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Home>FFTC Document Database>Extension Bulletins>Invasion of Exotic Weed Seeds into Japan, Mixed in Imported Feed Grains
Invasion of Exotic Weed Seeds into Japan, Mixed in Imported Feed Grains
Shunji Kurokawa
Department of Forage Production,
National Institute of Livestock and Grassland Science,
National Agricultural Research Organization,
768 Senbonmatsu, Nishinasuno, Tochigi, 329-2793 Japan, 2001-08-01


Recently, many kinds of foreign weeds have caused serious damage to farmers all over Japan. The source of the invasion was grain imported from countries overseas. Numerous foreign weed seeds mixed in with the grain have invaded ports and colonized fields. The viability of most foreign weed seeds is not likely to be affected by the processes of invasion. To control such weeds, several methods have been tried, both chemical and non-chemical. So far, composting manure is the only effective way of preventing weed seeds from germinating. There is a need for a complete solution to change the structure of agriculture in society, because otherwise more foreign weeds are likely to invade Japan in future.


Recently, many kinds of exotic weeds have been found to cause serious problems in Japan. The invasion pattern was different from that of weeds which had been accidentally introduced in the past. These had spread gradually from the primary naturalized point along roads or railroad tracks. In contrast, the new exotic weeds appeared at the same time in all areas affected. They included many tropical species that grow up to three meters in height. These cause yield losses even in fields of forage crops, where comparatively tall crops are grown.

To determine what was responsible for the invasion of these new foreign weeds and their invasion route, Japan's Ministry of Agriculture, Forestry and Fisheries carried out a research project from 1993 to 1996 (Project Report 1998a). Some local government institutes also carried out a project to develop ways of controlling the exotic weeds and preventing their seeds from spreading (Project Report 1998b). This Bulletin gives the results of this study of the source of the weeds, and describes the weed species involved and their distribution. The Bulletin also discusses the problem in general terms, and the future outlook.

Distribution of Foreign Weeds in Japan

Questionnaires were mailed in 1993 and 1996 to at least one research station and/or extension station in each prefecture, in order to determine the distribution of each exotic weed and the extent of crop losses it had caused. The results showed that several exotic species have become major weeds (e.g. Abutilon theophrasti Medic., Solanum carolinense L. and Sicyos angulatus L.) in most of the regions investigated ( Fig. 1(1631)). Moreover, the survey revealed that these weeds had been causing problems, not only in fields of forage crops, but also in paddy fields and orchards.

Invasion Route of Foreign Weeds

In order to control exotic weeds, it is very important to specify the route of invasion. In our project, the principal source and route of invasion were determined (Project Report 1998a).

Source of the Weed Seeds

Since the weeds appeared suddenly at the same time in forage fields all over Japan, it was initially believed that fodder crop seeds or imported feed might be the source of the invasion.

Possibility of Invasion Via Fodder Crop Seeds

Because crop seeds for forage production are usually produced in foreign countries and exported to Japan, they provide an opportunity for exotic weeds to invade. However, fodder crop seeds are subject to strict inspection. It would be impossible for fodder crop seeds which were contaminated with weed seeds to pass the inspection. In fact, no weed seeds were found in imported maize seeds during our investigation.

Possibility of Invasion Via Imported Feed

The amount of feed such as concentrates and hay imported into Japan has been increasing recently, and it was noted that this increase coincided with the occurrence of foreign weeds (Fig.2).

Imported grain which was used as feed stock for concentrated feed was investigated to determine whether foreign weed seeds were present. All grain imported during one year at the port of Kashima was thoroughly tested for weed seeds. The results showed that many kinds of weed seeds were mixed in with the grain imports ( Table 1(1348)). Some of these species were noxious weeds (i.e. they were particularly damaging and/or particularly difficult to control).

An increasing amount of hay has also been imported into Japan in recent years, because many dairy farmers are unable to supply all their own feed. We checked samples from residues of imported hay remaining in the backs of trucks arriving at a dairy farmers' cooperative over six months. Although many seeds were included in each sample, they were not those of the recently observed exotic noxious weed species.

