Agronomy - Harvesting and Storage

This lesson will teach the reader how to determine the best time and most suitable method to harvest their crops and how to recognize physiological maturity characteristics of crops.

 

Lesson 1.8 - Harvesting and Storage

Objectives: After this lesson, participants will be able to:

1. Determine the best time to harvest their crops.

2. Determine the most suitable method to harvest their crops.

3. Recognize physiological maturity characteristics of crops.

Lesson Preparation: Before presenting the lesson, prepare the following:

1. Samples of corn or sorghum to show participants the "black layer" for physiologic maturity.

2. If not in harvesting season, make some drawings showing the "black layer" in corn.

Concepts to be Taught

1. Guidelines for Harvesting Corn:

   The "Black Layer" Method

   The dry-down rate of corn

   When to harvest

   Methods of shelling corn

2. Guidelines for Harvesting Sorghum and Millet:

   When to harvest

   Methods of threshing sorghum

3. Guidelines for Harvesting Peanuts:

   When to Harvest

4. Guidelines for Harvesting Beans (Soybeans, cowpeas):

   When to Harvest

   Threshing methods for beans

Harvesting

Good yields are an integral part of good farming practices, and harvest time is when the farmer gather what he has cultivated. Harvesting is one of the important steps in profitable crop production. The most basic and important knowledge a farmer needs to acquire is when, rather than how to harvest. The maturation characteristics of the crops used in the production unit may be affected by difference in climate or soil, and the intended use of the crop. There are crops that shatter prior to harvest due to weather, and timeliness of harvest.

Nearly all small farmers in developing countries harvest their cereal crops and beans by hand and thresh them later. In the case of peanuts, harvesting involves lifting the plants and attached pods from the ground, then allowing them to cure (dry) in the field for a period of from several days to four to six weeks before threshing.

Guidelines for Harvesting Corn

Most maize varieties reach physiologic maturity within 90 to 130 days after seeding emergence. As maturity nears, the lower leaves begin to yellow and die off. In healthy, well-nourish plants, this should not occur until the ears are nearly mature. More typically most of the leaves are dead by the time the plant matures.

The "Black Layer" Method

When a corn kernel reaches physiologic maturity (maximum dry weight), the outside layer of cells at its base where it connects with the cob will die and turn black, thus preventing any further cob-to-kernel nutrient transfer. This "black layer" provides and indication of maturity. The layer can be seen by detaching kernels from the cob and examining their bases. Newly-matured kernels may have to be slit lengthwise with a pocketknife to expose the black layer. However, with older kernels, the layer can be readily seen by scraping the base with the fingernail.

Keep in mind that physiologic maturity is not reached until all the kernel's milky starch has solidified. This process begins at the tip of the kernel and moves downward toward the base. The kernels at the ear tip are the first to mature, followed by those in the middle and finally the ones at the lower end and the difference is no more than a few days.

With healthy plants, kernel moisture at physiologic maturity will vary from about 28-36 percent. This is usually too high for damage free threshing or for mold free storage except in the form of husked ears placed in very narrow cribs. The black layer may form much earlier in the corn plant's growth cycle if growing conditions are adverse. Such kernels will be small and shrunken and have much higher moisture contents when the black layer forms.

The dry-down rate of corn

When corn plants are left standing in the field after maturity, the kernels lose about 0.25 - 0.5 percent moisture per day, but can range from 0.1 - 1.0 percent depending on weather conditions and whether the ears are pointing downwards to prevent water entry.

When to harvest

Harvest should begin as soon as is practical after maturity, but this depends on the farmer's harvest method and storage and drying facilities. Most corn harvested in developing countries will be done by hand. Since husked ears can be safely stored in narrow cribs at up to 30-32 percent moisture, harvest can be started at or soon after maturity if desired. Most small farmers prefer to let the corn dry down further in the field first, bending the corn plant letting the ears point downward to prevent water entry.

Methods of Shelling Corn

If done too roughly or at too high a moisture content, shelling can cause kernel damage such as tip loss, cracking, stress cracks, and pulverization. Studies have shown that damaged kernels spoil two to five times more rapidly during storage than undamaged ones.

Hi-lysine varieties and other floury types are more susceptible to damage. Shelling methods and guidelines for small farmers include these:

Traditional methods

1. By hand: This method is very tedious and labor-intensive, but causes little damage to the kernels. It is more thorough than other methods and also allows for separation of damaged and insect-infested grain. This method is best suited for small amounts of corn.

2. Beating: Dry ears are placed in bags and beaten with sticks. This is quicker but less thorough than hand shelling and may cause damage.

Improved methods

1. Wooden hand-held corn sheller: The model shown in the drawing was developed by the Tropical Products Institute and has an output of roughly 80 kg/hour. Other types of hand-held shellers are available commercially. Cobs must be husked first.

2. Motor driven shellers have outputs of about 1000-5000 kg/hour. Corn at too high or too low a moisture content is likely to be damaged, but this can be checked visually. Ears must be husked first.

Winnowing Methods

Reliance on wind is the traditional method, but hand-cranked or pedal-driven fans can be constructed easily. The larger models of the hand-cranked or pedal-operated shellers usually are equipped with blowers.

