The Biology of Triticum aestivum L. (Bread Wheat)


Appendix A Weed Risk Assessment of Wheat



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Appendix A Weed Risk Assessment of Wheat


Species: Triticum aestivum (wheat)

Relevant land uses:

1. Intensive6 uses (ALUM7 classification 5)

2. Production from dryland agriculture (ALUM classification 3.3)

3. Production from irrigated agriculture (ALUM classification 4.3)

Background: The Weed Risk Assessment (WRA) methodology is adapted from the Australian/New Zealand Standards HB 294:2006 National Post-Border Weed Risk Management Protocol. The questions and ratings (see table) used in this assessment are based on the South Australian Weed Risk Management Guide (Virtue 2004). The terminology is modified to encompass all plants, including crop plants.

Weeds are usually characterised by one or more of a number of traits, these including rapid growth to flowering, high seed output, and tolerance of a range environmental conditions. Further, they cause one or more harms to human health, safety and/or the environment. Although wheat has some traits associated with weeds, it is not considered as an invasive weed in Australia. Other than agricultural areas where it is cultivated, wheat is common along the sides of roads and railway lines that have acted as routes for its transportation. It is classified as being naturalised to agricultural areas in all states, except the Northern Territory, and in Western Australia has been recorded as a minor problem in some areas (Groves et al. 2003). Unless cited, information in this weed assessment is taken from this document The Biology of Triticum aestivum L. (Bread Wheat) v3 (OGTR 2015). This WRA is for non-GM wheat volunteers and includes non-GM herbicide tolerant varieties of this crop. Reference made to wheat as a cultivated crop is only to inform its assessment as a volunteer.



Invasiveness questions

Wheat

1. What is wheat’s ability to establish amongst existing plants?

Rating: Low in all relevant land uses

Wheat is a domesticated crop that grows best under agricultural conditions. It prefers soils with high fertility and responds well to irrigation, especially during tillering and flowering. Wheat volunteers, mainly derived from seed that is shed at or before harvesting, readily establish in disturbed lands of dryland and irrigated cropping areas, especially along the margins of fields. Seed losses can also occur during harvesting itself, as well as in intensive use areas involved in transport, storage and processing. Volunteers also readily appear in subsequent cereal and non-cereal crops sown on the same land where wheat has been grown and harvested. However wheat has little dormancy and, subsequently, dispersed seed is likely to germinate early and die in unfavourable environmental conditions or be consumed by predators. Wheat does not compete well with other vegetation.



2. What is wheat’s tolerance to average weed management practices in the land use?

Rating: Low in all relevant land uses

Weed management practices (preventive, cultural and chemical) aim to reduce the presence of weeds and loss in yields due to weeds. In dryland and irrigated cropping areas, where wheat is grown in rotation with other crops, these practices effectively control wheat volunteers. Nevertheless, seeds may germinate after herbicides have been broken down and volunteers may become established. In intensive use areas, such as land adjacent to grain silos and along roadsides and railway tracks, weed management practices minimise the spread of volunteers.

The degrees of susceptibility of the currently available wheat varieties to herbicides are available in the respective grower guides for each state8.


3. Reproductive ability of wheat in the land use:

3a. What is the time to seeding in the land uses?

Rating: < 1 year in all relevant land uses

Wheat is an annual crop that generally takes five to seven months to complete its lifecycle under standard agricultural conditions. Volunteer wheat behaves in a similar way.



3b. What is the annual seed production in the land use per square metre?

Rating: Low in all relevant land use areas (from volunteers)

As a crop in dryland and irrigated cropping areas, wheat seed yields vary greatly between countries, averaging approximately 300 g/m2 (FAO 2015b), or 7500 seeds/m2, assuming a weight of 0.04g per seed. In Australia, using data for the 2012-13 crop year, the average yield was 1.76 T/ha or 176 g/m2 or about 4400 seeds/m2. At a recommended rate of about 100 plants/m2 for a 2 T/ha harvest, the 2012-13 data represents a yield of about 44 seeds/plant. A 3% loss is assumed, therefore, approximately 132 seeds/m2 would remain on a field post-harvest. One study in Australia indicated that 0.7, 5.6 and 5.3 volunteer wheat plants were present per square metre (measured as 70, 560 and 530 plants per 100 m2) a few months post-harvest in no-till, cultivated stubble-retained and cultivated stubble-burn systems, respectively (Wicks et al. 2000). However, these plants are unlikely to persist and generate seed as typical management practices for follow-on crops or fallow include control of weeds (including wheat volunteers) through herbicide sprays, grazing or cultivation.