These results suggested that the source of exotic weeds is most likely to be imported feed grain, which is used as an ingredient in concentrated feed. Most of this imported feedgrain is from the United States.

Route of Exotic Weed Seeds from the Port to the Field

At ports

When pests are found in imported grain, the grain is treated with methyl bromide. However, this treatment does not affect weed seeds.

At the feed factory

Imported grains used as livestock feed are processed in several way. They may be subjected to:

  • Mechanical crushing (>2 mm);
  • Pelletized steaming at 70 - 80°C after crushing;
  • Heated under pressure to 130°C, 3 atm.

Many feed grains receive only the first treatment, which does not affect seed viability.

In the digestive system of animals

The effect on weed seeds of passing through the rumen of cows was studied. Only one species, Abutilon theophrasti, showed a decreased germination rate. Passing through the digestive species of livestock did not seem to reduce the germination rate of other imported weed species. In fact for many weeds species, it promoted germination.


Sometimes animal manure is composted by farmers, but sometimes it is spread directly onto fields. If the compost is well fermented, many weed seeds have reduced viability (Nishida et al. 1998). However, if fermentation is not complete or the manure is applied directly onto fields, no weed seeds are killed.

In the field

Many exotic weeds have resistance or tolerance to herbicides currently used in Japan. In many cases, these species have escaped from control programs carried out in their countries of origin. Current weed control methods in Japan tend to leave many exotic weeds unharmed. Thus, the number of exotic weeds invading Japan in imported grains and spreading out over farmers' fields is increasing.

Kinds of Weeds Mixed in Imported Grain

The seeds of one of the most common foreign weeds, Abutilon theophrasti, were found in soybeans produced in USA and in lupines produced in Australia. A large number of seeds from several Amaranthus species were found in a range of grains from several countries. Seeds of an unusual species belonging to this family, Amaranthus spinosus, were found in soybeans from USA. This may have contributed to the spread of this weed species into the northern part of Japan (Shimizu 1998). Surprisingly, it was found that a number of native species, such as Setaria faberi Herrm, had been "reimported". In addition, some species such as Digitaria sanguinalis or Echinochloa muricara, which do not have Japanese names because they were previously unknown in Japan, were also found mixed with imported grain.

Losses Caused by Major Exotic Weeds

Abutilon Theophrasti Medik.

This is an annual plant belonging to the Malvaceae family ( Fig. 3(1198)). It is tall, fecund, self-fertile and contains a great deal of fiber. This plant was grown as a fiber crop in Japan until the 1920s. However, the weedy ecotype is genetically different from the crop type ( Fig. 4(1592)) (Kurokawa et al. 1998a). Because the weed grows to about three meters in height and competes with crops, it causes serious yield losses. Moreover, the plant has a strong odor that may be transferred into milk if dairy cows take feed which includes Abutilon theophrasti.

Solanum Carolinense L.

This plant is a perennial species belonging to the Solanoaceae family, which propagates by both seeds and roots ( Fig. 5(1542)). This weed is very difficult to control, because most herbicides are ineffective and because it can propagate by its roots. It causes serious crop losses, while its sharp spines cause injuries to both livestock and people. It also contains alkaloids which are toxic to animals.

Cyperus Esculentus L.

This sedge is a Cyperaceae species ( Fig. 6(1153)). It can be spread by vegetative propagation as well as seeds. Most herbicides are ineffective against this plant.

Sicyos Angulatus L.

This vine belongs to the Cucurbitaceae family ( Fig. 7(1343)). It winds around the stems of crops such as corn and covers the plants. Even if it occurs infrequently, it can cause serious damage to crops.

Datura Stramonium L.

This is a poisonous weed, which is highly toxic to both livestock and human beings ( Fig. 8(1542)). It can be troublesome even when the frequency of occurrence is low.

Attempt to Control Foreign Weeds

Local governments in Japan have developed two approaches to the control of exotic weeds. One is based on chemical herbicides, and the other is based on non-chemical control methods (Project Report 1998b).

Chemical Control

Table 2(1399) shows the list of herbicides tested, and their effect in controlling important introduced weeds. It was found that Datura stramonium could easily be controlled with existing herbicides. However, it is important that control programs eradicate all plants, because they are so poisonous. Many weeds such as Abutilon theophrasti or Sicyos angulantus are very difficult to control with existing herbicides. Herbicides have no control effect at all over Solanum carolinense.

