Harvesting constitutes a major operation among agricultural activities including field crops. Considered for a long time as the last step in production, it can also be approached as the first activity in the postproduction system. Harvesting has a great influence on subsequent processing and preservation of the products.

This article will take you through the different processes of harvesting, threshing, cleaning, drying, grading and grain quality parameters of major field crops.

Harvesting of Field Crops

1. Stage of Harvest for Small Grains

Grains cease growing and gaining in dry weight when they reach approximately the hard-dough stage, or when the moisture content of the grain drops below about 40 per cent.

Further ripening consists of desiccation unaccompanied by transport of nutrients into the kernel.

Ripening is not entirely uniform among different heads or different grains within a head. Consequently, growth may continue until the average moisture content is appreciably below 40 per cent.

Small grains at the hard-dough stage have a moisture content of 25 to 35 per cent, the heads are usually light yellow, and the kernels are too firm to be cut easily with the thumbnail.

Effect of Premature Harvest

Premature harvest reduces both yield and quality.

Under hot, moist conditions, grain quality deteriorates rapidly, mould problems can develop within days and insect problems in a month or two.

Underdeveloped grains are low in test weight, starch content and market value.

Strategies for Handling Over-Moisture Grain

Blending with dry grain to meet moisture specifications.

Aeration cooling to prevent mould development until the grain is used or dried.

Aeration drying.

Hot air drying.

Drying grain in the paddock.

Effects of Delayed Harvest

Lead to loss of grains due to shattering, crinkling, lodging and leaching.

Cereals stems are likely to crinkle down or break soon after maturity, leading to insects attack, sprouting dirtying of the grain, which consequently lowered the grain quality.

Weathered or sun-bleach grain is unattractive and often brings a lower grade and the market value.

Ripe grains exposed to wet and dry condition for long periods in the field are lower in weight because the grains swell when damp and do not shrink to their original volume after drying.

Methods of Harvesting Field Crops

Harvesting methods differ according to the part of the plant to be used. As regards forage crops, the whole plant is cut, but for underground crops (e.g. groundnuts, roots and tubers), the crop is lifted while the soil sticking to it is removed.

With cereals, the crop is first cut either as a whole or partially (ears), and then threshed and cleaned to separate the grain from the ears and straw.

In the latter case, two main alternatives exist, separate harvesting and threshing, or combined harvesting and threshing.

1. Manual Harvesting

In developing countries the first alternative is generally the most widely applied. Although harvesting and threshing are still frequently done by hand, their mechanization has begun to develop during recent years, especially where the crop is produced not for self-consumption but rather for commercial purpose.

Nevertheless, such mechanization has not developed everywhere to the same extent but according to the type of crop concerned, because labour requirements remain high for handling the produce before threshing.

2. Mechanized Harvesting

In industrialized countries, attempts have been made since the beginning of the 20th century to devise machines which would both harvest and thresh grain, so as to reduce the labour requirements involved.

Combine harvesters (‘combines’) which can cut, convey, separate and thresh the grain were the product of this development work. They are in widespread use, and have been used already on large grain production schemes in a number of developing countries.