In intensive use areas, seed production per unit area is likely to be considerably less than that in dryland and irrigated cropping areas, due to suboptimal germination and growth conditions (e.g. lack of moisture and nutrients), and competition by other plants.


3c. Can wheat reproduce vegetatively?

Wheat cannot reproduce by vegetative propagation.

4. Long distance seed dispersal (more than 100 m) by natural means in land uses:

4a. Are viable plant parts dispersed by flying animals (birds and bats)?

Rating: Occasional in all relevant land uses

There is no evidence that flying animals play a major factor in the dispersal of wheat seeds. Some introduced species such as ring-necked pheasant, mallard duck and rock pigeon do not excrete viable wheat seeds, but may disperse viable seed from their crop/oesophagus or gizzard if they were killed shortly after consuming the grain. Viable seed can also be transported on the muddy feet/legs of birds (Cummings et al. 2008). Amongst Australian birds, corellas have been shown to excrete some viable wheat seeds after passage through the digestive tract. Corellas readily consume wheat seeds under laboratory conditions with 2% of consumed seeds surviving to germinate under laboratory conditions (Woodgate et al. 2011), thus they may have a minor role in seed distribution. It has also been reported that wheat seeds will germinate after passage through an emu’s digestive system, although no experimental evidence was provided (Davies 1978).



4b. Are viable plant parts dispersed by land based animals?

Rating: Unlikely to Occasional in all relevant land uses

Wheat seeds do not possess adaptations for dispersal on the exterior (fur) of animals (e.g. hooks or spines). Nonetheless, wheat seeds may be dispersed in the wool of sheep. Seed dispersal by ingestion and excretion by land based animals has been reported. Wheat seeds are known to survive digestion and germinate in the dung of cattle and sheep. Dispersal in the hooves of animals is also probable, but not well reported, thus the frequency is not known. Rodents which hoard seeds could disperse wheat seed from crop production areas (e.g. after harvest) or from volunteers.



4c. Are viable plant parts dispersed by water?

Rating: Unlikely to Occasional in all relevant land uses

Dispersal by water is possible, but no data is available. Generally, the presence of a non-brittle (non-shattering) rachis reduces the opportunity for long distance seed dispersal by water Wheat seeds/ears are heavy and not adapted for water dispersal.



4d. Are viable parts dispersed by wind?

Rating: Unlikely to Occasional in all relevant land uses

Dispersal by high winds is possible, but no data is available. Wheat seeds are heavy and do not possess appendages that are designed to facilitate wind dispersal (e.g. they are not “winged”). Generally, the presence of a non-brittle (non-shattering) rachis reduces the opportunity for long distance seed dispersal by natural means.



5. Long distance seed dispersal (more than 100 m) by human means in land uses:

5a. How likely is deliberate spread via people

Rating: Common in/from all relevant land uses

Wheat is a crop species that is purposely transported and cultivated for the production of seed that is part of human food and animal feed. Where wheat is present as a volunteer, it is managed like other weeds. In those instances, wheat would not be spread deliberately.



5b. How likely is accidental spread via people, machinery and vehicles?

Rating: Occasional (to common) in/from all relevant land uses

In dryland and irrigated cropping areas, where wheat is planted as a crop, it is common for wheat to be accidently dispersed by people, machinery and vehicles. Seed is transported by humans after harvesting to silos, and further afield for processing, providing many opportunities for seed dispersal. Seed could be spread along roadsides and railway lines, as well as near storage facilities. Seed can remain on machinery after harvesting (e.g. in the header at the front of a combine harvester, reel, threshing drum, sieves). However, where wheat grows as a volunteer, it would be managed like other agricultural weeds. In those – suboptimal – growing conditions, fewer seeds are expected to be produced per plant than when wheat is cultivated as a crop. Therefore, accidental spread of volunteer seed is expected to occur occasionally. Accidental spread by people, machinery and vehicles may occur in or from intensive use areas. Practices such as the mowing of weeds along roadsides could lead to occasional spread of seeds by machinery.



