MSc

A field experiment entitled “Crop weather relationships of cocoa (Theobroma cacao L.)” was conducted at the Department of Agricultural Meteorology, College of Horticulture, Vellanikkara from April 2007 to March 2008. The location is situated at 10o31’ N and 76 o13’ E at an elevation of 25 m above the mean sea level in the central zone of Kerala. The experimental site is attached to the farm of Cadbury – KAU Co-operative Cocoa Research Project, Vellanikkara. The experimental cocoa trees were 20-year-old. A total number of 30 cocoa trees were selected, out of which 15 each were grown under shade and open conditions. The trees in shade were classified as plants giving yield of <15, 15-30 and more than 30 pods per tree per year and the plants in open were classified as plants giving yield of <60, 60-90 and more than 90 pods per tree per year. The biotic events viz., flowering and fruiting characters were recorded once in a week during the study period of one year. Daily meteorological data on maximum and minimum temperatures, relative humidity (morning and evening), rainfall and rainy days, bright sunshine hours, evaporation and cloud amount were collected from the Department of Agricultural meteorology, College of Horticulture, Vellanikkara. The investigations were undertaken with the objectives such as to study the seasonal influence on flowering and fruiting behaviour of cocoa, to find out the relationship between weather elements and biotic events of cocoa and to understand the impact of climate variability on cocoa production and productivity across the State of Kerala.
The biotic events viz., flowering, pod set and cherelle production in cocoa were seen throughout the year though weekly variations were significant and almost no biotic events were recorded in August. The pattern of biotic events in cocoa under open and shade were also similar. All the yield groups showed identical trend in flowering, pods set and cherelle production of cocoa irrespective of open and shade. However, the average number of flowers, pod set and cherelle was always lower in shade conditions when compared to the cocoa grown under open conditions. It is observed that the light availability on an average over cocoa under rubber (shade) was only 45 per cent, varying from 39 % (October) to 56 % (April). The overall percentage contribution of number of flowers, pod set and cherelle during summer was high when compared to that of the other seasons. The flowering appears to be very low during the heavy wet spell due to mechanical damage as well as low bright sunshine available to the crop. It reveals that the light availability is very important for better performance of cocoa in terms of flowers, pod set and cherelle production.
The correlations between rainfall and flowering indicated that rainfall had negative (-0.464) relationship with flowering, as no significant flowering was seen during the heavy rainfall period from June-October. Nevertheless, the summer rains (February-May) two weeks before flowering influenced the flowering favourably. The positive relationship of cocoa flowering with maximum temperature was only due to flowering pattern of cocoa but not due to high maximum temperature (35-37°C) that prevailed during the summer. There was a positive trend between the helio-thermal units (HTU) and biotic events such as flowering, pods set and cherelle production in cocoa and significant at 0.01 level. The helio-thermal units depend on growing degree days, which is a function of temperature [(Max+Min/2)-10oC] and number of daily sunshine hours.
The number of pods harvested was more in October and November, followed by summer (February-April) in contrast to the mean trend where the mean yield was more during summer followed by post monsoon season. The pattern of pod harvest was different between the habitats (open and shade) as the peak harvest in shade appeared during February – March while October – November in open. The coefficient of variation was very high (48.9-124.3 %) in monthly pod yield of cocoa while it was less (23 %) in the case of annual yield of cocoa. It indicated that the monthly cocoa yield is very sensitive to extreme weather conditions. The study also revealed that there was a sharp decline in cocoa area while increase in production and productivity due to technological interventions. However, the inter-annual variations in cocoa yield could be related to weather aberrations and it had no biennial bearing tendency.
The pods harvested during November (post monsoon season) was superior in pod weight (562g), pod length (17.03 cm) and bean weight (1.28). The pods harvested during September (rainy season) and January (winter) showed intermediary, having the pod weight of 555 g/pod in September and 524 g/pod in January. The pods harvested during summer recorded 44 and 29 per cent less in pod weight and bean weight, respectively when compared to post monsoon season. Hence, a harvest of five pods during the post and southwest monsoon seasons equals to nine pods harvested during summer season, 5.4 pods during winter while 5.1 pods in south west monsoon. The study reveals that the low pod and bean weights during summer were due to high number of pods produced, moderate to severe soil moisture stress and high maximum temperature including temperature range.
The maximum temperature from January to March had a profound influence on annual cocoa yield. The relationship between growing degree days (GDD) and yield also indicated similar trend as in the case of maximum temperature. The mean maximum temperature during summer was high in poor yield years while less in good yield years. There was an inverse trend between the annual rainfall and cocoa yield. It was found that the difference in cocoa yield during rainy months was very significant followed by post monsoon between good and bad yield years and thus the adverse influence of heavy rain on cocoa yield. On an average, the decline in yield was 45 per cent in bad yield years when compared to the mean yield while 45 per cent increase in good yield years during the southwest monsoon. The percentage increase in yield during good years when compared to that of bad years was 72 and 58 per cent during southwest monsoon and post monsoon, respectively. On examination through step wise regression, it was understood that the model explained 43 per cent variation in pod yield of cocoa due to maximum temperature alone. It revealed that high maximum temperature during summer with heavy rainfall during rainy season is likely to affect the annual cocoa yield adversely up to 40-50 per cent. Similar results were obtained when the secondary data on annual cocoa yield at the State level was subjected to crop weather analysis. From the above, it is clearly understood that high maximum temperature during summer, high rainfall and low light availability during the rainy season are the main factors limiting the cocoa production and productivity over the humid tropics.