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a b s t r a c tEfficient use of energy in agroecosystems will reduce environmental problems, prevent destruction of natural resources and serve to promote sustainable agriculture as an economical production system. The aim of this study was to investigate the energy use efficiency in four double cropping systems including: wheat-silage corn (W-SC), barely-silage corn (B-SC), barely-grain corn (B-GC) and barely-rice (B-R) in the arid regions of Isfahan province, Iran. Data used in this study were collected from 73, 45, 38, 18, 18 wheat, barley, silage corn, grain corn and rice farms, respectively, personal interview using semistructured questionnaire during 2010. The results indicated that the total energy consumed were 140,422, 128,979, 121,360 and 172,962 MJ ha −1 for the W-SC, the B-GC, the B-SC and the B-R cropping systems, respectively. The share of diesel fuel by 43.36% (W-SC), 43.93% (B-GC), 42.82% (B-SC) and 49.40 % (B-R) was the highest input. This was followed by fertilizer (W-SC: 24.70%, B-GC: 25.12%, B-SC: 27.05 and B-R: 16.11) and water (W-SC: 10.54%, B-GC: 11.76%, B-SC: 10.73 and B-R: 13.85), respectively. The energy use efficiency was found as 1.70 for W-SC, 1.65 for B-GC, 1.64 for B-SC and 1.03 for B-R double cropping systems, respectively. According to the research results the W-SC, B-SC, B-GC and B-R double cropping systems were more efficient in terms of energy, respectively.

Weed management in potato production is one of the main cost and time consuming practices. Understanding the most effective time of weed control could reduce the costs and increase potato yield. Field study was conducted in the west region of Iran during 2006 to evaluate the effect of weeds damage in potato fields. Twelve treatments used consisted of six initial weed-free periods in which plots were kept free of weeds for 0, 10, 20, 30, 40 and 50 days after crop emergence (DAE), and then weeds were allowed to grow until harvest, and six initial weed-infested periods in which, weeds were allowed to grow for 0, 10, 20, 30, 40 and 50 DAE, then the plots were kept free of weeds until harvest. Experiment was arranged as a randomized complete block design with three replications. The results showed effect of weed competition on crop dry matter started about 40 DAE and about 90 DAE reached its maximum. The weeds competition decreased dry matter accumulation, leaf area index, crop growth rate, leaf area index duration, light absorption, light extinction coefficient and radiation use efficiency (RUE) of potato. Weeds reduced the potato yield 54.8 percent. The beginning and the end of the critical period of weed control in potato (CPWC) was based on 5% and 10% tuber yield loss. The onset of the CPWC ranged from 486 to 572 GDD, at 5% and 10% yield loss level corresponding to 11 and 19 days after crop emergence, respectively. The end of the CPWC varied from 1372 to 1164 GDD, at 5% and 10% yield loss level corresponding to 65 and 51 days after crop emergence, respectively. RUE in the weed infestation treatment in comparison to the weed free treatment, reduced 11.8 percent.

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