Crop maturity from planting to grain harvest stage takes 4½-6 months, depending on variety, seasonal conditions and time of planting. Slower varieties tend to yield more than quick varieties.
Hybrids with loose husk cover dry down faster than those with tight covers.
While grain can physically be harvested at up to 25% moisture, this is not recommended as drying costs will be high. Where possible growers aim to harvest as close as possible to the required delivery moisture percentage.
Grain should not exceed 14% when delivered to merchants or storage. Some buyers specify 13.5%.
Harvesting corn to minimise hairline cracking/broken grain
Hairline (stress) cracking and broken grain can be minimised by keeping drum speed below 400 rpm. In heavy crops, try opening the concave or slow drum speed even further. Old rasp bars will usually give less hairline cracking/breakage of kernels than new ones.
Handling corn to minimise cracking/breakage losses
Handle (convey) grain as little as possible, especially if using augers.
Large diameter (greater than 20 cm) augers operating at low speeds damage grain less than when operating at higher speeds. Always keep augers full. Use belt conveyors rather than augers. Drop grain from low heights to avoid impact damage.
Chopping for silage
The best indicator of dry matter yield and silage quality is in the milk line score. At late grain fill stage, there is a visual line of separation between the starchy tip and the milky base of individual grains. The line moves down the grain towards the core of the cob over a six-week period (approximately) .
When this milk line is half way down the length of individual grains, its milk line score is said to be 2.5 (0 before the line becomes visible at the top of grains and 5 once the line is at the base of the grains). For most corn silage buyers, a milk line score of 2.5 is ideal.
A silage crop generally takes 3-4½ months, depending on the variety grown and seasonal conditions.
Grain: Dryland 2-5 t/ha.
Irrigated 5-13 t/ha.
Maize silage: To estimate silage yield, use the following formula as a rule of thumb:
Estimated grain yield (t/ha) x 6 = tonnes of silage chop/ha.
Grain Bulk Density
Normal merchant channels, or under contract to millers, starch manufacturers or corn chip or breakfast cereal manufacturers. Price premiums apply for grain of various processing quality. The feed market tends to have lower prices, but enjoys much more flexibility with regard to the range of hybrids used, allowing growers to make use of higher yielding hybrids.
Silage maize is normally sold direct to feedlots and dairies at an agreed price per tonne in-field. Farmers growing for stockfeed grain and silage need to be particularly aware of the stringent requirement of most buyers in relation to chemical residues.
Drying corn for the gritting millers
Hairline (stress) cracking of corn kernels must be within limits prescribed by millers. Stress cracks are cracks in the clear endosperm inside the kernel, which do not rupture the seed coat.
The following points on drying may also help growers to minimise stress cracking and obtain high gritting quality corn:
- Starting grain moisture levels over 17.5% make drying very risky for hairline cracking.
- Ensure that grain temperature does not exceed 38 ºC during drying.
- Cool the dried grain slowly. Avoid using blowers on grain after drying on cool, windy days. Dry grain early in the day to enable slow cooling as the day temperature slowly decreases. Drying grain under heat until late afternoon will mean the hot grain is too rapidly cooled, resulting often in excessive cracking.
- Do not leave grain uncovered overnight in field bins or open dryers. Rain or dew will predispose surface grain to cracking.
- Dryeration and combination drying systems minimise stress cracking. Dryeration involves a rapid initial drying followed by 6 to 10 hours of tempering to relieve stresses then a final cooling period. Combination or two-stage drying uses a first phase of high-temperature high speed drying followed by a second final drying with ambient or low temperature air.