A method of producing pollen that reduces fitness of at least one Amaranthus species of interest is provided. The method comprises treating the pollen of plants of an Amaranthus species of interest with an irradiation regimen selected from the group consisting of: (i) X-ray radiation at an irradiation dose of 20-1600 Gy； (ii) gamma radiation at an irradiation dose of 20-2000 Gy； (iii) particle radiation； and (iv) UV-C radiation at an irradiation dose of ???µ?/cm2 -50J/cm2, with the proviso that when the irradiation is X-ray the irradiation dose is not 300 Gy and wherein when the irradiation is gamma irradiation the irradiation dose is not 100, 300 and 500 G, and wherein when said radiation is UV-C the dose radiation is not 2 J/cm2.
