We have been trying to extend soybean in northern Bangladesh in Kharif II (Avg. Temp 30-35°C) and Rabi (Avg. Temp 18-25°C) seasons for several years. We are getting fluctuating yields often due to variations in environmental and climatic factors. We need to choose a suitable variety of soybean. We have come to know that crop heat unit (CHU) indexing system is used in temperate countries like Canada to select suitable crop varieties for a region. We are trying to find out whether such a system can be used in the subcontinent.
We would like to know:
· Can this system be appropriately adapted for tropical and subtropical countries?
· Is such a system in use India or any other country of Southeast Asia?
· Can this system be used to select crop varieties to avoid heat damage of excessively hot periods of summer?
· In case this system is used in this region, what are crop heat unit (CHU) values for different soybean varieties?
Crop Heat Units (CHU) provides an indexing system to assist farmers to select the most suitable hybrids and varieties for their area. This indexing system was originally developed for field corn and has been discovered in Ontario and is useful in many other countries for more than 30 years. The crop heat unit ratings are based on the total accumulated crop heat units (CHU) for the frost-free growing season in each area of the province. It describes how much CHU are available in different regions and how hybrids and varieties of some warm-season crops are rated.
The rate of development of crops from planting to maturity is dependent mainly upon temperature. Cool temperatures slow down the progress to maturity and warm temperatures hasten maturity. Other environmental factors – such as photoperiod (daily period from sunrise to sunset), soil fertility and available water in the soil – can also influence the rate of crop development.Air temperatures are usually expressed in terms of the daily minimum (min) and maximum (max).
Daily crop heat units are calculated from these min and max temperatures. There are separate calculations for day and night. The daytime relationship uses 10 °C as the base temperature and 30 °C as the optimum because warm-season crops do not develop when daytime temperatures fall below 10 °C and they develop fastest at about 30 °C. The nighttime relationship uses 4.4 °C as the base temperature and does not specify an optimum temperature because nighttime minimum temperatures very seldom exceed 25 °C.
he most common calculation for accumulated heat for corn in Canada is the ‘Ontario Crop Heat Unit’ (CHU). This calculation is based on the maximum (Tmax) and minimum (Tmin) daily temperature in degrees Celsius. This index reflects that growth rate of corn:
1. increases proportionately to minimum daily temperature (usually night-time temperature), at all temperatures above 4.4°C (40°F)
2. increases at increasing rates with maximum daily temperature (usually daytime temperature) from 10 to 30°C (50 to 86°F), then at decreasing rates to 50°C (122°F).
Agricultural Meteorology
Volume 26, Issue 3, May 1982, Pages 201-213📷
Heat units in relation to corn maturity in the Atlantic region of Canada,https://doi.org/10.1016/0002-1571(82)90031-0Get rights and content
Abstract: Field trials were conducted at five locations in the Atlantic region of Canada over seven years to determine relationships between climate and corn (Zea mays L.) maturity. Ear moisture at harvest was correlated with Corn Heat Units (CHU), growing degree-days above 10°C and soil temperature (r2 = 0.32 to 0.49, P ⪯ 0.01). Day/night temperature range and a latitude factor were also significant in regressions, although not likely due to ‘cause and effect’ relationships. Locally recommended hybrids required 2300–2500 CHU's to reach 30 to 35% whole plant dry matter, and 2500–2700 CHU's to achieve 35% kernel moisture. Corn grown at one location required an additional 150 CHU's to achieve maturity in comparison with the other test sites possibly due to coastal influences on climate.
Long-term weather records were used to calculate available CHU's and to construct a CHU zonation map for the region. Average CHU's available for silage corn production ranged from over 2500 in large inland valleys in New Brunswick and Nova Scotia, to less than 1900 in northern New Brunswick, the south shore of Nova Scotia, and Cape Breton Island. Production of silage is presently limited to regions with 2100 CHU's or more before frost, and grain corn to regions with 2400 CHU's or more before frost Coastal regions require an additional 150 CHU's for both silage and grain corn production.
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The heat units as used in maize works in tropical and temperate zones. Maize is photoperiod insensitive. Soybean is generally quite photoperiod sensitive and varieties most be selected based on their photoperiod adaptability. If your cropping is being severely limited by heat you may want to select your monsoon crop as a heat tolerant one and grow a cool season adapted rabi crop in a multiple crop sequence.
In tropics and subtropics maximum temperatures frequently overpass the crop maximum threshold, thus calculation oh heat units with classical residual method produces inaccurate phenological prediction
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