A field experiment was conducted during rabi 2016-17 at Research farm of Punjab Agricultural University, Ludhiana involving two barley (Hordeum vulgare L.) varieties: V₁ (DWRUB 52, Two-rowed) and V₂ (PL 807, Six-rowed) sown on three sowing dates viz. D₁ (25^th October), D₂ (10^th November), D₃ (25^th November) and three irrigation levels viz. I₁ (Recommended 4 post sowing irrigation), I₂ (Skip at vegetative stage), I₃ (Skip at anthesis stage) replicated thrice in factorial split plot design to investigate heat summation indices in relation to crop phenology under changed microclimate. Among different sowing windows, V₁ took more number of days (158) along with highest AGDD (2059.45 ^oC day) to attain physiological maturity as compared to V₂ (155 days and 1983.75 ⁰C day) under D₂. Phenological models explained 94 to 99 per cent variation in crop phenology due to heat units under three dates of sowing.V₁ produced more plant height and tiller numbers (89.26 cm and 332 per m² respectively) than V₂ whereas D₂ recorded maximum value of these attributes (91.53 cm and 362.06 per m² respectively) followed by D₁ and D₃ at harvest and 90 DAS respectively. Heat use efficiency reflected positive linear relationship with different growth components and economic yield of crop.

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