Effect of variable irrigation supply based diversification of Bt cotton (Gossypium hirsutum)-wheat (Triticum aestivum) system on productivity, profitability, soil fertility and water expense efficiency

A field experiment was conducted during 2010–2013 at Punjab Agricultural University, Regional Station, Bathinda, Punjab to study the options for diversification of existing Bt cotton (Gossypium hirsutum L.)-wheat [Triticum aestivum (L.) emend. Fiori & Paol.] cropping system under water scarce conditions. Three crop sequences, viz. wheat-Bt cotton, wheat-clusterbean [Cyamopsis tetragonoloba (L.) Taub], wheat-greengram [Vigna radiata (L.) Wilezek] in the main-plots and 3 irrigation regimes [optimum (O), sub-optimum (SO) and sub-sub optimum (SSO)] were taken in sub-plots with 3 replications under split-plot design. The sowing of the wheat was done according to the time of harvesting of previous crop (cotton, clusterbean and greengram) in same plots. The wheat grain yield was significantly higher where greengram and clusterbean were the preceding crop in rainy (kharif) season than cotton. There was 15.6% and 53.1% decrease in wheat grain yield, respectively under sub-optimum and sub-sub optimum irrigation supply than optimum level. Among the 3 wheat-based cropping sequences, cotton-wheat system gave significantly higher cotton equivalent yield (CEY), which was at par with clusterbean-wheat system under limited water supplies. The CEY in cotton-wheat sequence, where 4 irrigations (SO) were given was at par with the SO level (1 irrigation) of clusterbean-wheat system. At SSO irrigation, where 3 irrigations to cotton and no-irrigation to clusterbean, the CEY's of both the systems were at par. The highest water productivity (0.358 kg/m³), net returns (52.3 × 10³ /ha) and benefit: cost ratio (2.38) were found under clusterbean-wheat crop sequence. This sequence also sustained soil health with higher values of organic carbon (0.37%), available nitrogen (118 kg/ha), available phosphorus (18.9 kg) and available potassium (332 kg/ha) levels than other crop sequences. Significantly the highest yield was recorded in optimum water regime in all rainy season (kharif) crops. Under sub-sub optimum irrigation, the seed cotton yield decreased significantly compared to sub-sub optimum level of clusterbean, where no irrigation was given to clusterbean.