Paul Peterson, Jim Linn, and Dale Hicks
University of Minnesota Extension Service
Frost touched much of the state's corn and soybean acreage this past weekend. The degree to which the frost was a killing frost varies considerably, but a complete killing frost appears to have been the exception, not the rule. Where frost injury occurred without complete kill, it is too early to consider forage harvest because additional yield and forage quality accumulation is likely from surviving plant parts. However, where these crops were/are completely killed by frost before reaching optimum grain or even forage harvest maturity, harvesting as forage is a viable option. In addition, based on the delayed maturation of corn and soybeans to date, chances are good that there will many acres of these crops that will receive a killing frost before reaching maturity, so harvest as forage may still be one of the better options as the growing season plays out.
The Importance of Proper Moisture for Ensiling
Immature corn will be too wet to ensile properly within the first several days after a killing frost. Corn in the blister stage may be as high as 80% moisture. Proper moisture content, suited to the intended storage structure, has more influence on producing good quality silage than almost any other factor. Moisture content must be below 70% for good fermentation. To avoid excessive seepage from storage structures, moisture should be below 68% in horizontal silos and below 65% in upright silos. Plan to store the wettest silage materials in horizontal silos, since they can handle greater moisture contents with less risk of seepage and poor fermentation. Seepage occurs because dead, piled corn plant tissue can hold only so much water or moisture. Seepage from silos carries with it many water-soluble plant nutrients, such as sugars, simple proteins and organic acids. Loss of these materials results in 5-7 % loss in dry weight of the forage, but can be as high as 10-15%. But the impact of seepage on silage quality goes beyond just dry matter loss.
Seepage or effluent squeezed from corn silage tissue or other forages that are ensiled by pressures exerted within a silo creates a couple of problems. First, because of the high nutrient content, this seepage is a very strong pollutant. It should not be allowed to enter any water sources as it can kill fish. Secondly, the lost effluent carries with it organic acids that are produced during fermentation and necessary for the preservation of the forage crop. Forages ensiled high in moisture also have a dilution effect on the fermentation acids produced which can result in either the acids produced during fermentation not being concentrated enough for good preservation or extending the fermentation process resulting in increased loss of nutrients towards fermentation rather than for animal requirements. The loss of nutrients for extended fermentation will most noticeably be the soluble carbohydrates such as sugars and starches. These carbohydrates are the most digestible by silage and rumen bacteria and when they are lost, the remaining carbohydrate will be mostly fiber which has a lower digestibility than sugar and starch.
Good quality corn silage will have a pH between 3.8 and 4.2. The fermentation acids will be primarily lactic and acetic with a small proportion of propionic. In good quality silage, butyric acid should not be present. Silage ensiled too wet, above 70% moisture, often results in a high butyric and possibly acetic acid fermentation. This usually results in smelly silage with reduced palatability and digestibility compared to good correctly fermented silage.
What Yield and Quality Can We Expect From Immature Corn Silage?
The low point in forage quality of standing whole-plant corn is after flowering during the early stages of ear development (blister to milk stages). As ear fill progresses, grain percentage increases, and physiological maturity is approached; forage quality increases. In contrast, dry matter yield increases steadily throughout corn development to physiological maturity, so there is a larger reduction in potential milk yield per acre than milk per ton (forage quality) when harvesting at immature stages. Regardless, decent quality silage is achievable at immature stages, and to some extent, producers have to make the best of what they can get when climatic conditions prohibit harvest at optimum maturity.
During the dry down period after a killing frost, dry matter yield will decrease due to some leaf loss, plant lodging, and ear drop. Thus, a trade-off exists between optimum moisture for ensiling and dry matter yield going into the silo.
As harvest decisions are contemplated, growers who are covered by crop insurance should check with their company rep before they do any harvesting.
Frost-killed, immature corn should be allowed to stand in the field until reaching proper moisture for chopping and ensiling. The amount of time required varies with many factors. Warm, dry, windy conditions will accelerate the drying rate. Cool, wet weather will slow the rate of drying. Check moisture periodically and ensile as soon as target moisture has been reached. The best way to get an accurate moisture test is with chopped forage sent to a commercial testing lab. A Koster moisture tester or microwave oven can be used as well if you follow proper procedures for drying. The most critical thing is to use a chopped sample, less than 3/8 inch particle length. Large, long particles do not dry evenly and often lead to incorrect moisture values. Also, be sure to chop and sample all of the plant you are planning on harvesting to get the most accurate moisture test.
For corn frost-killed prior to the dent stage (eg. silking, blister, milk or dough stages), the moisture content will be too high for successful ensiling. The silage crop should be allowed to dry in the field for several days (5-10 days typically) and moisture content monitored.
Tissue death from frost may encourage mold growth in ears. Thus, corn frost-killed during the dent stage should be harvested quickly to reduce potential mold growth and yield loss due to leaf drop. To help control problems with excess moisture, wet silage can be mixed with a dry feed at ensiling. A good carbohydrate source such as corn, ear corn, small grains or dry beet pulp would be the best dry feed source. Chopped straw or chopped hay can be added, but lack the carbohydrates grains to aid fermentation. The rule of thumb is about 30 pounds of dry material per ton of silage to reduce silage moisture one percentage unit.
The use of a silage inoculant should be considered to help populate the ensiled material with the correct microbial population for fermentation and/or help expedite fermentation. Inoculants are not quick fixes for bad fermentation conditions and will work best when good ensiling practices are followed for moisture content, fast filling and good packing. Inclusion of buffered propionic acid or other antifungal compound at a low rate (2 -4 lb/ton of ensiled forage) at the time of ensiling may help reduce yeast growth in the silage and generally do not affect fermentation of the silage.
Grazing Immature, Frost-Killed Corn
Grazing is an option that may provide the means to capture the most nutritive value from standing, immature corn that is too wet to ensile. Grazing is also considerably lower cost than ensiling. Using temporary fencing to allocate small portions of a cornfield at a time will reduce trampling and waste of the standing corn and thus increase the number of grazing days per acre.
Like immature corn, soybeans will also be too wet to ensile immediately after a killing frost. However, in contrast to corn, soybeans intended for harvested forage should be mowed and conditioned and left in a swath as soon after the killing frost as possible to retain leaves.
It is too late to consider mowing-conditioning soybeans that may have been fully killed by the recent frost for silage or hay, but if a killing frost should occur before sufficient maturity for grain, ensiling after immediately mowing-conditioning into a swath and wilting or grazing may be viable options.
Soybeans can be difficult to wilt, so conditioning is essential. Oil content in soybean at more mature stages (R6 to R7, when seeds completely fill the pods and lower leaves have begun to turn yellow) can limit the fermentation and palatability of the ensiled product. In addition, seed loss can be high when run through a mower-conditioner. Soybean plants are not high in soluble carbohydrates like corn to start with and oil forms in the seed carbohydrates are even further reduced. Thus, mixing soybean forage with grass or corn silage during silo filling is a good option to achieve usage of a crop that normally does not lend itself to good ensiling through a) appropriate moisture content for ensiling with direct cut, b) better silage fermentation, and c) increased palatability, particularly at later soybean maturities (R6 to R7).
Silage is preferred to hay for soybean because hay requires considerable wilting, is dusty, and stems are brittle, increasing feed bunk refusal. If grazing is considered, watch for bloat. The authors are unaware of any reported cases of bloat on soybean pasture, but the possibility may exist, so grazing should be tested with lower value animals with that aren't too hungry first, especially after a recent frost.