5c. How likely is spread via contaminated produce?

Rating: Occasional in/from all land use areas.

Wheat farming in dryland and irrigated cropping areas is characterised by rotation with other crops, such as canola, lupins and beans. The amount of wheat seed left in the field prior to the planting of a rotation crop will depend upon the efficiency of the wheat harvesting, seed cleaning of machinery, and general weed management procedures. Growth of wheat volunteers within a rotation crop depends on the weed management procedures of the latter crop, while the spread of this wheat depends on the processing of the harvested plant material from the rotation crop. Long distance dispersal via contaminated hay and forage may occur from cropping areas and in or from intensive use areas (such as along roadsides) if harvested for hay or forage.



5d. How likely is spread via domestic/farm animals?

Rating: Occasional in all relevant land uses

If livestock are grazed in or adjacent to a wheat field, then it is possible that viable wheat seeds may be spread either in their hooves or fur. Wheat seeds can be dispersed in the wool of sheep. The separation of plant and animal farming minimises this possibility, but sometimes livestock are grazed on rotation crops such as legumes. Whole wheat, or that which has been processed (crushed or rolled), constitutes some livestock feeds (DPI NSW 2014). As noted above (4a & b), wheat seeds can germinate in the dung after passing through cattle and sheep, and can survive digestion by some bird species, but survival through other animals is not known. In the case of processed wheat (dry-milling or coarse grinding), it is expected that only a small amount of viable seed is present in the feed and this would further reduce survival of the seed during digestion.





Impact Questions

Wheat

6. Does wheat reduce the establishment of desired plants?

Rating: Reduces establishment by < 10% in all relevant land uses

Wheat is a domesticated and cultivated plant that typically establishes where land has been disturbed, most particularly in dryland and irrigated cropping areas. These areas are subject to standard weed management practices that would minimise the impact of any wheat volunteers on the establishment of desired crop plants.

In intensive use areas, such as along roadsides, desired species may range from native flora to introduced trees, bushes and shrubs. Such areas are often managed, for either aesthetic or practical reasons (e.g. maintaining driver visibility) by the removal of larger trees and invasive weeds. As such, wheat would be treated as a weed and managed accordingly. If left untreated, wheat is not competitive and would struggle to survive and persist amongst other vegetation. Dispersed wheat seed (e.g. along transport routes) is likely to germinate in unfavourable environmental conditions or be consumed by predators.


7. Does wheat reduce the yield or amount of desired plants?

Rating: Reduces yield/amount by < 10% in all relevant land uses

Wheat is typically used in rotation with other crops. The rationale behind crop rotation in cropping areas is the desire to break cycles of pest and pathogen infection, manage persistent weeds, and maintain soil moisture and quality. When used as a part of a rotation program, maximising the yield of the follow-on crop is of primary importance. Prior to planting the follow on crop, weeds (including wheat volunteers) would be managed by mechanical or chemical means, thus greatly reducing the density of wheat volunteers. Wheat plants are not competitive amongst other vegetation, are easily managed in follow-on crops and volunteers are effectively controlled in all relevant land use areas (see question 2, above).



8. Does wheat reduce the quality of products or services obtained from the land use?

Rating: Low in all relevant land uses

As discussed in questions 6 and 7 above, wheat (as a weed or volunteer) has a low impact on both the establishment and yield/amount of desired species (e.g. the follow on crop in a rotation or desired species along roadsides). Generally, because wheat volunteers are not competitive, their density is expected to be low and they are effectively controlled (see question 2), there is a low expectation that wheat would reduce the quality or characteristics of products, diversity or services available from any land use areas. However, for some follow on crops (e.g. red lentils) even a small amount of wheat seeds (2 seeds per 200 g of lentils) can lower the quality of the crop (Pulse Australia 2014).



9. What is the potential of wheat to restrict the physical movement of people, animals, vehicles, machinery and/or water?

Rating: Low in all relevant land uses

As a densely planted mature crop, wheat is never impenetrable and is unlikely to inhibit the passage of people, animals, vehicles, machinery and water. Standard management practices as well as environmental conditions would keep the density of wheat volunteers very low. Thus, the potential for wheat to restrict the physical movement of people, animals or water would be low.



