Research Article |
Corresponding author: Jesse M. Rubenstein ( jesse.rubenstein@lincolnuni.ac.nz ) Academic editor: John Ross Wilson
© 2023 Jesse M. Rubenstein, Philip E. Hulme, M. Philip Rolston, Alan V. Stewart, John G. Hampton.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Rubenstein JM, Hulme PE, Rolston MP, Stewart AV, Hampton JG (2023) A century of weed change in New Zealand’s forage seed multiplication industry. NeoBiota 85: 167-195. https://doi.org/10.3897/neobiota.85.100825
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International seed trading provides a significant introductory pathway for weed seeds, and many globally established weeds originated as contaminants in agricultural seed lots. Management of these trade systems helps minimize agricultural losses and is an important means of preventing future biological incursions. Forage crop seed lots could be considered higher risk than seed lots of arable and vegetable crops, as they have been found to have a higher percentage of contaminated seed lots. Two of the most commonly used temperate forage crops worldwide are perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). New Zealand is one of the top producers of these crop seeds globally, and both species are commonly used in New Zealand pastures. Using historical and current seed lot analytical purity test results, we examined the frequency, identity and temporal changes of weed seeds found within agricultural seed lots of perennial ryegrass and white clover grown in New Zealand from 1912 to 2019. Overall, the percentage of contaminated forage seed lots decreased between approximately three to sixfold over the study period, indicating that herbicide availability, seed certification and improved crop management have been effective for weed control. However, we identified a handful of annual weed species that could become more problematic in the future, either because they showed an increasing presence trend in seed lots or were identified as the most common contaminants. In 2019, Vulpia bromoides was the most common contaminant in perennial ryegrass seed lots, and Chenopodium album was the most common in white clover seed lots. Sherardia arvensis and Poa annua, both significant species with an increasing presence trend, had the largest increases in perennial ryegrass seed lots over the study period. Conversely, Rumex acetosella had the largest presence decline for both crop species. There was a significant difference between the percentage of contaminant species that were grass weeds between study crops, where perennial ryegrass seed lots had approximately four times more grass species than white clover. Considering New Zealand trades crop seed with approximately half of the world’s countries and contributes substantially to the global supply of forage seed, our study provides a unique insight into changes of the weed spectrum throughout the seed for sowing system over the last century.
Analytical purity, herbicides, Lolium perenne, perennial ryegrass, seed lots, Trifolium repens, weed seeds, white clover
Nearly all aspects of forage seed production are highly managed, including the use of agrichemicals, fertilizers, grazing, irrigation, harvesting, as well as post-harvest management, including seed drying and seed cleaning technologies (
This is especially true in New Zealand, which has strict biosecurity laws regarding the importation of agricultural seed and a long history of forage seed production dating back to the 1850s (
New Zealand is integral in the international trade of forage seed, as the world’s largest exporter of white clover seed and the fourth largest exporter of ryegrass seed (
The first tangible international quality control efforts for forage seed lots were not implemented until the 1920s (
Prior to seed certification, weed seed contamination was one of the primary concerns for forage seed producers (
With the introduction of mechanization on farms in the 1930s (small tractors, self-propelled combines, header harvesters), it became more practical for New Zealand growers to produce their own forage seed than it was to import seed lots from abroad (
This study aims to determine the following in regards to perennial ryegrass and white clover seed grown in New Zealand, as they are two of the most commonly used forage species in temperate pasture systems worldwide: 1) how seed lot contamination has changed over the last century; 2) the most common contaminant species historically and recently; 3a) whether there is a significant relationship between time and presence of a given contaminant species; 3b) whether significant contaminant species have an increasing or decreasing presence trend over time, and; 4) whether there is a significant difference in the percentage of contaminant grass species in seed lots of perennial ryegrass and white clover.
We could find no previously published New Zealand or international studies on long-term trends in agriculture weeds found within forage seed crops. Considering New Zealand contributes substantially to the global supply of forage seed and trades crop seeds with approximately half of the world’s countries, our study therefore provides a unique insight into changes of the weed spectrum not just in New Zealand, but throughout the forage seed for sowing system over the last century.
