Protecting peanut (Arachis hypogaea L.) from pest damage and providing adequate nutrition through applied fertilizer can increase yield and financial return (Naab et al., 2009). Access to crop protection products and fertilizers as well as ability to purchase these inputs by smallholder farmers in Ghana are challenges to realizing optimum peanut yield and quality (Idoko and Sabo, 2014). Aspergillus flavus and A. parasiticus presence in peanut contributes to aflatoxin contamination in food and can negatively affect human health (Gong et al., 2012; Jolly et al., 2006; Kew, 2012; Turner et al., 2003; Williams et al., 2003).
The majority of smallholder farmers in Ghana do not apply pesticides or fertilizers to minimize the negative impacts of pests or low soil fertility, respectively, to increase yield in part due to financial constraints (Naab et al., 2009; Osei et al., 2018). Some farmers do apply local potassium-based soaps that have been proven to suppress aphids (Aphis gossypii Glover) that transmit peanut rosette virus disease (Umbravirus: Tombusviridae) (Lamptey et al., 2014). These materials can also suppress leaf spot disease in some instances (Nutsugah et al., 2007). When labor is available, farmers could increase weed management through hand weeding. Preventing weed interference during the first 6 weeks of the growing cycle can prevent yield loss from weed interference (Everman et al., 2008). While commercial fertilizers are often considered too expensive for peanut production by smallholder farmers in Ghana, significant increases in peanut yield have been documented when fertilizers containing nitrogen, phosphorus, potassium, and calcium are applied (Naab et al., 2009). Calcium in the form of homogenized oyster shells is available in some areas of the country. In addition to increasing yield and kernel quality, calcium applied to peanut can reduce aflatoxin (Jordan et al., 2018; White and Broadly, 2003).
In southern Ghana, Appaw et al. (2020) reported that a production package that included one extra weeding, application of local soaps for suppression of aphids and peanut rosette virus, and calcium in the form of ground oyster shells at pegging resulted in greater peanut yield and higher financial return compared to the traditional farmer practice without these inputs. However, a comparison of these practices with a commercial fertilizer blend rather than calcium from homogenized oyster shells applied at pegging has not been done in Ghana.
Aflatoxin contamination is affected by practices in the field prior to harvest, during drying, and while in storage (Awuah et al., 2006; Guchi, 2015; Malaker et al., 2008; Villers, 2014; Waliyar et al., 2008 2015). Appaw et al. (2020) reported that drying on tarps rather than the ground and storing in hermetically-sealed bags rather than traditional poly bags lowered aflatoxin contamination in peanut. Using at least one improved practice decreased aflatoxin contamination after storage and resulted in greater financial returns than the standard farmer practice (Appaw et al., 2020). However, the study by Appaw et al. (2020) was conducted in southern Ghana where peanut production is less dominant than in northern Ghana. Although the bimodal rainfall pattern in southern Ghana allows farmers to potentially grow two peanut crops within the same year, rainfall can affect ability of farmers to harvest and effectively dry peanut prior to storage. In contrast, northern Ghana has a unimodal rainfall pattern and farmers plant as soon as possible after the initial rains so that the growing cycle is complete before the rains end (Abudulai et al., 2012). In some years, rainfall is adequate to produce relatively high yields and minimize aflatoxin contamination in the field (Jordan et al., 2018). However, when rainfall is limited during the final stages of peanut growth, yield can be reduced and aflatoxin contamination can be higher (Craufurd et al., 2006; Jordan et al., 2018).
Comprehensive approaches to managing aflatoxin at all steps in the supply chain are relatively untested in Ghana. While research by Appaw et al. (2020) addressed this need in southern Ghana, the majority of peanut is grown in the northern part of the country (Anonymous, 2011). Therefore, research was conducted to determine the impact of improved crop management during the growing cycle and improved practices for drying and storing. The first objective was to compare peanut yield, financial return, pest reaction, and aflatoxin contamination when two sources of calcium-containing fertilizer were applied and additional suppression of weeds and aphids was included compared to the traditional farmer practices that did not include these inputs. The second objective was to determine if aflatoxin contamination differed when peanut was dried on a tarp compared to drying on the soil surface following the different practices used during the growing cycle. The final objective was to determine if aflatoxin levels differed after 4 months of storage when peanut was dried on the soil surface and stored in traditional poly bags compared with drying on tarps and then storing in hermetically-sealed bags.
