Introduction

The three principle mechanisms involved in weed seed dispersal are wind, water and attachment to animals, equipment and people. Various physical characteristics have been developed to utilize these mechanisms. Those that utilize wind include those that have plumed seed (sowthistle), detach and roll (tumbleweed) or have wings (saltbush or many trees such as maple). Those that utilize water have low specific gravity or are flattened (many grasses such as canarygrass), or those that are corky (dock). Those that attach to people, equipment and animals include some of the most insidious seeds around. These include puncturevine, burclover, cocklebur, jimsonweed, desert thornapple, sandbur and many others. Sandbur is one weed that is as dreaded by growers and handlers of alfalfa hay as it is by the animals that consume it.

There are two species of sandbur. Field sandbur (Cenchrus pavciflorus) ( picture) is the most common of the two. It is native to Arizona and is easily distinguished from southern sandbur. The leaf blade is thin like that of bermudagrass and the burs are yellowish. It grows in a prostrate manner although it is more upright than the other species. It can root at the lower nodes and form a mat. Southern sandbur (Cenchrus echinatus) ( picture) ( drawing) has a much broader leaf that is red. It grows more prostrate to the ground and also roots at the node and forms a mat. The burs are also reddish and broader than those of field sandbur. Field sandbur burs usually contain 2 seeds while southern usually contain 4 seeds. Southern sandbur is not native and was introduced from South America.

Both field and southern sandbur are appropriately named because they almost always are found in sandy, well drained soil. The reason for this is unclear. Both are described as summer annual weeds although there are almost always some plants that survive even very cold winters. This has an important impact on the ability to control this weed.

Control

Frequent summer cutting and rapid regrowth make alfalfa very competitive with most weeds. This is the case more with broadleaf weeds than grasses which can withstand frequent cutting. Herbicides are needed most during stand establishment and in older, weaker stands but are often required even in second and third year fields during the “summer slump” period between July and September when sandbur is most prolific.

Preplant herbicides

There are two preplant treatments available for alfalfa in Arizona. These are Balan 60DF and Eptam 7E and 20G. Balan is the most active and least selective of the three dinitroaniline herbicides (Triflualin, Prowl) which are commonly used here. It must be mechanically incorporated or unacceptable injury will occur. Injury in the form of crop stunting and stand thinning is not uncommon even when mechanically incorporated although this rarely results in reduced yields. Balan is effective on seedling field and southern sandbur.

Eptam is normally applied after planting in the germination water when the 7EC formulation is used. The 20G formulation can be applied after planting and prior to the first irrigation. Eptam is extremely volatile and should be incorporated with water within 24 hours of application. The 20G is a more efficient means of applying Eptam and no more than 2 lbs. active ingredient per acre whould be applied. Up to 3 lbs. ai/AC of the 7EC can be applied in the germination water although crop injury in the form of stunting and stand thinning is common. Like Balan, this rarely results in yield loss. Eptam is effective on seedling field and southern sandbur.

Preemergent Herbicides

According to Arizona Department of Agriculture L1080 forms approximately 55% of the herbicides used in 2002 were preemergent herbicides. It is always best to control weeds prior to emergence with soil applied herbicides that have residual activity. However, the ability of sandbur to overwinter and regrow in the spring from established crowns greatly reduces the possibility of achieving high levels of preemergent control. Some sandbur plants always survive even very cold winters. Test results are presented here from a test conducted in 1992 to compare the two main preemergent herbicides used in alfalfa, trifluralin 10% granules and Eptam 10% granules. The formulation and availability of Eptam granules have changed although the 7E, active ingredient and rates have remained the same since the 1960’s.

This test was conducted on the Yuma mesa and southern sandbur was the species that was present. The test contained four treatments that included 1 lb.ai/A of Trifluralin 10% granules, 2lb.ai/A of Trifluralin granules, 2 lb.ai/A of Eptam 10% granules, 3 lb.ai/A of Eptam 10% granules and an untreated check. The Trifluralin treatments were applied once on February 5. Four applications of both Eptam treatments were made, one on February 5, April 10, June 14 and July 15 for a total of 8 and 12 lbs.ai/A. The herbicides were applied with a valmar airflow ground driven applicator with 4 replications of each treatment.

Table 1
Sandbur control from 4 preemergence herbicide treatments in a first year stand of alfalfa.

