Autumn frosted corn crops

Conditions causing frost damage

The severity of frost damage will depend largely on the duration and extent of freezing temperatures. Substantial frost damage of leaf, stalk and stem tissue may occur if temperatures drop below 0°C for 4-5 hours or when temperature drops to -2°C or less for even a few minutes. Frost damage due to rapid heat loss from within the crop can occur when temperatures are several degrees above zero if the air is clear and still. Under these circumstances, leaf temperatures can drop below the actual air temperature.

Where air temperature is close to freezing, frosting can occur in low-lying areas where the cold air tends to accumulate. Higher spots within the same paddock may not sustain any frost damage. A dense corn crop can trap considerable warmth within its canopy. Outer rows and areas of paddocks that are low in plant population are more frost prone. Corn plant parts vary in susceptibility to frost damage. Leaves are most susceptible because their thinness makes it difficult for them to hold heat. Thicker plant tissues such as stalks, husks and grain have greater heat retention capabilities.

Assessing frost damage

A visual inspection of frost-damaged corn should be made the morning after the frost, after the sun has risen and the crop has begun to thaw. At this time, damaged tissue will begin to wither and cell contents will be released as a sugary coating on the leaves and stem.

As you assess the crop, take into account the number of leaves that have been frosted, cob or stalk damage and the proportion of the paddock that has been affected. It is important to assess the stage of maturity of the crop in terms of how far away the crop was from harvest when it was frosted. Remember, as mentioned above, the outer rows and any hollows in the crop are likely to be more affected by frost than the rest of the crop. Always walk well into and through the crop to make an accurate assessment of damage. Generally, there is warmer air entrapped in the main body of the crop and the initial frost does not penetrate the crop canopy as far.

Silage crops - less than ½ milk-line

In most cases, frost damage will be limited to a proportion of the leaves. The leaf is normally only about 15-20% of the total yield. Frosted leaves will change from green to grey before turning brown and drying off. Some leaves that appear frost damaged may recuperate given time. Normally, remaining leaves along with the photosynthetic tissue in the stalk will be sufficient to allow the plant to continue to mature. Immature frosted corn silage crops will be very wet. It will be impossible to ensile these without a high risk of effluent run-off and subsequent energy and drymatter losses. The crop should therefore be left to mature and monitored carefully to ensure that it is harvested in the optimum whole plant drymatter range of 30-38%. Do not be tempted to harvest too soon. Yield will continue to increase even after a frost. Early harvest will mean decreased drymatter yields and a higher risk of a poor fermentation and wet, reduced quality silage.

In the rare case of an extreme frost killing all of the leaf and also the stalk, ear shank and husk, the crop will start to dry down relatively rapidly. Once again, monitor the crop carefully to ensure that it is harvested in the optimum range of 30-38% whole plant drymatter. As a last resort, grain or straw can be added to increase the drymatter of the silage. About 14 kg per tonne of grain, hay or straw per wet tonne of harvest material will be required to increase the drymatter by 1%. Since it is virtually impossible to assess drymatter of frosted crops visually, it is recommended that a sample is submitted to a laboratory or a microwave drymatter test is carried out.

Silage crops - ½ milk-line or greater

If the frost occurs after the crop has reached silage harvest maturity (30% drymatter), the crop should be ensiled as soon as possible. Frosted corn will dry rapidly especially during warm weather. Organise for your contractor to harvest at the first opportunity. Do not panic, you will normally have several days to harvest the crop before it becomes too dry. In addition, please consider the points made under the section Nitrate in Frosted Crops below. If the crop is over 38% drymatter at harvest, a finer chop length of 8-10 mm should be used. Particular care must be taken to ensure that the crop is well compacted. Further details on the ensiling of dry crops are given in the Pioneer Technical Insight - Harvesting Dry Corn.

Pioneer® brand 11c33 silage inoculant

Research has shown that nature does not always provide enough of the right bacteria for a fast, efficient fermentation. The use of Pioneer® brand 11C33 corn silage inoculant is recommended for frosted crops. 11C33 improves corn silage quality and drymatter recovery. It works by improving fermentation efficiency while reducing growth of yeast and mould responsible for heating and spoilage at feed-out. Note that while silage can be fed immediately after harvest maximum aerobic stability gains will be made when it is fermented 30 day prior to feeding.