Materials and Methods
The experiments were conducted on sandy loam soils common in northern Ghana during 2015 and 2016 seasons in farmer fields near Kpalbe (9° 06′ N, 0° 33′ W) in the Salaga North District, near Zankali (9° 50′ N, 0° 41′ W) in the Karaga District of the Northern Region, and Tanina near Wa (10° 3′ N, 2° 50′ W) in the Upper West Region. The cultivar Chinese, the most commonly grown cultivar in northern Ghana, was used in all experiments (Abudulai et al., 2018; Anonymous, 2014).
Improved field practices used in this research were similar to those compared by Appaw et al. (2020) in southern Ghana. The improved practices consisted of one additional hand weeding at 6 weeks after planting (WAP), application of a locally-derived potassium soap at 3 WAP (initiation of flowering), and application of either a commercial blend fertilizer (0% N, 18% P2O5, 13% K2O, 29% CaO) (Yara Legume Fertilizer, Accra, Ghana) at 370 kg/ha or homogenized oyster shells applied at the base of plants at 180 kg/ha at 4 WAP. Although farmer cooperators were the same for each year, the experimental fields were placed in separate areas by each farmer during the two years. The farmer practice did not include fertilizer or soap sprays and had only one hand weeding at 3 WAP. The improved practice for drying included peanut drying on a polyethylene tarp (Kotap America LTD, Lawrence, NY) compared with the farmer practice of drying on the soil surface. The comparison of drying treatments was included within each of the management practice treatments compared during the growing cycle in the field. Peanut pods dried on the soil surface were placed in poly bags while those dried on tarps were placed in hermetically-sealed bags (GrainPro, Inc., Boston, MA) and stored for 4 months. These drying and storage treatments represent the least and most effective approaches to maintaining kernel quality and minimizing aflatoxin (Appaw et al., 2020) and are referred to as the post-harvest farmer practices. Within each experiment, farmers served as replications with 10 to 12 farmers randomly selected by local Ministry of Food and Agriculture staff. Peanut rows were spaced 50 cm apart, with an intra row plant spacing of 20 cm. Plot size was 20 rows with a length of 20 m. A plot with the farmer practice and a plot with each of the improved practices were included in each farmer's field. The experimental design was a randomized complete black with 10-12 farmers serving as replications for each experiment. All treatments in the field, during drying, and in storage were compared by each farmer.
Visual estimates of canopy defoliation caused by leaf spot disease were determined at harvest using a scale of 0 to 100% where 0 = no canopy defoliation and 100 = all leaves had fallen from the plant. Termite (Microtermes spp. and Odontotermes spp.) (Isoptera: Termidae), millipede (Myriapoda: Odontopygidae), white grub (Coleoptera: Scarabaeidae), and wireworm (Coleoptera: Elateridae) populations were determined at harvest from 10 plants randomly selected by removing 60 cm3 of soil within the upper 15 cm of the soil profile. Plants were gently lifted from soil using a hoe. Arthropod density was recorded in situ. Scarring and penetration of pods caused by soil arthropods were determined at harvest by collecting 100 pods at random from each plot. Sample collection for aflatoxin included selection of five plants at random from each of five sections within the 12 bordered inner rows of each plot (total of 25 plants) at harvest (Mahuku et al., 2010). Twenty kilograms of unshelled pods were placed on tarps and on the soil surface for drying to 10% or less moisture prior to placing in poly bags or sealed bags for storage for 4 months. At each step of aflatoxin determination, approximately 2 kg of unshelled pods were aggregated from 12 randomly collected sub-samples for each treatment (150 g per sub-sample). Haulm yield was determined after the plants harvested for aflatoxin analysis were dried. Final weight was adjusted based on the number of plants collected and converted to kg/ha. Pod yield (kg/ha) was determined from the 12 inner rows of the plot and adjusted to 10% moisture. One hundred pods were collected from each plot and sorted into mature kernels and immature kernels. The percentage of mature kernels was determined by dividing the number of mature kernels by the number of total kernels in the sample.
Analysis of Aflatoxin Contamination.
