Thursday, August 25, 2016

Timing Corn Silage Harvest


Corn must be ensiled at the proper moisture to get fermentation for preservation. But, determining when to harvest corn at the right whole plant moisture is difficult. Each storage structure properly ensiles at slightly different plant moisture optimums. Harvesting corn too wet for the storage structure will result in reduced yield, souring and seepage of the ensilage, and low intake by dairy cows. Harvesting too dry reduces yield, can cause mold to develop, and lowers digestibility, protein and vitamins A and E.

Kernel milk is not a reliable guide for timing silage harvest

Dry matter content of whole plant corn varies with maturity. The position of the kernel milk-line is not a reliable indicator for determining harvest timing. Geographic location, planting date, hybrid selection, and weather conditions affect the relationship between kernel milk-line position and whole plant dry matter content.

Determining field harvest order and initial plant sampling

The first step to determine when a field is ready for harvest is to note the order in which you planted your fields. Next, note silking dates of the fields to project calendar days to when a field will mature. Once corn silks, approximately 55 to 60 days is required to achieve maturity at R6 or the "black layer" stage (Abendroth et al., 2011). Development during grain filling is influenced by temperature, but not as much as during the vegetative leaf emergence stages. Instead the number of days between pollination and a killing frost influence the time to maturity. So if an average killing frost occurs October 1, then subtracting 55 to 60 days means that the crop must be silking by August 2-7.

We know that kernel milk stage is not reliable for determining the actual harvest date, but it is a useful indicator of when to sample fields to measure plant dry matter. Silage harvest usually begins around 50% kernel milk which is 42 to 47 days after silking, so silking must occur by August 15-20 in order to mature before typical killing frost dates; but remember that the timing of silage harvest is dependent upon achieving the proper moisture for the storage structure (Table 1). Noting the order that fields silk will help plan the harvest queue of your fields and scheduling of custom choppers.


Table 1. Kernel milk stage "Triggers" for timing silage harvest
Silo Structure Ideal Moisture Content Kernel Milk Stage "Trigger"
  % %
Horizontal bunker 70 to 65 80
Bag 70 to 60 80
Upright concrete stave 65 to 60 60
Upright oxygen limiting 50 to 60 40
"Trigger": kernel milk stage to begin checking silage moisture.

Determining Silage Moisture

The only reliable method of determining the optimal time to harvest corn silage is to sample the crop and directly measure the % dry matter of whole plants. This information combined with average whole plant dry-down rates can be used to roughly predict the proper time to harvest corn silage.

The next plant indicator that determines the order of fields to harvest is movement of the kernel milkline. Once kernel milkline begins to move, measure moisture of fields intended to be harvested for silage (Table 1). Corn should be first sampled to measure dry matter shortly after full dent stage (80% kernel milk) for bunker silos and bags, at 60% kernel milk for conventional tower silos, and at 40% kernel milk for sealed (oxygen-limited) tower silos. It is important to begin sampling early as a precaution against variation in dry down.  You will likely be too wet, but you will have an indication of how quickly drydown is occurring when the next sampling date takes place.

Sampling a field for whole plant moisture 

Ideally the field to be harvested is uniform in development, but the reality is that uniformity is rarely achieved. Separate uneven fields into representative groups. Figure 1 describes the moisture drydown patterns of two locations in the same field. Knoll areas were as much as 20% units different from swale areas.

 Figure 1. Forage moisture of corn growing on a knoll and a swale at Arlington during 2003.

Sample two or more locations for each representative group in the field. Over time, sample the same locations - trying to determine the rate of drydown. Scott Hendrickson (Manitowoc county agent) measured whole-plant moisture over time at three sites in the county by always returning to the same location in the field (Figure 2). Depending upon year the average drydown rate ranged from 0.4 to 0.7 percent per day.
Figure 2. Corn silage drydown during harvest (Hendrickson, Manitowoc County, WI)

Procedure for measuring plant moisture
  1. Sample 3 to 5 plants in a row that are well bordered and representative.
  2. Put in plastic bag,
  3. Keep plants cool,
  4. Chop as quickly as possible,
  5. Measure moisture using NIR spectroscopy and/or by drying using a, Koster oven, microwave, or convection oven (Peters, 2000).
Predicting silage harvest date

Use 0.5% per day during September to predict the date when a field will be ready for the storage structure. For example, if a given field measures 30% dry matter at the early sampling date, and the target harvest dry matter is 35%, then the field must gain an additional 5% units of dry matter, thus requiring an estimated 10 days (5% units divided by 0.5 unit change per day). If weather is warm and dry, use a faster rate of drydown (1999 and 2000 in Figure 2). If weather is cool and wet, use a slower rate of drydown (1996 and 2001 in Figure 2). We are most interested in the rate of corn silage drydown. Wisconsin county agents have been accumulating corn silage drydown information since 1996. Results from county "Drydown Days" can be checked at the website http://fyi.uwex.edu/silagedrydown/ which averages and predicts area harvest dates.