Guidelines for Harvesting Sorghum and Millet

When grown under favorable conditions and good management, grain sorghum reaches physiologic maturity while the stalks and most of the leaves are still green. Like corn, sorghum kernels also develop a "black layer" at their base when physiologic maturity is reached. The layer can be checked by pinching off some kernels from the bracts that hold them to the head and examining their bases. If present, the black layer can be seen without splitting the kernel.

Sorghum flowers and pollinates from the tip of the seedhead downward, a progression which takes from four to seven days. The kernels mature in the same direction, with those at the bottom lagging about a week behind those at the top. Kernel moisture content is about 30 percent at physiologic maturity.

When to harvest sorghum

In most sorghum-growing regions in developing countries, maturity often coincides with the start of the dry season, and the crop may be left standing in the field to dry for a number of weeks before harvest. Crop losses during this period can be heavy. If dry conditions prevail, the crop can be harvested at or shortly after maturity and stored on the head with little danger of spoilage.

Sorghum can be harvested by hand or mechanical means once kernel moisture reaches 25 percent. However, loose grain that is this wet must be dried down to around 14 percent within a few days to avoid spoilage. If large amounts of grain are involved, some form of forced air or heated drying would probably be needed.

Methods of threshing Sorghum

1. Traditional methods: These include pounding, beating, and animal trampling and are very tedious except for small quantities. Kernel damage is possible unless care is taken.

2. Mechanical methods: Tractor or motor-driven stationary threshers come in many models with outputs of 600-3000 kg/hour. All but the simplest models will also clean the threshed grain by the use of shaking screens and/or blower fans.

NOTE: Millet is harvested and threshed much like sorghum.

Guidelines for Harvesting Peanuts

Peanuts reach maturity when the veins on the inside of the pods turn dark. However, since the plants produce flowers over a period of from 30 to 45 days, the nuts do not mature simultaneously. Unfortunately, harvesting can not be delayed until all nuts have ripened, because heavy losses may occur for two reasons:

1. By the time the last, many of those which mature earlier will have become detached from the plants due to peg rotting. This pod "shedding" can be especially serious when Cercospora leafspot causes premature leaf loss or when lifting occurs in dry, hard soils.

2. In Spanish-Valencia varieties, the early-maturing kernels may sprout if kept too long in the ground. The Virginia types have a lengthy seed dormancy period which prevents this.

Likewise, if harvesting occurs too early, an undesirably high proportion of the kernels will be immature, shrunken, low in weight, and inferior in flavor, The choice of harvesting date can easily make a 400-500 kg/ha difference on a high yielding crop.

When to Harvest Peanuts

The farmer should aim for a harvest date that will recover the largest number of mature kernels before excessive pod shedding or sprouting has occurred. This is often referred to as "peak maturity", and there are no easy rules for determining it. Peak maturity cannot be determined by looking at the above-ground portion of the plants. The best method is to carefully dig up a few plants every several days beginning near the end of the growing period and examine the pods. With experience, the farmer can learn to estimate quite accurately how many young pods will ripen before the matured pods begin to shed or sprout.

When traditional or modern methods are use, the harvesting process basically consists of four steps:

1. The taproots are cut and the plants are pulled (lifted) from the ground with the attached pods. The plants are pulled from the ground manually after loosening the soil with hand tools. It takes about 30 hours to pull and stack a hectare with this method. Lifting the crop when the soil is too wet can weaken the pegs. It may cause excessive amounts of soil to adhere to the pods which can also slow down curing. Lifting losses can be high in very hard, dry soils.

2. Usually the plants are cured (dried) in the field for up to 4-6 weeks or less depending on whether artificial drying is available, before threshing.

3. The pods are threshed from the plants.

4. The threshed pods are placed in bags for storage and possible further drying. In dry areas, the pods are often stored in outdoor piles.

Note: Shelling the nuts from the pods is not normally a part of the harvesting process, since the kernels dry and store better in the pod. Shelling damage can be high unless kernel moisture is at or below 10 percent.

Guidelines for Harvesting dry beans, soybeans, cowpeas.

The pods begin to turn yellow during the final stages of growth and become brown and rather brittle once maturity is reached. Determinate bush varieties and some indeterminate types have fairly even pod maturity, and the plants have usually lost most of their leaves by the time the pods have ripened. Most indeterminate vining types mature less uniformly, and a good number of pods may ripen while most of the leaves are still green. Seed moisture content is around 30 -40 percent at physiologic maturity.

When to Harvest

Indeterminate varieties with an uneven maturity are usually harvested in several pickings, while determinate bush types are harvested all at once when most of the pods are dry. The following methods apply to bush or semi-vine varieties with uniform maturity:

1. By hand: The mature plants are pulled from the ground and are placed in piles for drying. Pulling is best when the pods are moist to prevent shattering.

2. Mechanized: Two basic methods are used. The plants are cut or "glided" out of the ground using a tractor with front mounted horizontal blades with blunt cutting edges or rotating disks operated slightly below the soil surface. Several rows are combined into one windrow using a slide-delivery rake which can be rear-mounted behind the cutters. The windrows are dried for 5-10 days with tractor-drawn or self propelled threshers.

Threshing Methods for Beans

Beans can be threshed manually by beating the plants or bagged pods with sticks once they are dry enough. Whatever the method used, bean seed can be easily injured if threshed too roughly or when too dry. Injured seed, when planted, will produce weak, stunted plants and other abnormalities.

Winnowing beans

The procedure here is the same as for corn.

Continue on to the next lesson in this series

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