce in order to cut costs. However, it has to be understood that decreasing N applications will directly lower yields.

Sugarcane aphids:

Since originally finding sugarcane aphids (SCA) in Oklahoma in 2015, they have been a major pest in sorghum production. As SCA were very difficult to manage, they have been a major concern for growers since their arrival.  As such, they have been a reason for decreasing sorghum acres in recent years. Over the last several years, research programs throughout the southern Great Plains have learned a great deal about managing SCA. This research has provided information, which has allowed for easier management through agronomic practices, host-plant resistance, and insecticide applications.

Grain sorghum will continue to be a valuable crop for Oklahoma production systems, as it can be very efficient and productive in a wide variety of production systems across the state. Several production challenges have to be addressed in order to maintain the productivity and profitability of the system. It is because of some of these production challenges (namely, grassy weeds) sorghum cannot be grown continuously. Fortunately, grain sorghum serves as a very valuable rotation crop with many other crops grown in the state. Sorghum not only has the ability to improve yields in corn, soybean, and cotton production systems when grown in rotation,  but sorghum rotations can also help manage against several weed, insect, and disease pests.

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