Weed Control-76



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Project Leader and Principal UC Investigators

Glenn Nader, livestock farm advisor, UC Cooperative Extension Butte/Sutter/Yuba Counties


A hard look at past progress in weed control shows great strides - promising that the crucial problems remaining-which MUST be solved - CAN be solved. Screening of new chemicals continues. All in all, redoubled efforts in weed control are vital to rice production in California. The development of new materials and their registration will depend on pressure from the users, say the chemical companies.

The control of aquatic weeds has been a continuing costly and difficult problem for rice farmers since the industry began in California, in 1912. Watergrass has drastically reduced rice yields; slower-growing weeds such as lily and the sedges have reduced yields substantially. By 1915 the prolific-seeding watergrass and barnyardgrass had taken about 2,500 acres of rice land out of production in the Sacramento Valley and by 1925 had reached "alarming quantities." That is when UC researchers and the growers developed the continuous deep-water submergence method of rice culture, yielding practical control of watergrass.

That had little effect on cattails, arrowhead lily, water plantain, redstem, bulrush, and waterhyssop, but the chemical control following World War II saw 2, 4-D taking over, replaced around 1953 by MCPA, which damaged rice much less while giving equal control of broadleaved weeds. By 1975, four-fifths of California rice acres were treated with MCPA. _

There was a reason: increased yields returned the chemical application costs about twenty-five times! Propanil was estimated to increase rice growers' income by $14 million in 1968.

As propanil spraying increased between 1961 and 1966, reports increased of drift damage leaf spotting and dropping in prunes and almonds, sometimes at quite a distance from rice fields. Lesser damage was reported in walnuts, sugar beets, and tomatoes. After a hearing in 1968, the Director of the California Department of Agriculture banned propanil north of a line approximating U.S. Interstate 80 (except for a small pocket in the Natomas district north to Sankey Road). Improved spraying equipment and techniques have reduced the drift problem, allowing continued use of propanil for barnyardgrass control in the southern Sacramento and San Joaquin valleys.

The improved information on the use of propanil has not yet restored it to the banned area, but there is hope found in the spirit of intelligent cooperation and use of new knowledge that has brought MCPA back to a banned area. That chemical was gradually restored to Merced County growers through the concerted, close cooperation of the Merced County Agricultural Commissioner, local crop dusters, rice growers, and UC agricultural engineers, beginning in 1963 and extending through 1972. No damage was reported to surrounding crops there in 1976 when MCPA was applied to 7,500 acres and propanil to 8,300 acres. This is a clear demonstration that herbicides can be safely applied to rice.

The value of the research program becomes clearer if we consider the recent focus of weed-control research separately from the long-range objectives.

Research in recent years can be outlined on the basis of five basic areas:

  1. Aimed at making propanil safe in areas where now banned.
  2. Projects designed to help secure EPA registration of new herbicides.
  3. More effective and safer methods for aerial application of pesticides (including rice herbicides).
  4. Information on the fate of rice pesticides in the environment that will help us retain the use of MCPA, molinate, and others, and make possible the registration of Bolero (for grass control), Basagran (for sedges and broad-leaf weeds), and Du-Ter (for stem rot control).
  5. The coating of rice seed with fungicides, plant nutrients, and (probably ultimately) herbicides, thereby eliminating the costly seed-soaking procedure.

Long-range weed-control research naturally embraces the above - plus more. It interacts with other areas of rice research in such a way that some weed-control activity is covered in other sections and will not be duplicated here. Progress on long-range objectives is summarized next.

New herbicides are becoming available to growers and will provide outstanding results.

Hydrothol 191, registered for commercial use in 1976, has been particularly effective on American pondweed, increasing rice yields where infestations are heavy. Application at the proper time suppresses winter buds, making it unnecessary to treat for this weed every year.

Bentazon (Basagran) and benthiocarb (Bolero) show promise in test plots. Benthiocarb, a useful alternate to molinate (Ordram), was licensed for experimental field use in 1976 on about 1,000 acres in California. It controls sprangletop, ducksalad, waterhyssop, and southern naiad in addition to watergrass. Bentazon proved very effective against river bulrush, seedling cattails, and other sedges and broadleaved weeds, and looks promising in granular combinations with molinate. Information required by EPA is being developed to enable registration of bentazon and benthiocarb (though approval may be several years away).

Better timing in applying herbicides pays off in higher yields.

Competition from weeds is toughest on young rice. The first 28 to 30 days of growth is most critical for control of barnyardgrass, with benefits declining from later control.

The resistance of the rice plant to chemicals varies with age. Rice resistance to damage from MCPA was correlated with elongation of the first internode of the stem. Physiological tolerance is maximum when that internode is at least 1/a inch long. Even so, application at this stage has resulted in more leaf and tiller burn in recent years, for some reason. The burn is a necrosis of the tip 1 to 2 inches of older leaves, and sometimes complete kill of developing tillers. Symptoms are worse if temperatures are high during and immediately after MCPA applicaton - and worse yet with hot dry winds. Best results are obtained from 60-65F up to around 90F, with control declining above 95F.

