Effect of Weather Indices under Different Sowing Windows on Grain Yield of Sorghum

Background: Sorghum ( Sorghum bicolor ) is one of the major staple cereal crops after rice and wheat. Photoperiod (day length) sensitivity has been shown to affect the duration of plant’s vegetative phase based on sowing dates. Optimum sowing date is one of the most important factors that play an important part in yield. Methods: A field experiment was carried out during the summer season of 2022. in the Eastern block farm, field No:-37F of Tamil Nadu Agricultural University (TNAU), Coimbatore in sandy loam soil to study the phenological behaviour of sorghum (var. CO-32) as influenced by sowing window (D 1 - First fortnight of February, D 2 - First fortnight of March and D 3 - First fortnight of April) by analysing the Helio-thermal units (HTU), Photo-thermal units (PTU) and growing degree days (GDD) and to evaluate the performance of sorghum CO-32 cultivar. The experiment was laid out in strip plot design. Result: Among the different sowing windows, highest grain yield was recorded when sowing was done in I FN of April (2585 kg/ha) followed by I FN of March (2459 kg/ha). The attainment of phenophases was found early in February sowing. It was observed that accumulated heat units increased by 114 and 142.2  C days, respectively in March I FN and April I FN sown crop compared to February I FN sown crop. Among the sowing windows maximum photo thermal units were accumulated by April I FN sown crop (26787.2  C day hrs) followed by March I FN (25797.1  C day hrs) and February I FN (24459.8  C day hrs) sown crop


INTRODUCTION
Sorghum (Sorghum bicolor) ranks fifth among the important crops in the world after wheat, rice, maize and barley and is one of the main staple foods of world's poorest and most food insecured people across the semiarid tropics. In India sorghum is grown in an area of 4.3 million ha with the production of 4.8 million tonnes and productivity of 1099 kg/ha (MoA and FW, 2021).
The sorghum crop is characterized by its tolerance to the environmental conditions that are not suitable to produce other summer crops (maize and soybeans), especially towards heat, drought and soil salinity, hence, called as Camel Crop (Lamessa et al., 2016). In India, sorghum is cultivated during both monsoon (rainy) and winter (post rainy) seasons, mainly as a rainfed crop.
Since sorghum is grown as a rainfed crop, the climatic factors play a significant role in its productivity. W ith the threat of climate change looming large on the crop productivity, the tropics are the most vulnerable areas of the world, especially the semi-arid areas where higher temperatures and more variable rainfall might have significant negative effects (Parry et al., 2004). So, sowing window play an important role for achieving optimum yield and to enable the crop to avoid environmental stresses at critical growth stages.
The important meteorological variables associated with agricultural production are solar radiation, precipitation and air temperature (Hoogenboom, 2000).An estimation of the harvest date and the stage of crop production can be given with the knowledge of accumulated GDD. A minor change in temperature will adversely affect the phenophase duration. The present investigation was carried out to generate location specific information on duration of phenophases in sorghum variety CO-32.

MATERIALS AND METHODS
The experiment was conducted in the Eastern block farm of Tamil Nadu Agricultural University, Coimbatore. The experimental site is at an altitude of 420.7 m above mean sea level and is situated at 11N latitude and 7657E

Effect of Weather Indices under Different Sowing Windows on Grain Yield of Sorghum
longitude. The soil of experimental field is sandy loam in texture, low in available nitrogen, medium in available phosphorus and high in available potassium. The sorghum variety Co-32 in the experiment was released from TNAU during the year 2020.The varietal characters are presented in Table 1.
The field trail was conducted in summer season of 2022. The experiment was laid out in strip plot design with three different sowing windows (D 1 -First fortnight of February, D 2 -First fortnight of March and D 3 -First fortnight of April) as Factor A and six different crop geometries as factor B (S 1 -45  15 cm, S 2 -45  10 cm, S 3 -45  5 cm, S 4 -30  15 cm, S 5 -30  10 cm and S 6 -30  5 cm) and replicated thrice. All necessary package of practices were followed during the crop growing season. Daily maximum and minimum temperature, rainfall, relative humidity, wind speed, bright sunshine hours during the cropping season were retrieved from Agromet observatory of the Agro Climate Research Centre, Coimbatore. The weather graph of growing season was presented in Fig 1. The data was converted into standard meteorological weeks (D 1 -6 th SMW TO 21 st SMW, D 2 -10 th SMW to 25 th SMW and D 3 -14 th SMW TO 26 th SMW). Using this daily weather data, weather indices such as Helio-thermal units (HTU), Photo thermal units (PTU) and growing degree days (GDD) were calculated based on air temperature and used to describe changes in phenological behaviour and growth parameters (Paul et al., 2000;Girijesh et al., 2011;Prakash et al., 2015).The temperature based weather indices provide a reliable prediction for crop development and yield. A minimum temperature of 19C to 25.2C, maximum temperature of 31C to 36.5C, bright sunshine hours upto 10.3 hrs and a rainfall of 148.4 mm have been recorded during the growing season. The occurrence of different phenophases of the crop has been recorded when the crop reached the respective stages i.e., P 1 = Sowing to emergence, P 2 = Emergence to booting stage, P 3 = Emergence to 50% flowering, P 4 = 50% flowering to maturity and P 5 = Booting to 50% floweringP 6 = Emergence to maturity. The weather indices were calculated using the following formulae.

Growing degree days
GDD is the difference between the mean temperature of the day and base temperature. It is one of the best weather indices for assessing the development of crop. Base temperature of 10C has been used to compute GDD on daily basis for sorghum (Kumar et al., 2008).

Phenological phases
The results pertaining to phenophase attainment are presented in the

Growing degree days, Helio-thermal units and Photothermal units
The agro meteorological indices of GDD, HTU and PTU accumulated during different phenophases of sorghum are presented in  Th e maxim um P TU ac cu mu latio n (C day hrs) for attaining maturity also followed similar trend as that of P 1 = Sowing to emergence, P 2 = Emergence to booting stage, P 3 = Emergence to 50% flowering, P 4 = 50% flowering to maturity and P 5 = Booting to 50% flowering P 6 = Emergence to maturity. D 1 -I FN of February, D 2 -I FN of March, D 3 -I FN of April.

Grain yield
The grain yield of sorghum cultivar CO-32 was significantly influenced by the sowing time. The data regarding grain yield are presented in the  (Mishra et al., 2017). Azrag and Dagash (2015) reported that sowing date had greater effect on yield.

Leaf area index
Data pertaining to leaf area index was presented in Table 5

CONCLUSION
From the above study, it may be concluded that CO-32 sorghum variety recorded highest yield (2585 kg/ha) when