At present, and more so in the future, irrigated agriculture will take place under water scarcity. Owing to the global expansion of irrigated areas and the limited availability of irrigation water, there is a need to optimize water production and use efficiency (WUE). In South Africa, annual ryegrass (Lolium multiflorum) is one of the most widely grown cool season pasture species under irrigation. It is mainly used in dairy farming enterprises. Shortages of water and nitrogen can, however, be limiting factors for the production of this pasture. By using appropriate irrigation and nitrogen management tools, water and nitrogen productivity of the pasture can be improved. The objective of this study was, therefore, to determine the effects of different water levels in combination with different N fertiliser applications on the growth rate and dry matter production, quality, water use and water use efficiency of annual ryegrass. For two seasons, the trial was conducted under a rain shelter on the Hatfield Experimental Farm of the University of Pretoria. Higher frequency of irrigation coupled with high nitrogen application significantly improved the dry matter yield. Canopy size influenced the LAI and FI which in turn affects the yield. The study showed that the treatments that were irrigated twice weekly and top-dressed with 60 kg N ha-1 after each cut consumed the most water, and this resulted in the production of higher yield, maintenance of the largest leaf area index and higher interception of the incoming solar radiation. The increase in these parameters may be due to the sufficient water and nitrogen fertiliser that induces rapid cell elongation as a result of higher water potential, higher turgor pressure and higher photosynthetic processes. As hypothesized, the decrease in the frequency of water application resulted in an increase in the DMC, digestibility, ME and CP values. Nitrogen application had an effect on the WU, as less water was used in the treatments that received no nitrogen. Highest KC value recorded was in the optimal range and this indicates that the treatments were not over-irrigated. As the irrigation interval increased, more water was depleted from the soil profile. Depletion rates increased as the season progressed but generally it was minimal in the frequently irrigated treatments. Increase in WUE was achieved by reducing the frequency of irrigation from twice a week to once a week without causing significant yield loss. A possible reason for the increase in the WUE by reducing the irrigation frequency could be ascribed in part to reduced evaporation from the soil resulting from the lower wetting frequency of the deficit irrigation treatments. Within the same irrigation frequency, higher WUE was achieved by alleviating a limiting factor, N fertiliser, in this case, through increases in dry matter production. The highest WUE was achieved by irrigating once every two weeks. However, in some treatments, the WUE was not improved with the reduction in the frequency of irrigation as the water saved was overshadowed by yield loss. In summary, it can be said that the hypotheses that pasture production will be positively associated with soil moisture content, water stress can improve the quality of the pasture, N fertiliser will increase the DM response to soil moisture content and WUE will increase by alleviating a limiting factor, N fertiliser in this case were accepted. A logical extension of this work would be to do the trial in an open field to analyze the effect of irrigation and nitrogen fertilization on the growth, yield and quality of the pasture and then extrapolate the results to other sites and soil types using models.
Dissertation (MSc(Agric))--University of Pretoria, 2011.