Introduction

According to (Reynolds et al. 2012; Erenstein et al. 2022; Guarin et al. 2022), wheat (Triticum aestivum) ranks among the most crucial crops globally, providing approximately 20% of calorie and protein intake. Additionally, wheat is the most cultivated cereal crop, with over 220 million hectares planted annually as reported by (Shiferaw et al. 2013). To keep up with the increasing human population, the yield trends of the major crops must align with the rapid growth rate. Major variation in wheat yields annually is mainly attributed to environmental factors (Anderson 2010), which are only expected to become less predictable with the ever-changing climate conditions (Intergovernmental Panel on Climate 2014).

Several pests mainly of weed, insect, or fungal origin target wheat (Savary et al. 2019). Fungal pathogens cause several diseases on wheat, with yellow rust (Puccinia striiformis), brown rust (Puccinia triticina) and septoria tritici blotch (STB, Zymoseptoria tritici) being the most important (Figueroa et al. 2018; Savary et al. 2019). Current management practices rely heavily on high-energy inputs such as chemical fertilizers, herbicides, insecticides, and fungicides, mainly due to centuries of wheat breeding that focused on yield improvements resulting in crops cultivated in monoculture (Guarda et al. 2004; Voss-Fels et al. 2019). This approach promotes the emergence of new virulent pathogen strains, especially if disease resistance in the considered crop is based on a simple genetic structure (Brown 2015). Cultivating two or more constituent varieties together in the same field introduces diversity to the crop stand. This adds additional genetic, physiological, structural, and phenological heterogeneity, which can promote advantageous interactions between varieties and environments, thus introducing variation to the crop stand (Kiær et al. 2009). Wolfe (1985) suggests mixing varieties as an approach for growing several varieties simultaneously, which exhibit different traits such as yield potential, drought tolerance, disease, and insect resistance, and are similar enough in terms of maturity, height, quality, or grain type to allow for extensive agronomy and marketability.

Globally, fungal pathogens have the potential to cause yield losses of up to 15–20% (Figueroa et al. 2018). To address this issue, farmers rely on varietal resistance and fungicides. However, the efficacy of fungicides is often seen to decrease due to development of resistance by pathogens (Blake et al. 2018; Kildea et al. 2019; Jørgensen et al. 2020) and varietal resistance is also recognized as unstable (Singh et al. 2016; Brown et al. 2015; Kildea et al. 2021). To ensure a steady and reliable harvest of major crops, significant changes to current agricultural practices are necessary. One potential solution could be the adoption of variety mixtures.

In recent years, there has been a growing interest in the inherent strengths of wheat mixtures. Although there is almost no public data available on the proportion of variety mixtures grown, a few countries have released such information. Between 2017 and 2020, France saw a significant increase in the area devoted to variety-mixed bread, with the proportion doubling from 4.8 to 12.2% (Orellana‐Torrejon et al. 2021). In the United States, specifically Kansas state, winter wheat variety mixtures accounted for an average of 11% of the total planted acreage between 2013 and 2022 (USDA 2022).

Due to the increasing interest, this paper aims to provide a summary of the significant benefits and challenges related to growing winter wheat variety mixtures. Additionally, we will present a case study on Denmark, a country which has rapidly shifted from cultivating varieties in pure stand to mixtures, which now cover approximately 40% of the winter wheat area.

Global experiences from variety mixtures in wheat

Variety mixtures and impact on diseases

The benefits of variety mixtures are numerous, but the most well-established one is disease suppression (Wolfe 1985; Smithson & Lenné, 1996; Mundt 2002; Huang et al. 2012; Borg et al. 2018; Kristoffersen et al. 2020a). Studies conducted by various researchers have shown that variety mixtures can effectively suppress diseases in crops, which may also result in greater overyielding (Finckh et al. 2000; Newton et al. 2009; Kristoffersen et al. 2020a). For instance, delayed development of powdery mildew has been observed in several German studies (Strathmann 1984; Stuke & Fehrmann 1988; Manthey & Fehrmann 1993), while a decrease of 28% in yellow rust was found in a meta-study on wheat variety mixtures (Huang et al. 2012), and as high as 97% (Finckh & Mundt 1992). However, a reduction of STB appears to be lower, usually in the range of 10–15% (Mille et al. 2006; Vidal et al. 2017; Kristoffersen et al. 2020b). Mixture-derived disease reduction on wheat for several pathogens is presented in Table 1 based on published studies.