An extensive literature review was conducted to identify historical studies and datasets that reported analytical purity test results as they relate to plant contaminants within seed lots of perennial ryegrass and white clover grown in New Zealand. Both
Data fields from all seed lot analytical purity tests used for analysis included: sampling year, total samples tested, contaminant species and the percentage of seed lots with the contaminant, which we defined as the number of seed lots where the contaminant was present for a given year divided by total seed lots tested for the same year. Plant names were standardized using taxonomy listed in the International Plant Name Index (
Both
Up until approximately the late 1990s, New Zealand used a four component seed lot analytical purity reporting system for domestic seed analysis certificates (
The annual mean percentage of contaminated seed lots was determined by averaging all of the individual percentages of seed lots where each contaminant species was present (including zero values) in a given sampling year. White clover data from 1923 and perennial ryegrass data from 2003 and 2009 were excluded from this portion of the analysis because only six to eight common weed species were reported for these years. The most common forage crop weeds were the contaminant species with the overall top five highest values with regards to the mean percentage of seed lots they were reported in. To determine whether there was a significant relationship between time and presence of a given contaminant species, Kendal’s Tau-b correlation tests (two-tailed) were conducted using the percentage of seed lots containing a contaminant and the corresponding sampling year as variables. Kendall’s Tau-b was used because it is well-suited for small sample sizes and non-monotonic data with ties (
Number of seed lots tested and number of contaminant species reported for perennial ryegrass and white clover. Note that datasets from 2003 and 2009 only reported eight common and/or problematic species.
Sampling year | Perennial ryegrass seed lots tested | Number of contaminant species reported in perennial ryegrass seed lots | White clover seed lots tested | Number of contaminant species reported in white clover seed lots |
---|---|---|---|---|
1912 | 52 | 31 | 27 | 78 |
1923 | 1537 | 39 | 325 | 62 |
1925 | 1178 | 24 | 237 | 29 |
1962 | 100 | 16 | No Data | No Data |
1984 | 1445 | 40 | No Data | No Data |
1989 | 1289 | 33 | 537 | 19 |
1993 | 2537 | 33 | 1715 | 19 |
1994 | 1563 | 33 | 934 | 19 |
2003 | 100 | 8 | No Data | No Data |
2009 | 100 | 8 | No Data | No Data |
2019 | 245 | 49 | 168 | 45 |
A Wilcoxon signed-rank test was used to compare the percentage of contaminant grass species in seed lots of perennial ryegrass and white clover, for coinciding study years. IBM’s SPSS software was used for statistical analysis and visualizations throughout this study (
Data related to contaminant species found within seed lots of perennial ryegrass and white clover are provided in the subsequent Tables and Supporting Information of this article.
Between recent and historical data, there were 79 contaminant species (63 genera) reported in seed lots of perennial ryegrass and 90 species (68 genera) in seed lots of white clover, of which 46 species overlapped in both crops (Suppl. material
Mean percentage of contaminated perennial ryegrass seed lots and white clover seed lots. Note: Y-axis only goes up to 20%.
Overall, Bromus hordeaceus was the most common contaminant in perennial ryegrass, where it was present in 68.4% of all seed lots (Fig.
The five most common forage crop weeds reported in seed lots of perennial ryegrass and white clover. Values based on the percentage of seed lots with contaminant. Contaminants are listed in alphabetical order.
Rumex acetosella was the most common contaminant in white clover, where it was present in 56.8% of seed lots. It was also one of the top five most common contaminant species in white clover for every sampling year except 2019, when its presence declined to 1.8%. Chenopodium album was the most common contaminant in white clover in 2019 (present in 61.9% of seed lots). Of the most common weeds in perennial ryegrass and white clover, Rumex acetosella and Sherardia arvensis were the only species that occurred as a top five contaminant in both crops. These contaminants also had similar temporal trends in both crops, with Rumex acetosella reaching its highest presence in 1912, while Sherardia arvensis reached its peak in the late 1980s/early 1990s. The overall mean percentage of seed lots where these contaminants were present was approximately three times larger in white clover than perennial ryegrass.
The Kendall rank correlation (time versus percentage of seed lots where contaminant was present) identified eight significant species in perennial ryegrass that showed an increasing trend over time regarding the percentage of seed lots where they were present (Table
Significant contaminant species with an increasing presence trend in perennial ryegrass seed lots. Contaminants are listed based on descending Kendall rank correlation values.
Contaminant | Common name in New Zealand | Kendall rank correlation coefficient | P-value |
---|---|---|---|
Phalaris minor | Lesser canary grass | 0.85 | 0.002 |
Polygonum aviculare | Wireweed | 0.67 | 0.016 |
Anthemis arvensis | Corn chamomile | 0.62 | 0.033 |
Galium aparine | Cleavers | 0.62 | 0.033 |
Veronica sp. | Speedwell | 0.62 | 0.033 |
Poa annua | Annual poa | 0.56 | 0.020 |
Lapsana communis | Nipple wort | 0.55 | 0.049 |
Sherardia arvensis | Field madder | 0.48 | 0.042 |
For perennial ryegrass, Sherardia arvensis had the largest increase over the study period in the percentage of seed lots where present (+20%), followed by Poa annua (+19%) (Fig.