Nitrate in frosted crops

Nitrogen is taken into the corn roots mainly in the nitrate form. When conditions are favourable for growth, it moves up the stalk into the leaves where it is built into proteins. Under normal conditions, as the plant nears maturity, nitrate levels gradually decline to zero. Nitrate levels of crops frosted prior to silage harvest are sometimes high. This can affect animal health and in extreme cases, may result in stock losses.

The following management options will decrease the risk of nitrate poisoning:

• Delay harvest where possible. Provided that the crop is not too dry, delay harvest for at least three fine days after a frost. This will decrease nitrate levels.
• If a frosted crop needs to be chopped immediately because it is dry, consider leaving a 30 cm stubble. Nitrate accumulation is highest in the bottom of the stem.
• Frosted corn silage crops should ferment a full three weeks before feeding. It is best to feed frosted crops as silage rather than green-feed or green-chop because field experience has shown that fermentation may reduce plant nitrate levels.
• A forage nitrate test is recommended when a frosted crop is green-fed, green-chopped or ensiled. For green-feed or green-chop, test immediately prior to feeding. For corn silage, test prior to ensiling. If the levels are high, the harvest height may need to be increased. Retest prior to feeding.

Table 1 gives guidelines as to the "safe" levels of nitrate in silage.

Feed value of frosted silage crops

The effect of a frost on the feed value of corn silage will depend on the magnitude of the frost and crop maturity. Plant digestibility is highest at about half milk-line but shows little change from when the kernels are in the dough stage (when the milk-line first appears) until the milk-line disappears. This is because corn leaves and stalks are high in non-structural carbohydrates (energy) late in the season and these are eventually transferred to the ear. When the whole plant is ensiled, it makes little difference whether the carbohydrates are in the ear or in the leaves.

Crude protein declines continually as the plant matures and is usually higher in immature frosted crops than in corn harvested at the correct maturity.

Frost may decrease the palatability of corn silage slightly. This is not normally of major concern. If necessary, molasses can be added at feed-out until the animals have become accustomed to the flavour of the silage.

Frosted grain crops

The effect of a frost on grain yield is determined by the amount of leaf that is killed and the stage of development when the frost occurs. The closer the plant is to black layer the lower the yield loss caused by a frost. Even if all of the leaves are killed, the plant can still continue to mature as sugars are mobilised from the stalks to the ear. Grain dry weight will continue to increase up until black layer as long as there are leaves (especially above the ear) or even if just the stalk and the husk cover are still green. If there are several days of cool temperatures (7°C -12°C) during grain fill, premature black layer may occur and grain fill will cease. This can happen even if there has not been a frost.

The potential grain loss from an autumn frost at different stages of grain development is shown in Table 2 (adapted from Afuakwa, J.J. and Crookston, R.K. 1984. Using the kernel milk-line to visually monitor grain maturity in corn. Crop Sci. 24:687-691)

Table 2: Potential losses in corn grain yield after frost

Grain crops frosted prior to 1/3 milk-line will have a low grain yield potential and the kernels will be chaffy. It is recommended that these crops are earmarked for corn silage and left to mature to silage harvest drymatter (as outlined above). When a grain crop is frosted at 1/3 milk-line the yields will be reduced by 50% unless some leaves survived the frost (Table 2). Severely frosted crops will need to be field dried for a long time and this will result in high field losses since corn which is frosted at an immature stage tends to have a greater incidence of stalk breakage. Ear moulds may develop when drying is slow and test weight is likely to be low. During combining, grain will be susceptible to breakage and the wet core may break into several pieces increasing the risk of grain rejection or discounting due to broken grain and foreign material. For many crops that are frosted prior to ½ milk-line, corn silage is the best option.

Corn grain crops that are killed by frost after ½ milk-line will have reduced yield (Table 2) and test weights will be below normal. If only a portion of the plant tissue is killed and if the grain was close to black layer, the yield loss will be small and test weights will be closer to normal.

A study in the USA simulating frost at milk-line (approximately 55% grain moisture) indicated that corn killed immaturely by a frost dried normally when environmental conditions were favourable. The dry down was delayed for a few days after the frost and this resulted in 4-9 more days to reach a harvest moisture level of 22-30% compared to non-frosted corn. This will delay harvest and/or increase drying charges. Severe frost will not affect grain yield or quality after physiological maturity (black layer).

Reducing the risk of frost damage

The risk of frost damage in the autumn can be reduced by planting early. Shorter maturity hybrids should be used if planting is delayed. Paddock selection is important in frost prone areas. Choose a paddock that is sloping and/or has a northerly aspect if possible. Avoid flat paddocks surrounded by higher ground.

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