₂Average of 4 replications.

All of the treatments in this test, except for the low rate trifluralin, significantly reduced the sandbur population but were less than completely effective. This was a first year stand of alfalfa although the field had previously contained a heavy infestation of sandbur and some of the plants may not have been destroyed during ground preparation. Normally, when partial control like this occurs it is because some of the sandbur overwintered. Control levels of 70 to 85 percent from preemergent herbicides have been typical in both our tests and grower experience.

Pendimethalin (Prowl, Pendimax) has also been evaluated and found to be highly effective in controlling both sandbur species that are coming from seed and not established crowns that have overwintered. Water-run applications at 1 to 2 pounds active ingredient per acre produce excellent sandbur control. Pendimethalin (Prowl, Pendimax) is not registered for use in alfalfa although efforts are underway to gain this registration.

Postemergence Herbicides

Two grass herbicides are labeled for the postemergence control of sandbur in alfalfa. These are Poast and Select/Prism. Raptor, mainly a broadleaf herbicide with activity on some grasses, also has field sandbur on the label. I have had these and other grass herbicides in tests or sandbur control since the early 1980’s when the selective post emergence grass herbicides were first being developed and registered in alfalfa. Some of the other postemergence grass herbicides that have been tested are Fusilade which is registered for use on many crops including vegetables, cotton and soybeans but not alfalfa, and AssureII which is used on cotton, soybeans, sugar beets and other crops but also not registered on alfalfa. Table 2 summarizes the results of three tests conducted in 1994, 1995 and 1997 to evaluate Poast and Select/Prism for sandbur control in alfalfa. Both field and southern sandbur were present in these tests and the control levels reflect the activity on both. Sandbur was from two leaf to heading in these tests.

Table 2

Sandbur control in alfalfa with Poast and Select.

The control levels in these tests are representative of those achieved in other tests and grower experience. Control was poor and far below what would be commercially acceptable. I have seen these herbicides kill seedling sandbur at the 1 to 2 leaf stage. Once these weeds reach the 3 leaf stage they become tolerant. In practice, it is difficult for growers to treat when all of the sandbur is at the 1 to 2 leaf stage because of the multiple emergences of this and other summer annual grasses and the difficulty in treating between frequent summer cuttings and irrigations. Cuttings occur every 28 to 35 days and lay in the field for 5 to 7 days. Irrigations occur every 10 to 14 days. It is difficult to fit herbicide applications into this schedule.

A more recent test was conducted last year to evaluate these older herbicides and some new experimentals. In addition to Poast, Select, Fusilade and AssureII, raptor, Pursuit and a Valent experimental, V-0139 were evaluated. The results are presented in Table 3. Although some differences between treatments were measured, it is clear that none of these herbicides and combinations were effective in controlling field sandbur. These treatments were applied on 8-19-03 when the sandbur was from 2 leaf to heading. The plots measured 20’ X 14’ and were replicated three times. All the treatments were applied with a CO2 backpack spray calibrated at 20 gallon per acre. A visual evaluation of control was made on 10-7-03.

Sencor is a broad spectrum herbicide with activity on many grass and broadleaf weeds. It is most effective as a postemergence treatment but causes moderate to severe crop injury if much foliage is present at the time of application. We have evaluated sencor pre and post emergence as a sprayable treatment, impregnated on fertilizer and as a granule. Control was partial and inconsistent.

Table 3

The evaluation of 8 herbicides and herbicide combinations for field sandbur control in alfalfa.

LSD(0.5)=8.3989
*All treatments included 3 lb./AC ammonium sulfate and 2 pts./A crop oil concentrate.

Summary and Conclusions

Sandbur is one of the most dreaded and difficult to control weeds in alfalfa. Fortunately, it is confined almost always to sandy soils. Complete control with both pre and postemergent herbicides is difficult and this weed has continued to be a serious problem for many years. Sandbur has the ability to survive even cold winters and preemergent herbicides typically produce 60 to 85% control. Postemergent herbicides are ineffective once this weed reaches the two leaf stage. It is difficult to apply these herbicides at that stage of growth due to multiple weed emergences and frequent irrigations and cuttings. The best control option currently available is the use of a preemergent herbicide in the spring followed by postemergent herbicide applications as soon as possible after escapes emerge in the summer.

Full Disclaimers

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.

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