Table 1 Disease reduction in wheat variety mixtures compared to the average of pure stand varieties as stated in the literature
Full size table

Varietal diversity within a field is now widely acknowledged to aid in limiting the spread of pathogens (Zhan and McDonald 2013). (Borg et al. 2018) have summarized the major established mechanisms of disease reduction as a cause of (i) dilution, (ii) barrier, (iii) premunition (induced resistance), (iv) disruptive selection, and (v) compensation.

When dealing with emerging virulent pathogens, variety mixtures can provide a faster response and substitute the need for laborious crossing and selection that is required for the genetic pyramiding of resistance genes (Wuest et al. 2021). It is expected that the long-term viability of disease resistance allows mixtures to potentially have longer commercial viability than any sole variety.

Variety mixtures impact on yield

Variety mixtures tend to outperform monocultures of the constituent varieties in most cases, in terms of yield productivity; however, it is often restricted to a moderate range of one to five per cent (Kiær et al. 2009; Borg et al. 2018; Kristoffersen et al. 2020a).

Kristoffersen et al. (2020a) conducted a Danish meta-study, comprising 406 trials spanning 19 years. The study revealed that there was a significant yield increase of 1.4%. Furthermore, the study highlighted that untreated trials demonstrated a greater yield increase of 2.4%. The study also found that mixing different varieties not only outperformed the average yield of those varieties grown in pure stands but also surpassed the average yield of the four most widely grown varieties each year. This demonstrates that mixing varieties is a highly competitive solution.

Borg et al. (2018) analysed 120 wheat-dedicated studies and found a significant global average yield increase of 3.5%. The yield increase was even higher, reaching up to 6.2%, in areas with high disease prevalence. Similarly, (Smithson & Lenné, 1996) examined the yield data from over 100 studies and found that mixtures yielded significantly higher (5.4%) compared to the constituent varieties. A meta-study of 26 studies on wheat variety mixtures (Faraji 2011) showed an overall meta-estimate of the mixing effect at 2.7%. Furthermore, Kiær et al. (2009) found that mixtures with diverse weed suppressiveness reported improved yields compared to those without. A Chinese study (Kong et al. 2023) showed that wheat variety mixtures increased yield during four seasons with varying climatic conditions. The study demonstrated that under arid conditions, wheat mixtures effectively alleviated the impact of high-temperature stress on thousand-grain weight and improved grain water use efficiency. While most studies exhibit an overall positive effect on mixing varieties, (Hoang et al. 2022) found that two-way variety mixtures yielded lower than the mean of the constituent varieties.

Optimizing the composition of variety mixtures

Mille et al. (2006) demonstrated that mixtures of four varieties outperformed mixtures of two varieties in reducing STB disease severity. Kristoffersen et al. (2022) also confirmed this. However, (Finckh et al. 2000) showed in their review that the proportion of resistant varieties in a mixture plays a more crucial role than the number of varieties, based on findings with both splash-dispersed and wind-dispersed diseases. Kristoffersen et al. (2022) found no significant effect on yield based on the number of varieties, which is in contrast to previous findings by Finckh et al. (2000) and Kiær et al. (2009). However, based on the results on yield and disease reduction, it is recommended to grow variety mixtures with differing susceptibility, ideally at least three varieties.

Farmers often consider variety mixtures as inferior to the most resistant top-yielding varieties. However, to minimize the risk of a particular variety failing in a given season and environment, they might choose to grow multiple varieties on their land, which could be even more resilient if grown as mixtures. A French study conducted by Vidal et al. (2020) found that agronomic heterogeneity for traits such as plant height, yield potential, or earliness of the varieties in mixtures did not affect disease severity and yield relative to pure stands.

Additional aspects of variety mixtures

Only a few studies investigated if variety mixtures could have better efficiency in water and nutrient uptake, affected by the different varieties’ rooting systems (Newton et al. 2012; Wang et al. 2016). Since each variety has a distinct nitrogen uptake pattern, both in terms of different soil depths (Von Felten et al. 2012), different periods (Zhang et al. 2017), different forms (Spehn et al. 2002), and at different concentrations (Griffiths & York 2020), mixtures potentially limit nitrogen leaching and ensure better utilization of applied fertilizer.

