Differential Responses of Weeds and Barley to Soil Nitrogen Management in Semi-Arid Environments.

Barley is the second most important crop grown in Jordan, nevertheless, productivity per unit area is very low. Poor barley production is mainly attributed to limited and uneven distribution of rainfall, improper fertilization, and to magnified weed interference. Most farmers grow barley in areas receiving 200-300 mm (approximately 8-12 in) of annual precipitation. Although recommended rates, timing and methods of N fertilizer application have been developed, producers are reluctant in using N fertilizers or are using minimal amounts due to uncertainties and economic factors. Split N application is a recommendation that aim to maximize N use efficiency by the crop. Increased uptake of N can promote greater stem extension, branching, and leaf area proportions. Many weed species are considerably more effective than crops in capturing nutrients applied in fertilizers. Increases in soil N fertility may alter canopy relations in mixtures in favor of the weed component, thus, N application may have a neutral or even negative effect on crop yield. N application practices must be harmonized with weed management. However, the potential for increases in N to exacerbate rather than diminish weed problem emphasize the importance of achieving better understanding of weed-crop interactions. Models can be used to predict the performance of plant species in mixtures based on knowledge of how the individual species respond to variations in environmental conditions while grown in pure stands. Results of field surveys in representative semi-arid barley planting locations indicated that the most abundant weeds are Hordeum spp. (Poaceae), Avena spp. (Poaceae), Trigonella spp. (Fabaceae), Anthemis spp. (Asteraceae), and Senecio vernalis (Asteraceae).  The first weed species is a wild relative of barley, whereas the second species share the same taxonomical plant family with barley.  This indicates that competitiveness with the crop for N will be at its max. The three other weed species are broadleaves with recognizable variations in morphology and phenology compared to barley. The presence of these two distinct groups of weeds implies that several strategies should be imposed to increase N use efficiency in barley. Ongoing research aims to pinpoint cases where the additions of N increase weed surface area in upper canopy levels, which greatly increase the weeds’ ability to shade the crop. Weed infestation density in relation to N rates and time of application is another aspect that is under consideration. In many incidences, N application increased seed production of weeds and decreased crop yields compared with unfertilized treatments. One possible approach will be to delay application of N fertilizers, in an attempt to starve the weed during the critical initial stage and better match the timing of crop nutrient demand, which is very applicable if interspecific competition prevails with weeds of short stature and relatively early growth and development. Later N application can reduce the negative effect of such weeds on yield by more benefiting the crop.