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Field experiment was conducted to investigate the impact of sowing dates on thermal utilization of grass pea at Instructional Farm, Bidhan Chandra Krishi Viswavidyalaya, West Bengal. Grass pea was sown on nine different dates (D1 to D9) from 26th October to 21st December at weekly interval. Accumulated thermal indices viz. Growing degree days (GDD) or Heat units (HU), Heliothermal units (HTU) and Photothermal units (PTU) were computed for pre-flowering phase, post-flowering phase and entire growing period. Thermal use efficiencies were evaluated both in terms of grain yield and leaf and stem weight. D4 (sowing on 16th November) produced highest grain yield (99.44 g m-2) and leaf and stem weight (33.63 g m-2). Grain yield was positively correlated with the thermal indices accumulated during pre-flowering phase wherein GDD showed highest correlation (r = 0.81) followed by PTU (r = 0.75) and HTU (r = 0.44). Thermal use efficiencies (HUE: Heat unit use efficiencies; HTUE: Heliothermal use efficiencies and PTUE: Photothermal use efficiencies) varied form one treatment to another as affected by varying dates of sowing. D4 showed highest thermal use efficiencies both in terms of grain yield (HUE: 0.0592 g m-2 day0C-1; HTUE: 0.0095 g m-2 0C hour-1; PTUE: 0.0054 g m-2 0C hour-1) and leaf and stem weight (HUE: 0.0200 g m-2 day0C-1; HTUE: 0.0032 g m-2 0C hour-1; PTUE: 0.0018 g m-2 0C hour-1). Results revealed that sowing time greatly influence accumulation and utilization of thermal indices in grass pea.

Key words: GDD, Grass pea, HTU, Sowing date, Thermal use efficiencies.