Canopy structure and coverage are also variety-specific and influence both the amount of absorbed light and weed competitiveness. Few studies have shown no or limited improvements from variety mixtures in reducing weed biomass (Kaut et al. 2009; Lazzaro et al. 2018). Kong et al. (2023) showed that growing variety mixtures effectively alleviated the negative effect of high temperature and water shortage by improving grain size and water use efficiency in the drier years. Mixtures also reduce lodging tendencies, which subsequently lowers the reliance on plant growth regulators (Sarandon & Sarandon 1995; Jackson & Wennig 1997; Vidal et al. 2020; Li et al. 2023). They may also directly inhibit insect pests through the dissemination of volatile organic compounds that potentially impact the growth rate of aphid populations (Shoffner & Tooker 2013; Duan et al. 2021) or by a combination of resistance genes, as shown to apply for wheat midges (Vera et al. 2013). Based on Danish variety testing data, we have also observed improvements in grain quality from mixture cultivation (unpublished data). The results imply that the interactions among different wheat varieties strengthen the robustness of wheat cultivation.

Danish case study

Between 2014 and 2019, farmers in Denmark grew mixtures on only one to two per cent of the wheat area. However, in 2023, the proportion increased significantly to approximately 40%, according to SEGES (Table 2). The winter wheat area in Denmark is just under half a million hectares. Approximately 90% of the area is planted with certified and seed-treated seeds and traditionally 3–5 varieties dominate. However, in the last three seasons, mixtures have become the most commonly grown “variety”. Less than ten per cent of Danish wheat production is used for milling, while over 90% is used for animal feed. As a result, few Danish studies have investigated the impact of mixtures on baking quality. Stakeholders such as the Danish National Advisory Board (SEGES), the Danish Environmental Protection Agency, the Scientific Community, the private foundation responsible for national variety testing and certification of seeds (The Tystofte Foundation), and the Danish breeding companies and grain industry (Fig. 1) have all expressed a keen interest in encouraging integrated pest management (IPM)-based production by expanding the area grown with variety mixtures. These stakeholders have been promoting the benefits of variety mixtures through several years of publicly available variety trials and mutual advertisement, which has contributed to the significant increase in the area grown with mixtures. All of which was facilitated during various stakeholder meetings.

Table 2 Winter wheat area grown in Denmark and per cent area grown with variety mixtures compared with the most popular mono-variety. The area grown is based on the percentage of certified seeds of winter wheat variety mixtures sold in Denmark out of the total market share (Tystoftefonden)
Full size table
Fig. 1
figure 1

Main stakeholder groups and the process of advocating cultivation of variety mixtures in Denmark

Full size image

Commercial variety mixtures have been included in the Danish national trials and the results from these trials are published together with the results of individual varieties on www.sortinfo.dk. This has been a major step in advocating the cultivation of commercial variety mixtures, as the yield, quality, and disease severity of the mixtures are readily available to the growers. In the season of 2022/23, three of the top ten highest-yielding varieties were variety mixtures (www.sortinfo.dk), which can be regarded as a competitive alternative to monovarietal farming.

Moreover, variety mixtures have also been shown to produce financial gain. Based on 105 trials that included mixtures, Danish data from 2021 and 2022 have given an average yield benefit of 1.4 dt/ha. This benefit should be viewed in conjunction with a reduction in cost for fungicides as noted by Kristoffersen et al. (2020b). Although mixed seeds are slightly more expensive, growers still see a positive net gain of 40–50 €/ha depending on grain price.

In Denmark, the reference variety mixture (RVM), which comprises the four most widely multiplicated varieties from preceding seasons, is included in the national variety testing. The RVM has been part of the national trials since 1995 and is modified annually, with the substitution of a single variety within the mix. All commercial varieties and mixtures are compared to the RVM.

The substantial data set generated since 1995 with RVM along with more specific studies (Kristoffersen et al. 2020a, 2020b, 2022) have underlined reductions in disease pressure and minor yield gains from variety mixtures. The more recent field trials carried out by SEGES (SEGES 2022) and supported financially by the Danish Environmental Agency have similarly supported the cultivation of mixtures based on a very significant trial activity. The latest disease control benefits from this testing are displayed in Fig. 2, confirming prior results of significant disease reductions and a 1.4 dt/ha yield increase.

