Section 6: Harvest Groundnuts at Optimum Maturity
Harvest Groundnut at Optimum Maturity
Kernel Maturity and Yield
Deciding when to dig or lift groundnuts is one of the most important management decisions farmers make. Pod yield and kernel quality and economic value improve as maturity increases. Research in Zambia and Mozambique has demonstrated the importance of allowing groundnut to reach maturity to optimize yield.
However, at some point pods will naturally shed from the plant or in the case of Spanish varieties seed can sprout. The longer groundnuts remain in soil, the greater the risk they will be infected by mold and aflatoxin. The timing of harvest impacts yield and quality, as well as seed viability, if the groundnuts were grown for seed.
Since all of these factors interact, farmers should carefully consider the optimum time to lift or dig peanuts.
Optimizing Groundnut Yield and Minimizing Aflatoxin Contamination
Since groundnut is an indeterminate crop (i.e., it flowers continually throughout its life cycle), every pod on an individual plant will be at a different maturity. In addition, plants in different parts of the field can also be at different stages of growth due to environmental and other factors.
A farmer can evaluate the maturity of groundnuts from the field by looking at a layer of the skin on the pod. Groundnut pods have three layers of skin: the outer exocarp, the inner endocarp and a middle layer, the mesocarp. The color of this middle layer (mesocarp) can be used to determine the maturity. As the pod matures, the mesocarp will darken in color, from an initial white color (immature) to brown (intermediate) to black (mature). In a mature crop, the majority of the pods will show the reddish brown/dark black color in the mesocarp.
There are three basic methods to see the mesocarp and evaluate the maturity of groundnut (see below), but all the methods begin with taking a sample of 3 to 4 plants randomly selected throughout the field. Strip all pods from the plants and take a random sample of around 50 pods.
Once the mesocarp is exposed, a farmer can arrange a groundnuts by color, estimate the maturity of the majority of the nuts and approximate the number of days/weeks that remain before the crop will be ready to harvest. A maturity board (poster) has been developed and is used in the USA and other countries. The board provides a color chart for arranging the pods and then indicates the maturity.
Shell-out Method
The “shell out” method is one of the simplest ways to reveal mesocarp color differences. Shell the sample of 50 pods carefully so that the shell remains intact as two halves. Lay out the shells with the inside of the pods facing up. Group pods into three sections representing white to yellow, orange to light brown, and dark brown to black coloration. When only a few pods are in the white to yellow grouping and the remaining pods are evenly divided among the darker categories, groundnuts are at optimum maturity (Figure 6.1). Leaving groundnut in the field after this point does not increase yield and can reduce yield as plants shed pods and kernels sprout. The potential for aflatoxin contamination also increases when groundnuts are left in the field beyond optimum maturity.
The shell-out method is one of the more practical approaches to determining pod maturity. A handout was prepared for farmers to see the relationship of pod maturity, yield and aflatoxin contamination. (See addendum.)
Hull-scrape Method
The “hull-scrape” method requires scraping the outside of the pod near the saddle (middle of the pod) with a knife to reveal the mesocarp color. Remove approximately 50 pods from plants across the field and carefully scrape the shell in the middle of the pod (near the saddle) with a small knife. You do not need to scrape away a lot of the outer layer, only enough to be able to see the color underneath (this will be visible on the older mature pods). Lay out the scraped pods into three sections representing white to yellow, orange to light brown, and dark brown to black coloration. When only a few pods are in the white to yellow grouping and the remaining pods are evenly divided among the darker categories, groundnuts are at optimum maturity (Figure 6.2). Leaving groundnut in the field after this point does not increase yield and can reduce yield as plants shed pods and kernels sprout. The potential for aflatoxin contamination also increases when groundnuts are left in the field beyond optimum maturity.
Pod-blasting Method
For growers with larger land area and access to electricity, the exocarp can be removed using a pressure washer with a rotating nozzle to expose the mesocarp. As in the other methods, select 50 pods from plants across the field. Place the pods in a metal basket inside a larger plastic bucket. Power wash the pods for 2 to 3 minutes using a rotating nozzle and a power washer that generates around 1500 psi. Remove basket with pods, shake dry and lay out pods according to color. Examples of pod mesocarp color using the “pod blasting” method are presented in Figure 6.3.
The change in pod mesocarp color over a 21 day period for two fields in the US are presented in Figures 6.4 to 6.9.
One field (Figures 6.4, 6.5 and 6.6) exhibit a “split crop” in that maturity of pods is generally in the yellow and early stages of the orange while a similar percentage in the brown and black categories. This scenario makes the decision on when to lift a challenge. Depending on weather conditions and disease severity, it is often available to dig when the first group is mature rather than waiting on the pods that are slowing more slowly to reach greater maturity. Damage for pests and greater aflatoxin contamination can occur the longer groundnuts remain in the field.
Figures 6.7, 6.8 and 6.9 present a more typical distribution of pod maturity. A stress much earlier in the season can cause pollination and pod set to be disrupted. When the stress is removed pollination and pod set may resume. A period of drought during the middle of the season is often the cause of a “split crop” making the decision of when to dig more challenging. In this case groundnut should be dug as soon as the dark brown to black color is reached even though this may only be only half of the pods. Yield will be lower than normal because there are no orange to light brown pods represented, however, yield would be even lower if digging is delayed because the more mature pods will shed or be damaged.
If groundnut plants are infected with leaf spot, a farmer should harvest the crop when 40% of leaves have fallen regardless of the maturity shown by the internal pod color. Groundnuts also should be dug early when drought has been excessive during the latter part of the season and no additional rain is forecast. Digging one week earlier than optimum maturity can decease aflatoxin contamination significantly without sacrificing yield since pods and kernels generally do not mature in drought-stressed plants. Dry soil conditions and brittle pegs may also make harvesting more difficult.