Non-Chemical Control

Sometimes chemical control is difficult to apply. There may be no existing effective herbicide, while new herbicides take a long time to develop. In addition, it may be difficult to rely on herbicides alone from the viewpoint of sustainable agriculture. Ways of controlling major foreign weeds without herbicides were examined during the project by local governments. An example of these attempts in shown in Fig. 9(1399).

Preventing Invasion by New Foreign Weeds

A list containing 316 weed species which may possibly invade Japan in the future was drawn up by Konnai et al. (Project Report 1998a). After this list was made, it was found that Emex australis (included in the list) had already invaded Japan. This weed, which is one of the worst in Australia, produces a number of spiny fruits ( Fig. 10(1203)). It is clear that we can expect many additional weeds to invade in future, unless precautions are taken.

The only way to prevent the germination of weed seeds is to compost animal wastes adequately. The relationship between the temperature of fermenting compost and seed viability has been studied (Nishida et al. 1998, Nishida et al. 1999). It was found that seeds cannot survive in compost which reaches a maximum temperature during fermentation of more than 60°C ( Fig. 11(1303)). Ensuring that the temperature of wastes reaches a sufficiently high level during composting is very important in preventing the invasion of new weeds.

Some new techniques have been tested lately to prevent weed invasions at ports. One of these is electron irradiation (Kurokawa et al. 1998b, Kurokawa et al. 1999). The susceptibility of different species to electron irradiation was found to vary widely ( Fig. 12(1374)).

Circumstances Surrounding the Foreign Weeds Problem

The circumstances surrounding the problem of foreign weeds are shown in Fig. 13(1397). In Japan, a recent fall in milk prices has brought a financial crisis to dairy farmers. In this difficult situation, farmers have had to raise milk productivity in order to survive.

There were two ways of raising productivity. One was to increase the number of animals reared, and the other was to increase the average milk yield per cow. The first method produced animal wastes in too large a quantity for adequate composting. Consequently the seeds remained viable. The second method encouraged farmers to use more feed concentrates, which increased the chances that foreign weeds would invade, mixed in with imported grain. The combination of these factors resulted in the sudden appearance of a serious foreign weed problem.

What Will Happen in the near Future?

We can see that the causes of the foreign weed problem are complex. If the situation does not improve, what will happen in the near future? This will depend mainly on the exporting countries. It is their actions, rather than those of Japan, which will determine the types and numbers of foreign weeds that will invade.

For example, a tolerance gene for non-selective herbicides has been introduced into some crops grown in the United States. In the future, weeds tolerant to non-selective herbicides may appear. Consequently, such herbicide-tolerant weeds may invade Japan. It is important to prevent this, and to find a way of changing the social and economic situation of agriculture.


  • Enomoto, T., N. Shimizu and S. Kurokawa. 1996. Identification of alien weeds contaminated with feed material cereals imported. II. Remarkable species. Journal of Weed Science and Technology 41 (Supplement I): 214-215. (In Japanese).
  • Kurokawa, S., T. Komatsu, N. Shimizu, Y. Yoshimura and S. Uozumi. 1998a. Genetic variation of Abutilon theophrasti based on DNA polymorphism and its growth characteristics. Journal of Weed Science and Technology 43 (Supplement I): 122-123. (In Japanese).
  • Kurokawa, S., K. Nagakura, Y. Yoshimura and S. Uozumi. 1998b. Effects of electron irradiation on the germination of alien weed seeds. Journal of Weed Science and Technology 43 (Supplement I): 278-279. (In Japanese).
  • Kurokawa, S., O. Watanabe, K. Nagakura, T. Hirano, Y. Yoshimura and S. Uozumi. 1999. The electron irradiation to prevent from an invasion by the exotic weed. Journal of Weed Science and Technology 44 (Supplement I): 296-297. (In Japanese).
  • Nishida, T., N. Shimizu, M. Ishida, T. Onoue and N. Harashima. 1998. Effect of cattle digestion and of composting heat on weed seeds. Japan Agricultural Research Quarterly 32: 55-60.
  • Nishida, T., S. Kurokawa, S. Shibata and N. Kitahara. 1999. Effect of duration of heat exposure on upland weed seed viability. Journal of Weed Science and Technology 44,1: 59-66.
  • Project Report. 1998a. Development of Technology to Prevent Spreading of Noxious Naturalized Weeds. Agriculture, Forestry and Fisheries Research Council Secretariat. (In Japanese).
  • Project Report. 1998b. Development of Urgent Technology to Prevent Damage by Noxious Alien Weeds in Forage Fields. Gunma Animal Industry Institute, Chiba Livestock Research Center, Nagano Animal Industry Institute and Mie Agricultural Research Center. (In Japanese).
  • Shimizu, N. 1998. Invasion and spread of recent alien weeds and control measure. Japanese Journal of Ecology 48: 79-85. (In Japanese).
  • Urakawa, S. and Y. Deguchi. 1998. Effect of paddy-upland rotation on the viability of velvetleaf seeds. Journal of Weed Science and Technology 43 (Supplement I): 124-125. (In Japanese).