Fig. 2
figure 2

Results from the Danish National trials from 2021 to 2022. Trials were prompted to support the promoting of variety mixtures (SEGES 2022)

Full size image

In Denmark, variety mixtures are mixed and sold directly by the seed companies, commonly comprising three varieties of their portfolio. Commercial mixtures are trialled and authorized in Denmark by the Tystofte Foundation. To be authorized for sale and distribution in Denmark, the mixture must fulfil a list of specific criteria. These criteria are modified annually following the release of the first commercial mixtures, to specify and set realistic criteria. These criteria are the following. Firstly, disease severity in the mixture must be below a certain threshold for each of the four most prominent diseases on winter wheat under Danish conditions, namely STB, yellow rust, brown rust, and powdery mildew. The threshold is defined based on the average disease severity of the five most widely grown varieties in the previous season. Secondly, some physiological and morphological characteristics may not differ too greatly between the individual constituent varieties. These are the plant height (no more than 20 cm difference), the plant maturity date (no more than 5 days difference), and the yield of each constituent variety must be above an index value of 95 compared to the RVM (index 100) across the five preceding years of national trials or above an index value of 97 if a new variety is included in the mixture. Thirdly, the varieties must be on the official Danish variety list or the European common variety list, and the mixture must consist of three to four varieties, with each variety being equally represented in terms of proportional weight. In the season of 2022/23, eleven commercial variety mixtures were submitted to the Danish National testing (Tystoftefonden 2023). Four of the mixtures did not pass the thresholds of at least one of the listed criteria. For the coming season of 2023/24, sixteen winter wheat variety mixtures have been submitted for testing, of which three consist of four-way mixtures. Additionally, three winter barley mixtures have also been submitted. As of date, wheat mixtures for milling have not been established. To assemble a variety mixture valid for the millers, certain additional criteria would have to be set. The criteria would relate to grain quality characteristics, such as gluten, protein, and starch content, thousand-grain weight, specific weight (hectolitres per kilo), and more specific flour qualities, such as sedimentation value, HFN and the bread volume. As of the 2023/2024 season, the very first commercial Danish winter bread wheat variety mixture has been authorized, comprising three winter wheat varieties, all of which are listed on the national list and approved for bread wheat production, which under Danish legislation allows farmers additional fertilizer input.

Discussion

The cultivation of variety mixtures has been shown to provide many positive aspects. To list a few, variety mixtures increase yield stability (Kiær et al. 2009; Wuest et al. 2021), reduce the risk of crop failure due to weather or disease (Borg et al. 2018; Kristoffersen et al. 2020a), and provide better utilization of resources such as water and nutrients (Barot et al. 2017). Variety mixtures also increase genetic diversity (Tooker & Frank 2012; Joshi et al. 2023), which may result in improved adaptation to changing environmental conditions over time (Alsabbagh et al. 2022; Kong et al. 2023).

Disease-resistant varieties are currently the best tool available for disease control. However, they often do not provide complete disease control, and their resistance typically erodes over time when grown at a large scale (Singh et al. 2016). Although countries in Northern Europe grow different varieties, the actual number of varieties grown is relatively low (Perronne et al. 2017a, b; Perronne et al. 2017a, b) and they are often founded on similar pedigrees. The diminishing diversity in European winter wheat germplasm increases the risk of breakdowns in qualitative resistance, which subsequently advances the frequency of virulent strains leading to the varietal boom and bust cycles (Cowger et al. 2000; Singh et al. 2016).

In search for more resilient cropping systems, variety mixtures have frequently been proposed as a solid option that could potentially help mitigate plant diseases, lower the selection of new virulent strains, and reduce the dependency on fungicides, thereby slowing the development of fungicide resistance (Kristoffersen et al. 2020b; Orellana‐Torrejon et al. 2021, 2022). Recently, researchers have proposed that variety mixtures might also improve tolerance to extreme climate conditions (Kong et al. 2023). Policy-driven initiatives such as the EU’s Farm to Fork programme (EU) have highlighted the need to boost integrated pest management (IPM) and reduce dependence on pesticides. Adopting variety mixtures can be seen as one of the simplest ways to achieve part of these goals.

