Harvesting and Post Harvest Practises
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8/18/2019 Harvesting and Post Harvest Practises
5 Harvesting and post -harvest practices
5.1.1 Determining the time of harvest In order to determine the optimum time of harvest, the following factors could be considered:
• Variety’s growth duration and its capacity to be prolonged • Environmental conditions (water supply, soil condition, weather, etc) • Pest and disease incidence • Market demand • Market price • Need of land to plant the consecutive crop • Economic value of the consecutive crop
Sweetpotato roots are ready for harvesting between three to eight months after planting, oftenmuch sooner than other root and tuber crops. Most sweetpotato varieties are normally harvested at five months after planting. Harvesting too early or too late can result in low yields. If the crop is harvested too early the roots will not had enough time to develop to their maximum size. If the crop is harvested too late the storage roots may have become fibrous, and the chances of the roots having been attacked by sweetpotato weevils or diseases (particularly root rots) increases. In order to determine the optimum harvest time, farmers should weigh the various factors listed above in the context of the prevailing conditions, and find a balance between advantageous and disadvantageous factors, in relation to the potential and needs of each individual farm enterprise.
5.1.2 Harvest ing methods In several places where sweetpotato is mainly grown basically for home consumption, staggered/ piecemeal harvesting is practiced, whereby a few large roots are dug up and taken back to the home for cooking while the other roots are left on the plants in the field, and will be gradually be removed depending on the pattern of the households consumption. In East
Africa, women typically do the piecemeal harvesting, and they move around the field looking for cracks on the mounds or ridges, which they perceive as being indicative of a sizeable root. Mature roots are selected carefully during the harvesting and the earth is heaped up over the remaining ones to allow them to continue bulking, the heaping up of earth will also protect the roots from sun damage and reduce the chances of weevil access to them. Harvesting is usually done carefully with locally made sharp sticks, rods or machetes in order to avoid injuring the remaining roots. Other farmers harvest all the roots from one area of the field at once using a hand hoe. Some farmers use both methods. Farmers usually harvest enough sweetpotato for one or more meals for one or two days. Piecemeal harvesting can start as early as two/ three months after planting for some varieties. Farmers do not usually harvest large quantities at once, in order to avoid the roots rotting and being wasted. Harvesting a large field all at once is usually only done when sweetpotato is destined for the market.
Different varieties respond differently to piecemeal harvesting; some produce larger or smaller roots as a result of it, some have a longer time period over which piecemeal harvesting can occur. Varieties with longer maturity periods are usually more suitable for piecemeal harvesting than varieties with short maturity periods where all the storage roots tend to mature at the same time.
The duration of piecemeal harvest varies by location and agro-ecology, harvesting can start from three to eight months after planting and might go on for a period of one to nine months depending on the variety. Harvest duration is a function of different factors such as variety, soil type, availability of other foods, household size, disease and pest infestation, and weather conditions. Under high sweetpotato weevil population pressure and during times of water
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stress, piecemeal harvesting could last only a month or less (as plants tend to dry up due to disturbance of the root system, weevils infest any exposed roots and necks through the soil cracks and multiply rapidly). At high altitudes with very low weevil population pressure, piecemeal harvesting can last for six to nine months.
In some areas where commercial sweetpotato growing is common, market competition results
in some sweetpotato roots being harvested before they are mature. In the humid tropical partsof East and Central Africa, it is possible to grow two crops of sweetpotato per year.
Harvesting of sweetpotato is mainly done manually and care needs to be taken to avoid wounding the roots during harvesting. After harvest the roots are usually sorted manually. Sorting involves the selection of roots into different lots based on shape, colour and other physical parameters. Sorting can take place at different levels (field, roadside, market, consumer, catering etc) depending on the intended use of the roots. The sweetpotato roots are then usually covered with plant materials to protect them against direct sunshine which can cause shrinkage and shrivelling.
In some parts of the world such as Japan and China where sweetpotato commercialisation is advanced, mechanical harvesters are used. In this case, vines are removed before
harvesting, then the roots are dug out by machine. The roots are sorted and loaded onto trucks. In many cases, sorting is done by hand. The vines are cut off prior to harvesting the roots as this process allows the skin of the roots to become firmer making them less vulnerable to bruising during harvesting.
Since the yields increase rapidly right up to the time the plant matures, harvesting before maturity means a loss of market quality as well as yields.
5.1.3 Assessment of yield and crop value If farmers are able to assess the sweetpotato yield in their field and are informed about the market price at harvest time, they will be in a stronger bargaining position with the traders. An experienced trader can estimate the number of roots in the soil by looking at just a few plants. Farmers need to be able to do the same in order to calculate the value of their crop and to ensure they don’t sell it either directly from the field at a low value, or harvest it only to discover the deal they agreed on is a poor one.
The quantity of storage roots in field can most easily be assessed by estimating the average root weight per plant and multiplying this by the number of plants in the field. Another way is to assess the average root weight per square meter and multiply this by the field area (in m2). The average root weight per plant or per square meter should be assessed by observing (and weighing if possible) a representative sample across the field. The second method using weight per square meter is more difficult than the first one using weight per plant, particularly when the field has an irregular shape and the exact area is difficult to measure. Therefore, only the first method is discussed in detail here.
The weight of the roots sweetpotato plants produce can vary widely. Therefore it is necessary to estimate the average weight of roots per plant. In order to do this we need to observe a sample of at least 10 plants per 1,000-2,000 m 2 to represent the whole crop, this example assumes that the whole field is planted with the same variety of sweetpotato. These ten plants must be selected randomly across the field. Digging up and weighing the roots is the most reliable way to determine the weight per plant, but this is time consuming and might not be acceptable to potential buyers. In that case, the soil covering the roots is partly removed to observe the storage roots and estimate their total weight. Estimating the weight of roots in the soil is a skill that can only be developed with practice. After having assessed the total sample, the average weight per plant is calculated by totalling the weight of the individual plants and dividing this by the total number of plants observed.
The plant population in the whole field can be determined by counting all productive plants,which is the most accurate way, but very time consuming. A simpler slightly less accurate way is to determine the average number of plants per step along a ridge, hence to count the total
8/18/2019 Harvesting and Post Harvest Practises
number of steps across all ridges or mounds in the field. If the shape of the field is regular and allows easy division into equal parts, we only need to count the number of steps along the ridges of half or quarter of the field, and then multiply that by two or four respectively and then by the number of plants per step along the ridge to get a good estimate of the number of plants in the whole field.
Example: A. Weight of roots per plant Plant 1 – 0.35 kg Plant 2 – 0.50 kg Plant 3 – 0.40 kg Plant 4 – 0.75 kg Plant 5 – 0.50 kg Plant 6 – 0.55 kg Plant 7 – 0.60 kg Plant 8 – 0.20 kg Plant 9 – 0.65 kg Plant 10 – 0.40 kg
If we work out the average weight of roots per plant, we add up the total weight of the roots of the 10 plants, then divide the total by 10 Total weight of roots of the ten plants = 4.9 kg
Divided by 10 = (4.9 ÷ 10) = 0.49 kg
So the average weight of roots per plant is 0.49kg
B. One step along a ridge contains 5 plants. C. The field can be divided into four equal parts; a quarter of the field contains 125 steps
along the ridges, which means the whole field contains 125 * 4 = 500 steps. D. If there are 5 plants per step, and there are 500 steps in the whole field there are 5 *
500 = 2,500 plants in the whole field. Each of these plants has an average root weight of 0.49 kg, so the root weight of the whole field is 0.49 * 2,500 = 1,225 kg.
E. The total (underground) crop value can be cal