This procedure provides only a rough estimate for the harvest date. Many factors affect dry down rate, including hybrid, planting date, general health of the crop, landscape position, soil type, and weather conditions. In general, corn silage that is slightly too dry is worse than corn silage that is slightly too wet. Therefore, starting harvest a little early is usually better than waiting too long.

Literature Cited 

Peters, J. 2000. On-Farm Moisture Testing of Corn Silage [Online]. Available at http://fyi.uwex.edu/forage/files/2014/01/CSMoistTest-FOF.htm_.pdf (verified 25 August 2016). Focus on Forages, UW-Madison.

Abendroth, L.J., R.W. Elmore, M.J. Boyer, and S.K. Marlay. 2011. Corn growth and development. PMR1009. Iowa State University.

Sunday, July 31, 2016

What is happening in the corn plant during the month of August?

By August two of three corn yield components, ear number and kernel number, have been determined. The final yield component, kernel weight, will largely be determined during the month of August. Preliminary yield estimates can be made and depending upon the success of pollination, decisions regarding harvest use strategies can be planned.

Corn kernel development begins with silking (R1) and is marked by the blister stage (R2), milk (R3), dough (R4), and dent (R5) stages. The final stage called black layer formation (R6) marks the end of kernel development. The corn kernel accumulates weight in a sigmoidal pattern over a 55-60 day period beginning with a 7-10 day "lag" phase and ending with a 7-10 "maturation" phase (Figure 1). The linear phase of the sigmoidal curve lasts about 40 days.

Figure 1. Kernel weight accumulation pattern of corn.

For a 200 bushel per acre yield level about 5 bushels per day (200 / 40) accumulates during the linear phase of kernel development. About 60% of the starch that accumulates within the kernel is produced by the ear leaf. Leaves above and below the ear are also important sources for developing kernels, but as the distance from the ear increases less starch is translocated to kernels and more to other plant parts. The stalk serves as a temporary storage organ during the day and photosynthate will be translocated to the kernels throughout the night.

Photosynthesis is maximized at about 1/3 of full sunlight, so even cloudy days can produce the starch needed to sustain accumulation in the kernel. Other plant parts (leaves, stalk and roots) demand photosynthate for respiration and are competitors with kernels. Temperatures that are comfortable for us (65-80 degrees F during the day and 50-65 degrees at night) provide the best trade-off between maximizing photosynthesis production and minimizing respiration in corn.

About 0.25 to 0.30 inches of water is being transpired by the plant during August. Every day that corn plants are stressed can lower yields 5% per stress day. Nutrients (N-P-K) are still being taken up by the plant until about the R3 to R4 stages. Brace roots are acting as a nutrient scavenger system in the upper layers of the soil profile, while roots deeper in the profile are used primarily for water uptake. During August it is important to protect the ear leaf since that is the plant part where most of the photsynthate is produced for a developing kernel.

Wednesday, June 29, 2016

What is happening in the corn plant during the month of July?


The corn plant during July transitions from developing vegetative structures to reproductive structures. It is significant for yield in that two of the three components of yield are set up during this month. In the first half of the month, the number of potential ovules that could develop into kernels is determined. In the second half, the number of potential cells in the kernel endosperm, which ultimately affects kernel weight, is determined. However, everything is predicated on the success of pollination and fertilization of the ovules on the topmost ear from pollen released by the tassel (see "Methods for determining corn pollination success").

During early July ear development is rapid and prior to tasseling (V18). The upper ear shoot is developing faster than other shoots on the stalk. Brace roots are now growing from nodes above the soil surface. They will scavenge the upper soil layers for water and nutrients during reproductive stages. Moisture deficiency will cause lag between pollen shed and beginning silk ("nick"). Usually the largest yield reductions will result from this stress. The plant is using 0.30 inches of water per day. Lodging will cause 12-31% yield reduction. Frost (<28 F) will cause 100% yield loss due to plant death (see "Frost"). Hail will cause 100% yield loss when completely defoliated (see "Hail damage on corn"). Drought will cause 4% yield loss per day due to drought or heat when leaf rolling occurs by mid-morning (see "Drought"). Flooding (<48 h) will not affect yield, however, other management options need to be considered (see "Flooding effects on corn").

At the silking (R1) stage the actual kernel number and potential kernel size is determined. R1 begins when any silks are visible outside the husks. Pollen shed begins and lasts 5-8 days per individual plant. Silk emergence takes 5 days. Silks elongate from base of ear to tip of ear. Silks elongate until pollinated. Silks outside husks turn brown. The plant has now reached its maximum height. First 7-10 days after fertilization cell division occurs within kernel after which kernels begin to fill with starch.

The plant must have a healthy root system because proper uptake of moisture and nutrients are critical at this time. Hot and dry weather results in poor pollination and seed set. Drought dehydrates silks (delaying silking) and hastens pollen shed causing plants to miss window nick for pollination. Drought decreases yield 7% per day (leaf rolling by mid-morning). Rootworm beetle clips silks which prevents pollination if less than a half-inch of silk is showing

Nitrogen applied through irrigation water, should be applied by V18. Rootworm beetle control should be implemented if 4-5 beetles are observed feeding near ear tip. Stresses that reduce pollination result in a "nubbin" (an ear with a barren tip).