The entire sample of shelled peanut was used in the aflatoxin extraction based on the USDA-GIPSA 2013-041 protocol (USDA-GIPSA, 2015) using RevealQ+ aflatoxin lateral flow strips (Neogen Corp., Lansing, MI) for quantitative test with Mobile Diagnostic Reader (mReader™) (Mobile Assay Inc., Boulder, CO). Shelled peanut kernels (2 kg) were milled using a blender (Preethi Mixer-Blender, Sholinganallur, Chennai, India). Ten grams of milled product were placed in 50 ml extraction tube and 30 ml of 65% ethanol was added and vortexed for 3 min. The mixture was filtered through a 0.45 μm filter paper. One hundred μl of sample extract was added to 500 μl of Reveal Q+ sample diluent. The test strip was removed after 6 min and the level of toxin quantified using the Mobile Detection Reader. Aflatoxin levels greater than 50 μg/kg (threshold determination level for the lateral flow strips) were diluted and re-analyzed. Aflatoxin concentration was also determined using the High Performance Liquid Chromatography (HPLC) based on the AOAC (Association of Official Agricultural Chemists Method 2005.08) (AOAC International, 2006) with minimal detection level of 0.5 μg/kg.
Base cost of production was set at $140/ha including land preparation, seed, planting, and one weeding. Cost of the additional hand weeding was $50/ha. Cost of calcium applied as oyster shells and commercial fertilizer was $6/ha and $148/ha, respectively. Cost of the improved practice during the growing cycle prior to harvest included the local soap for aphid and rosette suppression, one additional hand weeding, and either oyster shells or commercial fertilizer for a combined total of $52/ha or $150/ha, respectively. Cost of removing pods from vines was set at $0.075/kg farmer stock. Shelling cost was set at $0.075/kg shelled peanut.
Peanut price was set at $1.20/kg assuming an estimated shell out rate of 65% of unshelled pods. Estimated financial returns were determined for each combination of the improved and farmer practices during the growing cycle from the gross return (product of unshelled yield in the field with a 65% shell out rate) and the price of $1.20/kg minus the costs of each combination of practices. Unlike results reported by Appaw et al. (2020), kernel quality after 4 months of storage was not documented. This prevents determining the impact of post-harvest practices on financial return.
To address Objective 1, data for pod yield, haulm, kernel maturity, financial returns, canopy defoliation caused by leaf spot disease, percentage of pods expressing visible scaring and puncturing from arthropod feeding, populations of arthropods, and aflatoxin concentration at harvest were subjected to ANOVA using the GLIMMIX Procedure in SAS (SAS Software Version 9.4, Cary, NC) to compare the three treatments administered in the field during the growing cycle. To address Objective 2, data for aflatoxin concentration after drying but before storage were subjected to ANOVA using the GLIMMIX Procedure in SAS considering the 3 (field treatments) by 2 (drying treatments) factorial arrangement of treatments. To address the final objective, data for aflatoxin concentration after 4 months of storage were subjected to ANOVA for a 3 (field treatment) by 2 (post-harvest treatment of drying and storage) factorial treatment arrangement using the GLIMMIX Procedure in SAS. In each analysis, data are pooled over experiments. Means of significant main effects and interactions were separated using Fisher's Protected LSD test at α = 0.05. Data for aflatoxin concentration was transformed to natural logs prior to statistical analysis. Pearson correlation coefficients were determined for pod yield, kernel maturity, financial return, canopy defoliation caused by leaf spot disease, percentages of pods expressing pod scaring and penetration from arthropod feeding, and populations of arthropods at p < 0.05. The data used in the correlation analysis were based on values per individual farmer, thus local effects of disease and insect effects on yield were evaluated. This allowed a more thorough attribution of yield response to arthropod pests that were not controlled by the management practices employed.
Results and Discussion
Applying local soaps to suppress aphids and rosette virus, applying either calcium in the form of ground oyster shells or a commercially blended fertilizer, and weeding one additional time resulted in higher values for pod yield, kernel maturity, and financial return when compared to the farmer practice (Table 1). Haulm yield was similar when comparing the farmer practice to the improved practice when either fertilizer was applied while haulm yield following commercial fertilizer exceeded yield following application of oyster shells. Pod yield, kernel maturity, and financial return were greater when commercial fertilizer was applied compared with oyster shell only.
Scarring and penetration of pods caused by arthropods, canopy defoliation caused by early and late leaf spot disease, and termite number were similar for both improved practices, and lower than the farmer practice (Table 1). These results were not unexpected. Additional weeding most likely reduced weed interference and protected yield for the improved practices compared with the farmer practice of a single weeding. In addition to suppression of aphids, local soaps can play a significant role in suppressing leaf spot disease (Nutsugah et al., 2007). While calcium is important for overall plant growth and pod and kernel nutrition, applying a commercial fertilizer that contains N, P2O5, and K2O as well as calcium likely improved plant health and contributed to both an ability to withstand biotic stresses and increased plant nutrition, resulting in an increase in yield (Jordan et al., 2018). However, the mechanism causing less pod damage was not determined in these experiments. Based on financial returns, the increase in yield more than compensated the farmer for the additional costs associated with fertilizer, local soaps, and the additional hand weeding.
