Specialist offers advice on protecting soils and fall cover crop grazing concerns

Sara Bauder photo: Plant diversity in cover crops is important for healthy soil.

Sow and Grow

Recently, I’ve heard many farmers express frustration with late-summer or fall planted cover crops that didn’t take off as expected. 

Of course, the main reason we witnessed slow fall growth in much of the state was lack of moisture, a stark comparison from many areas last year! 

Now that frost is moving in, there isn’t a lot of “fix” to this issue, but there definitely are a few things producers can do to protect their soils.

Protecting soils

In South Dakota we have encouraged adoption of five basic “Principles of Soil Health”. These include:

1. Soil cover – keeping plant residue on the soil surface at all times.

2. Limited disturbance – minimize or completely avoid tillage in order to allow soils to build aggregates, pore spaces, organic matter, and improve biological activity.

3. Plant diversity – mimic nature by using cool and warm season grasses and broadleaf plants. For example, this includes adding a mixture of cover crops and/or small grains to a strictly row crop rotation.

4. Living roots  allowing cover crops to grow in the off-season provides carbon to the soil and serves as a food source for microorganisms.

5. Integration of livestock – allowing livestock to graze cash crop and cover crop residue provides more forage options for producers at a time of year when forage quality and supply begins to drop. In turn, livestock grazing increases soil biological activity, improves nutrient cycling, and helps improve overall soil health if grazing is properly managed.

Although it is getting closer to winter, there is still an opportunity to make management decisions based on the health of your soils. For example, if your cover crop establishment was poorer than expected, rather than grazing or tilling, it would be best to avoid any fall tillage or grazing and allow the plants to remain on the soil surface over the winter as a protective “coat of armor.” In the spring, any residue present on the soil will likely decompose and there should be little to no concerns planting into it. On the other hand, if your cover crops did well and you intend to graze, it is very beneficial to use the “take half leave half” approach. 

Simply put, the goal is to graze about 50% of the standing cover crop and leave the other half to help cover soils and keep living roots as long as possible. For more details on these approaches, please visit the South Dakota Soil Health Coalition Website (sdsoilhealthcoalition.org) and click “technical resources.”

Grazing concerns

When grazing fall cover crops during a dry year like this, nitrate toxicity can become an issue. 

Although grasses such as oats, sorghum, barley, wheat, etc. are known for accumulating nitrates, many other cover crop species may also accumulate excess nitrates in their tissues during a dry year. Risk may be worse if nitrogen fertilizer was applied to the cover crop. Should the intake of nitrates in a ruminant animal get too high, toxic levels of nitrite can be absorbed into the bloodstream; this can cause asphyxiation. In order to know if grazing is safe, it is best to take a sample and arrange testing. All SDSU Regional Centers and select county offices offer a free nitrate “QuickTest” that can detect whether nitrates are present or not. If nitrates are detected, further testing at a commercial lab is recommended for quantifying the amount. Contact a SDSU Extension livestock specialist nearest you for assistance interpreting test results to determine if grazing is safe.

It is always a good practice to avoid introducing hungry animals to new cover crop fields. Introduce animals slowly and place a bale of hay in the field with them or allow access to other dry matter feeds. 

Other helpful tips include avoiding over-grazing as lower plant parts typically contain higher concentrations of nitrates. 

Aside from nitrate concerns, prussic acid poisoning is another issue to watch for this time of year, especially after sub-freezing temperatures. 

Prussic acid restricts animal oxygen utilization, which results in suffocation. This toxin accumulates in the plant after frost or freezing, or crushing/trampling/chopping of the plant; it can also be found at high concentration in plants following environmental stress, grazing, or in plant regrowth after drought. 

With fall frosts upon us, prussic acid risks must be taken into consideration before grazing.

Ruminants are quite susceptible to prussic acid poisoning, specifically in sorghums and sudangrass species. Waiting at least a week to ten days after a frost allows gases to leave plant tissues and greatly reduces risks when grazing. 

Prussic acid is less of a concern when haying or chopping, as ensiling/fermentation, and curing greatly reduces the formation of toxic compounds. For more information on prussic acid and safety levels, visit extension.sdstate.edu/prussic-acid-poisoning.

Keep in mind that many toxic compounds can build up in commonly grazed cover crops; however, this does not mean that these crops should not be grazed. It simply implies that growers need to be watchful of crop and environmental conditions. For a list of SDSU Extension nitrate “QuickTest” locations visit extension.sdstate.edu and search “nitrate QuickTest”; for a list of feed testing labs search “feed testing labs.”

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