Assessment of Maturity over Seasons using Various Indices in Groundnut (Arachis hypogaea L.)

Oilseed crops have been the backbone of agricultural economy of India from time immemorial. On the oilseed map of the world, India occupies a prominent position, both in regard to acreage and production. The important oilseed crops grown in this country in order of importance are groundnut, rapeseed and mustard, sesame, linseed, safflower, castor, sunflower and Niger (Hedge, 2009). Groundnut oil is edible oil. It finds extensive use as a cooking medium both as refined oil and Vanaspati Ghee. It is also used in soap making and manufacturing cosmetics and lubricants, olein stearin and their salts. Kernels are also eaten raw, roasted or sweetened. They are rich in protein and vitamins A, B and some members of B2 group. Their calorific value is 349 per 100 gram. The H.P.S. (Hand Picked and Selected) type of groundnut kernels is exported to foreign countries. The residual oilcake contains 7 to 8 per cent of N, 1.5 per cent of P₂O₅ and 1.2 per cent of K₂O and is used as a fertilizer.
       
Groundnut pod development takes place in the soil making it difficult to correctly judge the maturity of the crop. As the groundnut crop starts flowering from 25-30 days after sowing (DAS) to 60-70 DAS, varied maturity level is quite common. It is good to have considerable experience and great vigilance to carry out the harvesting operations efficiently without much loss of quality and yields. A proper time to commence the harvest is when a good number of pods are fully developed and are fairly intact. Maturity of pods is normally achieved when the vine begins to turn yellow and leaf shedding starts. Due to inter specific hybridization to incorporate many desirable traits from wild species especially foliar disease resistance, the stay green during maturity is also very common in the recently evolved varieties.
       
A fully mature pod can often be difficult to split open with the pressure of the fingers. An immature pod can be split easily revealing the white surface of the inside pod which appears also to be spongy in texture. These criteria may help in assessing the correct stage of the harvest of groundnut crop. Delay in maturity may occur because of late-season drought stress.
       
Long periods of rain immediately prior to harvest may result in both yield loss and deterioration of quality of groundnuts. Harvesting at the proper time ensures that a high percentage of mature pods remain on the plants and the maximum number of pods has attained their greatest weight.
       
Over the past few decades, several methods have been used to assess groundnut maturity with varying degrees of success and logistical application. These include: days after planting; Langley’s Index; internal hull color (Shell out Method); oil color; methanol extraction; kernel density; seed/hull ratio (SHMI); arginine maturity index (AMI); physiological maturity index; and the hull scrape method (Sanders et al., 1982a,1982b). While these methods of assessing groundnut maturity have been used to varying degrees by groundnut growers in the history of groundnut production, the most easier, non-destructive and currently accepted method for the determination of groundnut maturity practiced by groundnut farmers in developed countries like USA are days after planting, shell out Method, seed/hull ratio, the hull scrape method and the ‘‘maturity profile board’’ method based on the work of Williams and Drexler (1981).
       
Considering the importance of predicting the correct date of harvest of groundnut crop the present study was taken up to optimize the maturity index calculations of selected varieties at Coimbatore condition.
       
In this study, 10 varieties of Spanish bunch groundnut that differ in days to maturity were subjected to maturity index calculations during rabi and kharif  by shell out, seed hull ratio and hull scrap methods. The range of maturity indices with maximum efficacy in predicting the optimum date of harvest was determined.

Spanish bunch groundnut varieties with days to maturity of more than 110 days viz., CO 7, ICGV 07222, VRI (Gn) 6, VRI 8 and GPBD 4 and early maturing varieties viz., VRI 3, Chico, Gangapuri, ICGV 91114 and ICGV 93468 were taken up for the study. The varieties were sown in 3 replications in a row of each 3.0 m length with spacing of 30 × 10 cm.  and cultivated based on the recommended agronomic package of practices during rabi 2018-19 and kharif 2019 at Department of Oilseeds, Tamil Nadu Agricultural University, Coimbatore.
 
The days to maturity of the 10 varieties were collected from the related literatures. A range of days to maturity check was planned keeping the maturity duration of the varieties obtained from the literatures as the mean. In such a way 5 harvests for maturity check for each variety was done with five days interval. In each harvest of all the varieties the maturity index by shell out method, seed hull ratio method, hull scrape method and Maturity profile board was calculated along with average pod yield per plant.
 