The Kendall rank correlation identified five significant contaminant species in perennial ryegrass and six significant contaminant species in white clover that showed a decreasing trend over time in regards to the percentage of seed lots where they were present (Table
Significant contaminant species with a decreasing presence trend in seed lots. Contaminants are listed based on ascending Kendall rank correlation values.
Contaminant | Common name in New Zealand | Kendall rank correlation coefficient | P-value | Crop species with decreasing presence trend |
---|---|---|---|---|
Cerastium sp. | Mouse-ear chickweed | -1.00 | 0.000 | White clover |
Plantago lanceolata | Narrow-leaved plantain | -0.83 | 0.002 | Perennial ryegrass |
Cuscuta sp. | Dodder | -0.82 | 0.012 | White clover |
Rumex acetosella | Sheep’s sorrel | -0.81 | 0.011 | White clover |
Prunella vulgaris | Selfheal | -0.78 | 0.042 | White clover |
Rumex acetosella | Sheep’s sorrel | -0.78 | 0.004 | Perennial ryegrass |
Silene noctiflora | Night-flowering catchfly | -0.78 | 0.042 | White clover |
Plantago lanceolata | Narrow-leaved plantain | -0.71 | 0.024 | White clover |
Trifolium hybridum | Alsike clover | -0.62 | 0.033 | Perennial ryegrass |
Holcus lanatus | Yorkshire fog | -0.61 | 0.025 | Perennial ryegrass |
Hypochaeris radicata | Catsear | -0.61 | 0.028 | Perennial ryegrass |
Rumex acetosella had the largest decline in the percentage of seed lots where present for both crop species, decreasing approximately 61% in perennial ryegrass seed lots and 95% in white clover (Fig.
Of the contaminant species reported from perennial ryegrass and white clover seed lots, 22.0% were grass species (Suppl. material
In comparison to other weed management tools, the introduction of herbicides in the 1940s had the largest impact on decreasing seed lot contamination (
Up until the 1960s, herbicide chemistry had primarily focused on broadleaf weeds, and as such, early herbicides were mostly effective in perennial ryegrass but not white clover seed crops. However, the widespread introduction of grass herbicides in the early 1960s, such as paraquat (
During the 1970s, selective grass herbicides that were safe for use in ryegrass crops were introduced in New Zealand. These included ethofumesate (
Also released in the mid-1990s was the diflufenican-based herbicide Jaguar (
While herbicides have reduced weed seed contamination in seed lots in New Zealand, herbicide resistant weed species have increasingly been reported, the majority in forage and arable crops (
Of the eight significant species that had an increasing presence trend over time in perennial ryegrass seed lots, both Sherardia arvensis (+20%) and Poa annua (+19%) had the biggest increase over the study period, indicating that these species could become more problematic in the future. Their presence increases can partially be explained by the end of the opportunist ‘catch cropping’ system (focused on livestock production) in the late 1980s, before New Zealand’s forage seed multiplication industry finished transitioning into an annual arable system with five to ten-year crop rotations that incorporated forage species like perennial ryegrass and white clover (
Another reason for changing weed spectrums can be attributed to a greater availability of irrigation that began in the 1980s in New Zealand’s major seed production region, Canterbury (
Although there were eight significant contaminant species in perennial ryegrass that showed an increasing trend in seed lots, no increasing contaminant species were identified as significant in white clover. This can partially be explained by the fewer degrees of freedom used in Kendall rank correlation tests for the white clover analysis. Unfortunately, there were fewer analytical purity studies published on seed lots of white clover when compared to perennial ryegrass, resulting in about one-third fewer years’ worth of available study data. Even though it was not identified as significant in our study, Chenopodium album is a weed of particular concern in the production of forage and arable crops (
Trifolium glomeratum had the second largest increase over the study period in the percentage of white clover seed lots it was present in (+13%). For white clover crops, other Trifolium spp. are often the most common contaminant species reported in their seed lots (
In perennial ryegrass seed lots, Bromus hordeaceus was the most common contaminant species over the study period, and it was one of the top five most common weeds for each individual sampling year. This contaminant species is an annual grass and is common in a variety of forage crops (
In white clover seed lots, we identified Rumex acetosella as the most common contaminant species for all years combined, and in perennial ryegrass seed lots it was one of the top five most common weeds. Aside from being one of the most common contaminant species in both crops, it was also identified as a significant species with a decreasing presence trend in both crops, with presence levels dropping below 2% by 2019. This is worth noting, because while Rumex acetosella was very common in both crops, this does not preclude the weed from steadily decreasing over time. In fact, Rumex acetosella had the largest decline in both crops of any of the significant contaminant species, based on the percentage of seed lots where present. Because of this, additional recent sampling years would be useful to determine if this trend continues after 2019.
