5 Herbicide resistance
 | 5.1 Stellaria media |
| 5.2 Brassica campestris |
5.1 Stellaria media
In Denmark, eight weed species have evolved resistance to atrazine (Andreasen & Jensen, 1994) and one weed, Stellaria media (common chickweed), has evolved resistance to sulfonylurea after 8 years of consecutive use of sulfonylurea herbicides (Kudsk et al. 1992). Use of the herbicide atrazine has now been banned, which leaves the sulfonylurea resistant Stellaria media the only significant herbicide resistant weed species yet found in Denmark.
Inheritance
In most cases of evolved sulfonylurea resistance, the trait is inherited by a single dominant gene (Holt et al. 1993), and the resistance mechanism does not seem to cause reduced fitness (Jensen, 1993). In the models the sulfonylurea tolerant S. media is, therefore, behaving exactly like its non-resistant counterpart.
Characteristics
S. media is an annual, winter annual and sometimes perennial herb, which emerges throughout the year. It is the most common Danish dicot weed (Andreasen, 1990), and the species is native to Europe. The species is self-pollinated and cleistogamous (Holm et al., 1977). In the models the sulfonylurea resistant S. media is treated as a separate population that is homozygous in the resistance loci, and its offspring do not segregate in a mendelian fashion. If S. medias, descending from the resistant individual, were assumed heterozygous in the resistant locus, then the offspring would segregate into homozygous resistant, heterozygous resistant and sensitive individuals. However, the fraction of sensitive individuals would rapidly diminish over the first generations (in F1 25% is non-resistant, in F2 17%, in F3 10% etc.).
5.2 Brassica campestris
Area grown with oilseed rape
Figure 5.1 shows the area grown with oilseed rape from 1976 to 1995. It is obvious that the area has increased almost exponentially during the first 10 years followed by a small decrease during the next 10 years. A survey on the occurrence of weed species in Danish arable fields, conducted in the period from 1987 to 1989, showed a low infestation of Brassica campestris in different crop types (Table 5.1). However, it will most likely take several decades for a weed species to stabilize its occurrence and density according to the land-use, and 10 years of intensive oilseed rape cropping is hardly enough time to expect a stabilized weed flora.
B. campestris as a weed
The herbicides used in cereal crops have traditionally been effective against B. campestris, and today large areas of cereal crops are sprayed with a sulfonylurea herbicides which are known for their efficacy against crucifeers (Chapter 4). The species is, however, tolerant to the same range of herbicides as oilseed rape, and it is therefore expected that an increased acreage with oilseed rape will increase the numbers of B. campestris.
Table 5.1
Frequency and constancy of B. campestris and oilseed rape as weeds in Danish crops (Source: Andreasen, 1990).
| Crop | B. campestris | Oilseed rape |
| Barley | 0,7 | 8 |
| Fodderbeet | 3,4 | 6,3 |
| Sugarbeet | 3,9 | |
| W. wheat | 2,4 | |
| Constancy | 6.4% | 27% |
Hybridization
B. campestris
and oilseed rape are known to hybridize with relatively high frequencies. Jørgensen and Andersen (1994) found crossing frequencies of 13% on B. campestris and 9% on oilseed rape when the plants were growing in a 1:1 mixture, and if individuals of B. campestris were surrounded by oilseed rape the hybridization frequencies were up to 93% (Figure 5.2). B. campestris is known as a self-incompatible species, whereas oilseed rape is approximately 66% self-pollinating. Therefore, it is not surprising to find high hybridization frequencies on solitary B. campestris .Fitness
Oilseed rape has a higher fitness (measured as survival in the field and seed production) than B. campestris, and the hybrid plants (F1) seems to be intermediate the parental type (T. Hauser personal communications, 1997) but morphologically closer to oilseed rape (Jørgensen & Andersen, 1994). Seed production of the F1 generation is not different from seed set on B. campestris. However, the next hybrid generation, F2 or backcross BC1, has a significantly lower seed production of only 16-28% of the seed set on B. campestris (Mikkelsen, 1996). Further backcrossings in the model are considered to be similar to B. campestris in growth habit and seed production.
Seed dormancy
Mikkelsen et al. (1996) showed that the hybrid generation has little seed dormancy, and in that respect it resembled oilseed rape. However, already in the first backcross several of the individuals were morphologically close to B. campstris and significant seed dormancy appeared in the BC2.This means that the seed longevity in the soil will be prolonged, and thus that seeds of transgenic herbicide resistant individuals may survive several years after the seed shedding.
The model
In the model we have allowed for two hybridization scenarios: 1. The transgenic oilseed rape is homozygous for the herbicide resistance gene (used in the oilseed rape model), and 2. The transgenic oilseed rape is heterozygous for the gene. In reality we can expect a mixture of heterozygous and homozygous individuals (Kirsten Klitgaard, AgrEvo, personal communication). In the model we have simplified the process of gene transfer by only including one hybrid generation, after which, the seeds, which have the gene, are transferred to the seedbank for B. campestris. To account for the depression in seed production in BC1 and F2, we included a low harvest index for the F1 generation (similar to the F2-BC1 generation). Oilseed rape and the F1 have the same growth parameters and sensitive individuals of B. campestris, oilseed rape and oilseed rape volunteers are controlled with the same efficacy for each herbicide (for model equations and parameter values, see Appendix 2).