Methodology
 
Shell out (SO) method
 
The shellout method is one that has been historically used by growers as a general measure of crop maturity. This method involves shelling pods to observe the interior color of the pod (Sanders et al.,1980). Mature pods have darker interiors due to aged veins and cell death. Some coloration of the seed coat may also occur in the most mature pods (Miller and Burns 1971). On the day of maturity check random plants are uprooted so as to get nearly 200 pods. The matured pods identified by shelling are counted and the maturity index in percentage is calculated by the following formula:
   
Seed hull weight ratio maturity index (SHMI) method
 
The maturity of individual seeds is classified subjectively on the basis of physical and morphological characteristics of the hull and testa (Pattee et al., 1977). The plants are uprooted on the day of maturity check and all the pods are removed. The pods are cleaned off the mud, dried and shelled out. The hull and seed fractions are weighed.  The seed weight is divided by the hull weight. The value obtained is indicative of the average maturity level of the groundnut.
  Hull scrape (HS) method
 
In 1981, Williams and Drexler proposed a manual method for classifying groundnut development stages based on the mesocarp colour of the groundnut pods. First, around 150-200 pods of from the plants for maturity check are collected and the exocarp is removed using water jets or sand abrasion, or hand scraping to expose the mesocarp. The brown and black pods are counted as the matured ones and the maturity percentage is arrived at by the following formula:
  Maturity profile board (MPB) method
 
The MPB, based on the work of Williams and Drexler (1981), developed is the most popular and widely used method. The MPB is a visual tool for pod classification that graphically interprets pod maturity. It consists of 25 categories based on mesocarp color variations within the five major color classes (listed from most immature to mature): white, yellow 1, yellow 2, orange, brown and black.
       
The exocarp must first be removed in order to access the mesocarp and visually classify pods according to their mesocarp color. Pods are placed in colored columns corresponding to the color of the saddle region of pod. At the bottom of the columns on the MPB, the estimated days until digging are projected, corresponding to the distribution of pod maturity. After placing groundnuts on the MPB, the grower typically has to determine when to dig by choosing the column on the MPB farthest to the right that has at least three pods categorized as this single color class. The number below this column represents the recommended number of days before digging should commence.
       
The methodology of calculating maturity index in groundnut namely shell out method, seed hull ratio method, hull scrape method and Maturity profile board are illustrated in Fig 2.
 
Data analysis
       
Season wise separate analysis was carried out. Factorial analysis of variance (ANOVA) was determined for the differences in yield of the 5 harvests of each variety. Significance of the yield of five harvests was tested using the critical difference obtained and grouped (Fig 1). Maturity indices were correlated with yield for the methods viz., shell out method, seed hull ratio method and hull scrape method. Coefficient of correlation was calculated among days after sowing, maturity indices and average pod yield and tested for its significance.
 

The average yield of the 10 varieties in all the 5 harvests scheduled the respective maturity indices calculated by shell out method, seed hull ratio method and hull scrape method during rabi and kharif are presented in the Table 1 and 2. The average yield of all varieties under study shows significantly increasing trend initially in both the seasons. In the varieties that are reported to mature within 100 days viz., VRI 3, Chico, Gangapuri, ICGV 91114 and ICGV 93468, the average yield increased up to the third harvest and there on decreased significantly for fourth and fifth harvest. The trend of increasing yield in the varieties CO 7, ICGV 07222, VRI 6, VRI 8 and GPBD 4 stabilized to some extend in third and fourth harvest while it significantly decreased in the fifth harvest.
 

 

       
Harvesting groundnut earlier to physiological maturity resulted in a reduced number of pods per plant, immature pods with shriveled kernels which in turn resulted into low pod yields among the varieties during the initial harvests.
       
The decrease in average yield after the optimum maturity period is attributed to in situ germination observed in Chico, an early maturing variety commenced on 100 days after sowing. This is one of the important reasons for low yield (Shelar et al., 2014).
       
According to research conducted by Emmanuel Zuza et al., (2017), on groundnut harvesting time, the kernel yields tend to decline with harvesting 10 days before and 10 days after physiological maturity. Other reasons for yield loss in delayed harvests were found by Singh and Oswalt (1995) that insect damage to pods due to an increase in insect population with time. Another factor for lower yield after physiological maturity was adverse effects of dry weather making weakened pegs due to over maturity and other pods were physically damaged.
       