10. What is the potential of wheat to negatively affect the health of animals and/or people?

Rating: Low in all relevant land uses

There is no evidence that wheat is toxic to humans. A minority of people do suffer from wheat induced allergies, chiefly caused by -amylase inhibitors and seed storage proteins. Coeliac disease (gluten intolerance), characterised by damage to the intestinal wall and a failure to absorb the nutrients found in food, is an autoimmune disorder induced by an intolerance to cereal storage proteins. Like many plants, excess production of nitrate can occur in wheat, which upon digestion by animals (in particular ruminants) can be converted to nitrite, an ion that is tenfold more toxic than nitrate. Elevated levels of phytic acid in wheat can also be a problem, chelating minerals and preventing their dietary use after digestion by animals. Standard management practices as well as environmental conditions would keep the density of wheat volunteers very low. The proportion of volunteer wheat in animal feed (e.g. hay) is unlikely to be great enough to cause toxicity. Thus the potential for wheat to negatively affect the health of animals or people is considered low.



11. Major positive and negative effects of wheat on environmental health in each relevant land use:

11a. Does wheat provide food and/or shelter for pathogens, pests and/or diseases in the land use?

Rating: Minor or no effect in all land uses

In crop rotation regimes, wheat can provide a disease break, resulting in a decline in the numbers of any pathogen, pest or disease that attacks the follow on crop. However, wheat is associated with a number of insect pests that infect multiple crops. Wheat is susceptible to a range of pathogens, such as bunt, rusts, and nematodes. Infected wheat volunteers could act as a reservoir of these pathogens that can infect crops in subsequent years. Most of these pathogens are specific to wheat or cereals in general, and do no infect plants that are more distantly related.

However, the density of wheat volunteers is expected to be low and thus may have only minor or no effect.


11b. Does wheat change the fire regime in the land use?

Rating: Minor or no effect in all relevant land uses

The number and density of wheat volunteers is expected to be low for all relevant land uses, and would not be expected to affect fire regimes.



11c. Does wheat change the nutrient levels in the land use?

Rating: Minor or no effect in all relevant land uses

The number and density of wheat volunteers is expected to be low for all relevant land uses, and would not be expected to affect nutrient levels.



11d. Does the species affect the degree of soil salinity in the land use?

Rating: Minor or no effect in all relevant land uses

The number and density of wheat volunteers is expected to be low for all relevant land uses, and would not be expected to affect soil salinity.



11e. Does the species affect the soil stability in the land use?

Rating: Minor or no effect in all relevant land uses

The number and density of wheat volunteers is expected to be low for all relevant land uses, and would not be expected to affect soil stability.



11f. Does the species affect the soil water table in the land use

Rating: Minor or no effect in all relevant land uses

The number and density of wheat volunteers is expected to be low for all relevant land uses, and would not be expected to affect the soil water table.



11g. Does the species alter the structure of nature conservation by adding a new strata level?

Rating: Minor or no effect in all relevant land uses

The number and density of wheat volunteers is expected to be low for all relevant land uses, and would not be expected to add a new strata level.





1 Nullisomics: 2n-2 chromosomes; lacking one of the chromosomes normally present in the species. In hexaploid wheat the four homologous chromosomes apparently compensate for the missing pair of homologs.

2 Monosomics: 2n-1 chromosomes. A chromosome present only in a single dose. Occurs when an ‘n‑1’gamete occurring as a result nondisjunction during meiosis is fertilised by an ‘n’ gamete.

3 Telocentrics: a chromosome in which the centromere is located at the terminal end and which only has one ‘arm’.

4 Isochromosomes: a chromosome with identical arms. Present due to mis-division or strand exchange during meiosis or mitosis.

5 Potential costs (or severity or incidence) of damage are those which would occur in the absence of control measures. Present costs are the costs (or severity or incidence) which occurs with current control measures.

6 Intensive use includes areas of intensive horticulture or animal production, areas of manufacture or industry, residential areas, service areas (e.g. shops, sportsgrounds), utilities (e.g. facilities that generate electricity, electrical substations, along powerlines) areas of transportation and communication (e.g. along roads, railways, ports, radar stations), mine sites and areas used for waste treatment and disposal.

7 ALUM refers to the Australian Land Use and Management classification system version 7 published May 2010.

8 Source: Grains Research and Development website; accessed on 21 January 2016.

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