If weed competition requires MCPA before the first internode begins to elongate (35 days after seeding), apply only to rice that is not stressed and has a good root system. (Light or moderate leaf burn rarely affects rice yields seriously.)

Improved nozzles and understanding of spray behavior have increased the safety of spray applications of chemicals (from air or ground) but have not eliminated windborne danger to other crops.

A UCD agricultural engineer summarizes the prospects for future use o propanil as follows:

  1. Propanil cannot be used within one mile of prunes without causing small yellow spots on the prune leaves.
  2. Propanil cannot be used within 1000 feet (for extra safety) of young sensitive prune trees without danger of defoliating them.
  3. Propanil has been windborne from treated fields for at least 37 days after application (probably as very small crystals). The only logical way of retrieving the full use of propanil appears to be through acceptance of these nondamaging leaf spots by prune and almond growers (almond is less sensitive than prunes). With large areas of rice sprayed with propanil, some leaf spotting appears inevitable.

Know your enemy - then hit his vulnerable spots. That is the formula for final victory over the weeds of rice of many species, varieties, and ecotypes.

Barnyardgrass, we now know, is not one weed but many - varying in seed size, germination date, depth of germination and emergence, and date of maturity. Such information about this weed and the other weed pests of rice should prove of crucial value in improving weed-control programs.

Varieties of watergrass or barnyardgrass are the most serious weeds in most rice fields of California. Previous workers have indicated that a number of different varieties or forms are present, but a satisfactory classificaiton has been needed. Although the barnyardgrasses Echinochloa crus-pavonis and E. muricata occur in California, they do not grow in rice fields. The most widespread types in and around rice fields are E. crus-galli var. crus-galli and E. crus-galli var. oryzicola (= E. oryzicola).

Variety crus-galli is of Eurasian origin and was introduced into California during the Spanish colonial period. It was a serious threat to early drill-seeded rice culture in California, but now is mainly controlled by continuous flooding and is thus a problem only on high spots and levees, or where early drainage enables it to start growing. Many separate introductions from different parts of the Old World account for its variable or polymorphic appearance. Variety oryzicola is considered by some taxonomists to be a separate species and can be distinguished from var. crus-galli by its larger spikelets and seeds, shorter stature, and lighter green color. The seeds of var. oryzicola are 2 to 3 times as heavy as seeds of var. crus-galli and enable this variety to germinate and emerge through water depths up to 12 inches. Because of this difference in germination characteristics, var. oryzicola grows in continuously flooded fields, where it is the most serious rice weed in the state. However, its lack of dormancy exposes serious infestations to ecological attack. Repeated flush irrigation followed by rotovation or plowing during summer fallow seasons can substantially reduce the seed load in the soil.

A second form of var. oryzicola is common in Butte County and occasional in Sutter and Colusa counties. This form closely resembles the rice plant and heads at the same time as early rice. For this reason botanists refer to this form as a "crop mimic." Both forms of var. oryzicola were introduced into California before 1920. Their introduction probably was in contaminated rice seed from Asia. Other Asian rice weeds that probably were introduced in the same way include dopatrium (Dopatrium junceum), Indian toothcup (Rotala indica), and monochoria (Monochoria vaginalis).

Potentially serious weed problems were the focus of a statewide survey in 1974-76 of the distribution of rice weeds.

The most widely distributed rice weeds in California include varieties of watergrass or barnyardgrass (Echinochloa crus-galli), bearded sprangletop (Leptochloa fascicularis), roundleaf waterhyssop (Bacopa rotundifolia), California arrowhead (Sagittaria calycina), redstem or purple ammania (Ammania coccinea), smallflower umbrella plant (Cyperus difformis), and southern naiad (Najas guadalupensis). More localized problems, though often as severe, are caused by river bulrush (Scirpus fluviatilis), roughseed bulrush (Scirpus mucronatus), common cattail (Typha latifolia), common waterplantain (Alisma triviale), American pondweed (Potamogeton nodosus), horned pondweed (Zannichellia palustris), and the alga chara (Chara sp.).

Ducksalad (Heteranthera limosa) and Eisen waterhyssop (Bacopa eisenii) are two of the weeds which have spread markedly in recent years. Ducksalad was first recorded in 1948 in Glenn County. It is now common in Glenn and Butte counties, where it persists in rice fields throughout the season because of its partial resistance to MCPA. Eisen waterhyssop resembles the more widespread roundleaf waterhyssop, but its showy white flowers are larger. Unlike roundleaf waterhyssop, which was introduced from southern rice-growing states, Eisen waterhyssop is a native species that was formerly confined to San Joaquin County. This year, Eisen waterhyssop was recorded in northern Sutter County and was very common in rice fields between Sacramento and Dunnigan. Eisen waterhyssop grows more vigorously throughout the season and appears more resistant to MCPA than roundleaf waterhyssop, so its spread calls for caution.


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