The analysis of variance for yield in the five harvests for each of the varieties (Table 3) was done in both rabi and kharif season data separately. The mean sum of square of the treatment in each variety was significant indicating significant differences present among the average yield 5 harvests of the varieties. In all the varieties the average yield of both the seasons clustered around the third and fourth harvests which were scheduled to be the mean maturity duration of the varieties.
 

       
Using the critical difference calculated at 5%, par diagram was constructed (Fig 1). During rabi the average yield of all the varieties were on par in any of the two harvests indicating an extended time taken for maturing. The varieties CO 7, ICGV 07222 and VRI 8 showed significantly high average yield in one of the single harvest while other varieties showed a range of period for maturing in kharif season. On a whole the maturity duration of the varieties were higher during rabi season on comparison to that in kharif season. This result is in concurrent to the study by Bhagat et al., (1992), in which it was reported that seed germination, days to flowering is slowed down due to lower temperatures prevailing in rabi season leading to delayed maturity.
       
In the maturity profile board (MPB) method, the mesocarp colour of all the varieties under study was able to fit into the colour chart of the Maturity Profile Board (Fig 2). During first and second harvest there were very less mature pods than the immature one. Towards the third and fourth harvest bimodal maturity of the pods in all the varieties was observed rather than uniform maturity during rabi season indicating a situation of extreme weather events when the maturity of the crop slows down. In kharif season maturity of pods were more or less uniform. The days until digging indicated on the board for the right most columns with at least 3 pods were recorded in all the 5 harvests for each variety (Table 4).
 

 

       
The result from the MPB is similar in the last three harvests also indicating that it does not count the pods that germinate in situ or over mature. As a part while using a MPB, earlier maturity check has to be done and harvesting has to commence when higher proportion of pods reach the matured column indicating the lowest days until digging. The groundnut grower has to wisely decide the date of harvest with the distribution of the samples on the board with the help of the days until digging given in the bottom (Ethan Carter et al., 2016).
       
Thus, from the study the optimum maturity index for predicting the date of harvest for groundnut are summarized in the Table 5 and the corresponding maturity duration and maturity classes of the varieties under study are furnished in the Tables 6 and 7.
 

 

 

       
Correlation between the yield and maturity index and days after sowing and between maturity index and days after sowing was studied (Table 8). The results revealed that the maturity indices calculated by shell out method and hull scrape method showed significantly positive correlation with yield while that calculated by seed hull ratio method showed lower significant correlation. Similar report was stated by Rowland et al., (2006), in groundnut that the maturity index calculated by the mesocarp color showed strongest correlation with yield than other methods.


       
In the present study the hull scrape method of maturity index showed highest positive correlation of 55% with average pod yield while shell out method stands second with 48% correlation. The seed hull ratio maturity index showed the lowest significant relation with yield as observed that the method’s result may mislead when pod damages occur.
       
Maturity indices of all the 3 methods show non-significant correlation with days after sowing. This is due to the fact that the index values initially increase with days after sowing and then tend to decrease and thus do not show a linear relationship. The relationship of days after sowing with yield is significantly positive indicating that pod yield increases with the increase in days to harvests. The value of correlation is 41% which less than 50%, which reveals that the increase of yield with days after sowing attains a plateau and there on may remain static or even decrease. This result is on par with that reported by James et al., (2014). They concluded in their study of relationship of yield and stages of maturity in groundnut that yield per plant increased initially up to physiological maturity date and thereon the crop faces yield loss.

The maturity indices by the three methods: shell out method, seed hull ratio method and hull scrape method was calculated and mature pods were arranged on MPB to get the days until digging in all 5 harvests for each variety to arrive the days to maturity of the varieties under study at Coimbatore condition. Among the maturity indices calculations hull scrape method and Maturity Profile Board method - the maturity assessment based on mesocarp colour were most reliable. A groundnut cultivator is suggested to examine the mesocarp colour and calculate the maturity index according to the Hull Scrape method. If the maturity index is under 70%, the samples may be placed on the MPB to determine a prediction for harvest date. Hence harvesting at the proper time ensures that a high percentage of mature pods remain on the plants and the maximum number of pods has attained their greatest weight.