Less aflatoxin was observed at harvest when the improved practices were used compared with the farmer practice (Table 1). However, this difference was small and the biological significance unknown. The number of experiments and replications within experiments contributed to greater power in comparing these treatments (96 observations for each treatment in the field). The interaction between the field treatments and drying treatments was not significant for aflatoxin concentration after drying (p = 0.8697 and p = 0.4862, respectively). However, drying peanut on a tarp rather than the soil surface resulted in less aflatoxin (Table 2). No difference in aflatoxin concentration was observed after 4 months of storage when comparing peanut dried on the soil surface and then stored in poly bags vs. drying peanut on tarps and then storing in sealed bags (Table 2). This could be due to very low aflatoxin in peanut after harvest. Similar results were observed for field storage studies by Darko et al. (2018). They found, when the concentration of aflatoxin is low, the type of packaging did not influence the increase in aflatoxin content during storage. However, if the initial aflatoxin content is higher, use of hermetically-sealed packages provided more effective control of aflatoxin levels during storage. A major objective of this research was to determine the impact of practices in the field during the growing cycle and at the drying and storing stages on aflatoxin contamination in peanut just prior to consumption or marketing. Generally, aflatoxin contamination in peanut across northern Ghana and in particular at these experimental locations was relatively low in 2015 and 2016 (Sugri et al., 2017). This limited the ability to make adequate comparisons across treatments at each step in the supply chain relative to aflatoxin contamination.
Pearson correlation coefficients are presented for pod yield, haulm yield, kernel maturity, and financial returns vs. data for individual response to pests or their damage (Table 3). While these correlations are informative, they are challenging to interpret because in this research, peanut response to an improved production package was compared to the traditional farmer practice. This approach limits ability to establish cause and effect when considering individual responses for a particular pest. Additionally, the biological significance of reaction by some pests, especially arthropods, may have been limited in these experiments.
Results from these experiments document the positive contribution of production management packages in the field (increased suppression of weeds and insects, and improved plant and kernel nutrition) during the growing cycle to pod yield, pest reaction, financial returns, and aflatoxin reduction. Appaw et al. (2020) also demonstrated increased yield and financial returns when one extra weeding was performed, and local soaps and calcium were applied compared with the traditional farmer practice in research conducted in southern Ghana. Similar to other research (Appaw et al., 2020; Jordan et al., 2018), our results document the benefit of drying peanut on a tarp vs. on soil to reduce aflatoxin contamination. However, availability of labor and inputs and ability to access adequate financial credit to purchase inputs remain major challenges for smallholder farmers in northern Ghana (Quartey et al., 2012). None-the-less, results from this research provide information on solutions that could address poor pest control and low yields of peanut in Ghana.
This publication was made possible through support provided by the Office of Agriculture, Research and Policy, Bureau of Food Security, U.S. Agency for International Development, under the terms of Award No. AID-ECG-A-00-07-0001 to The University of Georgia as management entity for U.S. Feed the Future Innovation Lab on Peanut Productivity and Mycotoxin Control (2012-2017). The opinions expressed herein are those of the authors and do not necessarily reflect the views of the U.S. Agency for International Development. Appreciation is expressed to technical staff and farmers for assistance with this research.
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- Council for Scientific and Industrial Research-Savanna Agricultural Research Institute, Tamale, Ghana; [^]
- Council for Scientific and Industrial Research-Savanna Agricultural Research Institute, Wa, Ghana; [^]
- University for Development Studies, Tamale, Ghana; [^]
- Kwame Nkrumah University of Science and Technology, Kumasi, Ghana; [^]
- Council for Scientific and Industrial Research-Crops Research Institute, Kumasi, Ghana; [^]
- Department of Crop and Soil Sciences, North Carolina State University, Raleigh, NC; [^]
- Department of Entomology and Plant Pathology, North Carolina State University, Raleigh, NC; [^]
- University of Connecticut, Storrs, CT; [^]
- Agricultural and Biological Engineering Department, University of Florida, Gainesville, FL; [^]
- Department of Food Science and Technology, University of Georgia, Griffin, GA; [^]
- Department of Food Science and Nutrition, University of Minnesota, St Paul, MN; [^]
- Virginia Polytechnic Institute and State University, Blacksburg, VA; [^]
- Feed the Future Innovation Lab for Peanut, University of Georgia, Athens, GA. [^]