Don W. Morishita, Michael J. Wille, and Stephen L. Young

An important part of an integrated weed management system is the economic weed threshold level in a particular crop. This is usually based on the density, or number of weeds per unit area or length of row. The threshold level varies from one weed species to another because some weeds are more competitive than others. In addition to weed density affecting yield, the length of time weeds compete with a sugar beet crop and how long sugar beets must be kept free of certain weeds before a yield loss occurs is also important. Understanding weed emergence in sugar beets also is important to understand from the standpoint of timing postemergence herbicide applications. Typically, growers apply postemergence herbicides at 7 to 10 day intervals following the initial postemergence herbicide application. Three to five postemergence applications are made depending on the emergence of weeds.

Most of the weed competition in sugar beet studies have been conducted in Colorado and Wyoming. We have looked at the area of influence of redroot pigweed, common lambsquarters, and kochia as a means of determining whether or not the data from Wyoming and Colorado fit our conditions in southern Idaho. The result of the area of influence studies showed that kochia was more competitive than common lambsquarters, which was more competitive than redroot pigweed in sugar beets. Figure 1 shows the competitiveness of several weeds including the previously mentioned three species. These data show that the order of competitiveness in sugar beets is common sunflower > kochia > common lambsquarters > redroot pigweed. Our Idaho data closely fit the Colorado and Wyoming data.

Figure 1.  Comparisons of sugar beet yield loss from four weed species at different densities.  From Weed control in sugar beets in N. America.  E. Schweizer and A. Dexter.

The threshold levels of these same weeds and others are shown in Table 1. Of the most common broadleaf weeds, hairy nightshade is the least competitive and is about as competitive as the common grass weeds barnyardgrass, foxtail sp., and wild oat.

Table 1. Threshold levels of several common broadleaf and grass weeds in sugar beets.

Understanding the emergence characteristics and patterns of weeds can be helpful in determining the optimum time to apply postemergence herbicides. This becomes especially important when using micro herbicide rates. A comparison of the germination temperature of common weed species with those of various crops including sugar beets, shows why certain weeds may be more of a problem in one crop than in others (Table 2).

Table 2. Germination temperature ranges and optimum germination temperatures
of four crops and nine weed species.

Information on emergence times can be most beneficial in predicting when to begin postemergence herbicide applications. Table 3 lists the accumulated growing degree day requirements of sugar beet and seven weed species. This information is the result of some preliminary studies conducted at the University of Idaho. Additional field studies will be used to confirm these results.

Table 3. Average growing degree days required before emergence of sugar beet and several weed species.

Field studies were conducted in 1997 and 1998 to determine the effect of weed emergence and chemical control inputs during the growing season on sugar beet yield and quality. Weed emergence, weed control, weed seed production, and sugar beet yield were measured. In 1997, kochia began emerging on May 2, while common lambsquarters and hairy nightshade began emerging 5 days later. Redroot pigweed and green foxtail emergence followed kochia by almost two and four weeks, respectively. In 1998, which was a wet spring, all weeds began emerging by May 8. In both years, weed emergence peaked on or before June 11, and populations declined the rest of the season. Any later emerging weeds were suppressed or killed by the sugar beet canopy. Common lambsquarters, hairy nightshade, and green foxtail were controlled 96 to 100% with Nortron applied preemergence followed by two postemergence applications of Progress + UpBeet (Table 4). This same treatment controlled kochia and redroot pigweed 89%.

Table 4. Late season weed control evaluations for herbicide treatments in 1997 and 1998.