The Effect of Planting Date On Early Season Weed Interference In Sugarbeet.

A field study was conducted in southeast WY in 2009 to evaluate the effect of early season weed interference in replanted sugarbeet (Beta vulgaris). A split-plot RCBD was used with planting date as the whole plot factor and duration of weed interference as the split-plot factor.  Sugarbeet (cv. 'BTS 66RR60') was planted in 76-cm rows at a rate of 173 000 seeds ha^-1 on 22 Apr., 6 May, and 20 May. Within each planting date, eight weed removal timings were established ranging from a season-long weed-free period to full-season interference with no weed removal. Plots were 3 wide by 9m long with 3 replicates. A Sunscan Canopy Analysis System was used to collect light interception data on 8 Sept.  Sugarbeet was harvested on 30 Sept.  Weeds present in first and second planting dates were mostly common lambsquarters (Chenopodium album), while the third planting date consisted of an even mix of common lambsquarters, redroot pigweed (Amaranthus retroflexus), and common sunflower (Helianthus annuus).  Competitive ability of these three species were similar when growing in a low-stature crop such as sugarbeet.  Late season light interception was largely unaffected by the initial weed control timing.  Leaf area index and light interception tended to be lower for the second planting date, indicating that the competitive effects of weeds may have been greater for this planting date compared to earlier or later dates. Planting date had a strong effect on root yield; however, the percentage yield loss due to early season weed interference was similar regardless of planting date.  Root yield increased if weed removal was delayed until at least one week after emergence. Duration of weed interference required to cause a 5% yield loss was 5.3, 4.4, and 4.3 weeks after sugarbeet emergence for the 22 Apr., 6 May, and 20 May planting dates, respectively.