Grain yield is the most important factor when it comes to traits, characteristics, and agricultural outputs related to crop cultivation and harvesting. In Danish conditions, mixtures have proven to provide similar or higher yields compared to growing a single variety. Commercial mixtures usually contain high-yielding and well-recognized varieties. However, creating a mixture can be a time and resource-intensive process due to numerous possible combinations of varieties (Wuest et al. 2021). Companies usually use varieties within their portfolio, but more intelligent and robust mixtures could be generated by considering more details on the genetic background and hosting of resistance genes. If no commercial mixtures are available, farmers can also mix varieties on their own.

When growing a mixture of crop varieties, farmers must consider the potential negative or overlooked details that come along with it. One of the major concerns among farmers is the reduced uniformity in terms of maturity, height, and disease resistance, which can make it more challenging to manage the crop (Finckh et al. 2000; Bowden 2001; Barot et al. 2017). However, these factors can be managed by choosing mixing partners based on data from variety trials. Moreover, the variability in the composition of the grain quality is often considered a challenge for bread-making wheat. Few studies have investigated the effects of variety mixtures on grain quality (Jackson & Wennig 1997; Swanston et al. 2005; Hoang et al. 2022). Some studies have found that mixtures can improve grain quality or provide similar qualities as for the mean of the components (Osman 2006; Hoang et al. 2022), while others such as Clarke et al. (2006) found that the Hagberg falling number (HFN) was lower in mixtures compared to the mean of the component varieties.

Another risk which is often discussed is the potential development of "super-virulent strains" that could subvert the pool of resistance genes in several varieties simultaneously. The question is whether a cropping system based on variety mixtures would increase the prevalence of virulent strains after several years compared to monovarietal cultivation (Finckh et al. 2000; Mundt 2002). Alternatively, it is more likely that mixtures increase the durability of resistance by reducing the pressure exerted on the virulent subpopulation of the pathogen. A French study investigated mixtures’ impact on the selection of virulence on a specific stb resistance gene and found that mixtures reduced the size of the virulent subpopulation by affecting the frequency of strains displaying virulence against a major STB gene (Stb16q), relative to mono-varieties (Orellana-Torrejon et al. 2022). There is currently little knowledge about pathogen evolution and changes in virulence gene frequencies in variety mixtures, and this area requires more research.

Growing variety mixtures can reduce but not completely substitute the need for the use of fungicides, as their impact on leaf blotch diseases like STB is still relatively low compared to the effects of strong fungicides. In the Danish study, they could significantly reduce the need for fungicides, as 66% of the investigated cases did not experience a significant reduction in net yield after reducing the number of treatments by one (Kristoffersen et al. 2020a). Additionally, this reduction in fungicide use is likely to increase the longevity of fungicides by reducing the selection pressure, as also demonstrated by Kristoffersen et al. (2020b). Studies conducted in different climatic conditions have shown that cultivating wheat mixtures can increase genetic diversity and promote grain yield under drought and high temperatures (Kong et al. 2023), potentially buffering the negative impacts of climate change and enhancing productivity and stability. This is an area that requires further research.

Despite the large positive evidence generated over the years, variety mixtures are still not exploited largely on a global scale. Scientifically proven results are essential, but making variety mixtures a reality in practice requires the involvement of all stakeholders. Clear dissemination of plain and valid data to end-users is crucial. In Denmark, grain suppliers have played a key role in testing, producing, and selling mixtures, which has promoted the growth of variety mixtures in the country. The positive experiences from winter wheat in Denmark have intensified the desire to spread the use of variety mixtures to other crops, such as barley and other cereals.

Conclusion

Crop management can benefit significantly from growing variety mixtures, which is a sustainable and environmentally friendly approach. By emphasizing more IPM-based cropping, growing wheat variety mixtures can also bring substantial economic benefits to farmers. Many research projects and trial activities have confirmed the positive impact of variety mixtures, including reduced plant diseases, less dependence on pesticides, and a more robust cropping scenario that is less vulnerable to extreme climate conditions like drought and water stress. To promote the use of variety mixtures and encourage their adoption in the farming community, it is essential to involve all stakeholders and disseminate valid data.