After the implementation of seed certification in 1929, New Zealand farmers could be assured that forage seed lots met strict genetic and analytical purity standards (
Cuscuta sp. is a broadleaf contaminant which we identified as a significant species with a decreasing presence trend in white clover seed lots, after declining from approximately 15% of seed lots prior to the 1930s to 0% from the 1980s onwards. Aside from herbicide use, this decrease can also be explained by New Zealand’s industry-led policy changes in the 1980s. Along with thirteen other species, it was dubbed an ‘undesirable weed’ by what is now the New Zealand Grain & Seed Trade Association (
Seed cleaning technology has had a major impact on reducing seed lot contamination and the machinery used prior to seed certification (air screen cleaners and spiral separator), as well as those developed later (indent cylinders and gravity table separators), are still regularly used today (
Overall, the mean percentage of contaminant species that were grasses was approximately four times larger in seed lots of perennial ryegrass than white clover. As was previously discussed regarding Trifolium spp., this difference in values between study crops is to be expected since it is harder to clean or use chemical controls when a grass contaminant is present in a grass crop, especially since non-selective herbicides can be damaging to a wide array of genera within the same family. In perennial ryegrass seed lots, the percentage of species that were grasses decreased 2.5 times from the 1990s to 2019. This decline can partially be explained by the increased use of ethofumesate, an herbicide which controls a wide range of grass weeds in perennial ryegrass. It decreased in price in New Zealand in the late 1990s when generic versions become available (
Although we identified several annual weed species that could become more problematic in the future because they showed an increasing presence trend in seed lots or were identified as the most common contaminants, overall the percentage of contaminated forage seed lots has decreased between approximately three and sixfold since the early part of the 20th century. This indicates that herbicide availability, seed certification and improved crop management have been effective for weed control. However, while the percentage of contaminated seed lots has decreased, unless control measures are maintained, contamination levels can quickly increase. Additionally, weed seeds still remain a concern given the rise in reports of herbicide resistant weeds, a reduction in herbicide availability from a lack of new chemistry, decreases in herbicide use amidst regulatory bans, and the risk of introducing non-native species (
Because of New Zealand’s long history of forage seed production, large number of international trading partners, and its role as a primary seed producer of common forage crops, our study is in a unique position to identify weed trends occurring globally throughout the forage seed industry. However, we were only able to identify trends based on the absence/presence of a contaminant species, since previous studies did not report the number of weed seeds. This type of information would be useful for future studies investigating propagule pressure. Considering the lack of historic or current analytical purity data, we believe there is a need for the development and maintenance of a national (or multi-national) seed lot analytical purity database. This could be managed by individual country’s ISTA accredited seed testing laboratories, who could digitize the information from seed lot analysis certificates already being provided to seed companies.
The authors would like to thank the following seed companies for providing data: Barenbrug, Cropmark Seeds Limited, Luisetti Seeds, PGG Wrightson Seeds Ltd. and Seed Force Ltd. We would also like to thank the following people for their assistance: Jennifer Bufford from Manaaki Whenua – Landcare Research, Murray Kelly from PGG Wrightson Seeds Ltd., Trevor James from AgResearch Ltd., Kerry Harrington from Massey University, Will Godsoe from Lincoln University, Blair Galvin from University of Canterbury and Jennifer McCulloch from the Foundation for Arable Research.
Funding was provided by the Seed Industry Research Centre (www.sirc.co.nz), a partnership between research providers and major New Zealand seed companies. Funding was also provided by Plant and Food Research Ltd. via the Better Border Biosecurity research collaboration (www.b3nz.org.nz). This publication has been financed by the Lincoln University Open Access Fund.
Supporting information
Data type: Weed seed occurrences (word document)
Explanation note: table S1. Contaminant species reported in perennial ryegrass seed lots and corresponding percentage of seed lots where present. table S2. Contaminant species reported in white clover seed lots and corresponding percentage of seed lots where present.