Probability Analysis of Microclimatic Variables and Impact of some Biological Environments on Sorghum Varietal Response to Selection

Weekly interval intra-annual variability in rainfall, temperatures and relative humidity of Jima Research Center was expressed in probability terms. The objective was to assess the functional relationship between atmospheric variables and growth patterns of sorghum varieties from which appropriate experimental varieties (responsive varieties) might be identified through specific planning and selection. Three consecutive periods were determined in a year. Period one (PI) represents days from early November to January (week 44 to 5) during which precipitation drops to a practical minimum. While the probability of getting the critial minimum of 15 mm of rainfall on weekly basis is less than 30 percent, average Potential evapotranspiration (PET) and Moisture Availability Index (MAI) values are 33.7 mm and 0.35/week respectively, for this period. The period is termed dry at least for Jima and the outskirting areas. Period two (P2) constitutes week 6 to 15 (January to second week of April). The probability of receiving 15 mm of rainfall/week is only 50 percent. PET is averaged to be 35.4 mm/week and MAI is about 0.054. The name 'per-rainy season' is coined to this range of time. Period three (P3), of April and continues through to October (43 rd week). It is this period which constitutes the main growing season and therefore is suitable for sorghum cropping under rainfed condition. About 45.8 mm/week of rainfall is received during this period. PET drops to its lowest value (25.2 mm/week) and MAI rises to 1.7. The probability of receiving excess rainfall is 82 percent (value above 25 mm/week). Leaf and head diseases and bird damage are the most biological risks limiting progress from selection of high yielding sorghum varieties at Jima. The steadily increasing maximum temperature (25 deg C.day) and high relative humidity (73 percent) prevailing from September or onwards are identified as responsible for development of both leaf and head diseases.