Index of Images

Figure 1 Distribution of Foreign Weeds in Japan, Based on Recent Questionnaire

Figure 1 Distribution of Foreign Weeds in Japan, Based on Recent Questionnaire

Figure 2 Changes in the Amount of Grain Imported into Japan (Based on Fao Statistical Data)

Figure 2 Changes in the Amount of Grain Imported into Japan (Based on Fao Statistical Data)

Figure 3 <I> </I><B>Important Exotic Weedin Japan: </B><I>Abutilon Theophrasti</I> Suppressing the Growth of Corn<BR> <BR>

Figure 3 Important Exotic Weedin Japan: Abutilon Theophrasti Suppressing the Growth of Corn

Figure 4 Genealogical Tree of the Weed <I>Abutilon Theophrasti</I> Accessions, Based on Dna Analysis<BR>

Figure 4 Genealogical Tree of the Weed Abutilon Theophrasti Accessions, Based on Dna Analysis

Figure 5 <I> </I><B>Important Exotic Weedin Japan: </B><I>Solanum Carolinense</I><BR>

Figure 5 Important Exotic Weedin Japan: Solanum Carolinense

Figure 6 <B>Important Exotic Weedin Japan: </B><I>Cyperus Esculentus</I><BR>

Figure 6 Important Exotic Weedin Japan: Cyperus Esculentus

Figure 7 <B>Important Exotic Weedin Japan: </B><I>Sicyos Angulatus</I><BR>

Figure 7 Important Exotic weedin Japan: Sicyos Angulatus

Figure 8 <I> Datura Stramonium</I>

Figure 8 Datura Stramonium

Figure 9 Effect of Paddy-Upland Rotation on the Viability of <I>Abutilon Theophrasti</I> Seeds.<BR> <BR>

Figure 9 Effect of Paddy-Upland Rotation on the Viability of Abutilon Theophrasti Seeds.

Figure 10 The Fruit of the Weed <I>Emex Australis</I>, Which Has Recently Invaded Japan from Australia in Imported Hay

Figure 10 The Fruit of the Weed Emex Australis, Which Has Recently Invaded Japan from Australia in Imported Hay

Figure 11 Percentage of Weed Species Which Retained Their Ability to Germinate after Being Composted at Varying Temperatures

Figure 11 Percentage of Weed Species Which Retained Their Ability to Germinate after Being Composted at Varying Temperatures

Figure 12 Interspecific Differences in Susceptiblity to Electron Irradiation

Figure 12 Interspecific Differences in Susceptiblity to Electron Irradiation

Figure 13 Context of the Foreign Weed Problem

Figure 13 Context of the Foreign Weed Problem

Table 1 Exotic Weeds Which Have Invaded Japan, Mixed with Imported Grain

Table 1 Exotic Weeds Which Have Invaded Japan, Mixed with Imported Grain

Table 2 Effect of Herbicides on Some Major Foreign Weeds

Table 2 Effect of Herbicides on Some Major Foreign Weeds

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