The effects of weed competition and weed control with herbicides on growth and yield of fodder beet (Beta Vulgaris cv. Trestel)

Two field experiments were conducted at Lincoln College between 1980 and 1981 to test how fodder beet (Beta vulgaris cv. Trestel) yields were affected by allowing weeds to grow for a certain period at different stages in the life of the late-sown crop and the response of the crop to various herbicides, used alone and in combinations. The first experiment involved hand-weeding plots for 0, 2, 4, 6, 8 and 10 weeks after crop emergence and subsequently allowing the weeds to emerge and grow until harvest, after 30 weeks. In the other plots, weeds were left to compete initially with fodder beet seedlings for 0, 2, 4, 6, 8 and 10 weeks after crop emergence and the plots were then kept weed-free until harvest. In the second experiment, the control of weeds in fodder beet was investigated with 8 treatments which comprised of various combinations of pre- and post-emergence herbicides. The herbicide treatments were compared to the control plot which was left weedy throughout the growing season. Under a different environmental condition, another experiment was conducted between September and December 1981 to test the effects of herbicides on the control of weeds and the early growth of fodder beet. This experiment consisted of 10 herbicide treatments and these were compared to the controls which included a weedy and a weed-free treatment. In Experiment I, a significant reduction of 51.7 per cent root dry weight occurred at the final harvest when weeds were left uncontrolled throughout the growing season. In this late November sowing, the weeds reached their maximum density at 136 weeds m⁻² in the second week after crop emergence, and these did not affect the root yield if the crop was subsequently kept weed-free. To prevent any irreversible effects on growth and yield of fodder beet, weeding had to commence between 2-4 weeks after crop emergence. Once competition began, the crop produced fewer and smaller leaves than the weed-free crop. When the weeds were removed early in the season, the fodder beets were capable of recovery through the production of new leaves at a rate close to the weed-free plots. Weeds that managed to emerge after this period were effectively suppressed by crop competition. In this season, a 'critical period' of weed competition was absent and a single weeding towards the end of January 1981 was sufficient to give a yield close to that of the beet which had been weed-free throughout. In Experiment II, the amount of weeds that emerged was less in this early-sown crop (October 1980) when compared to that sown in late November 1980. Lenacil applied at 2.5 kg ha⁻¹ as pre-emergence treatment on moistened silt-loam Wakanui soil gave higher weed control of annual weeds than pyrazon. In the treatments where phenmedipham plus desmidipham and metamitron were applied post-emergence at 9.0 litres ha⁻¹ and 4.2 kg ha⁻¹ respectively, they controlled most of the weeds, which were mainly white clover (Trifolium repens) and ryegrass (Lolium spp.). At the time of application, most of these weeds were at their 2- 4 leaf stage and were most susceptible to phenmedipham plus desmidipham and metamitron. All the herbicide treatment combinations were also shown to be effective in controlling weeds. The root yield in the weedy control was slightly lower than those from plots under herbicidal treatments. The reduction in root yield by weeds was not significant and this was attributed to the low weed density that occurred in this early-sown crop. At the final harvest, there was no significant difference in root yield between the plots treated with herbicides although lenacil applied pre-emergence followed by pos emergence phenmedipham plus desmidipham gave a root yield of 14.7 tonnes ha⁻¹ and was the most outstanding treatment in this experiment. In Experiment III, the efficacy of lenacil and pyrazon, which were applied at 2.5 kg ha⁻¹ and 6.0 kg ha⁻¹ respectively, was reduced in the control of canary grass (Phalaris minor) and Californian thistle (Cirsium arvensis) . Their ineffectiveness were attributed to the adverse weather conditions and high clay content of the heavy-medium Wakanui soil series. In addition, these two weed species showed high tolerance to lenacil and pyrazon. The post-emergence herbicides such as phenmedipham plus desmidipham and metamitron showed poor weed control because the weeds at the time of application were past their most susceptible stage. Phenmedipham plus desmidipham was more toxic than metamitron and thus it reduced weeds more effectively. In sequential weed control, the degree of weed control depended largely on the post-emergence herbicides since the pre-emergence herbicides were ineffective in the control of the early emerging weeds. The treatment lenacil followed by phenmedipham plus desmidipham reduced weeds more than other herbicide combinations. The weeds, when treated with post-emergence herbicides, showed a reduction of dry matter, but were not killed. They soon recovered from this temporary check and grew above the crop canopy to compete for light. None of the herbicidal treatments affected beet population and leaf number per beet. Fodder beet seedlings from the unweeded treatment had slightly reduced leaf size with longer and thinner petioles than that in the weed-free plot. At this early stage of crop growth, a root yield production of 90.8 and 85.0 per cent of hand-weeded control with pyrazon and lenacil respectively was significantly higher than that of the uncontrolled weed treatment. Fodder beet plants treated with phenmedipham plus desmidipham or metamitron showed no difference between their root yields which were both significantly lower than that in the weed-free plots. This was due to the partial weed control and the toxic effects of the herbicides on the beet. In sequential herbicidal treatments, lenacil followed by phenmedipham plus desmidipham gave a root yield of 105.8 g m⁻² which was close to that of the hand-weeded plots. All the root yields resulting from the other treatment combinations were not significantly different from the hand-weeded control.