Amongst non-native grass weeds, the brome species (Bromus spp.), particularly sterile or barren brome (B. sterilis) have become increasingly important pests in British cereal and leguminous crops and may cause cereal yield losses of 45% (Peters et al. 1993). National studies show an average wheat yield loss of over 7% from headland infestations, rising to 11% for patch infestations and over 18% for more general infestations.
Similarly, wild oat (Avena fatua) has been ranked as one of the most important and competitive grass weeds of winter and spring cereals and winter rape by European scientists (Schroeder et al. 1993). Wild oat and wheat compete for the same resources and are
mutually exclusive, resulting in 5% yield loss from as few as 5 plants per m2. Defra reported that over 750,000 ha of cereals were sprayed annually for control of wild oats in 1995, which equated to roughly 23% of cereals grown at that time (Defra final report for project PT0211).
Italian ryegrass (Lolium perenne) is also becoming more of a problem in cereal crops as the species develops herbicide resistance. Ryegrass is widespread through the country and is found in at least 25 % of cereal fields (Bayer CropScience 2006), with some data indicating that populations of as little as 5-7 plants per m2 can lead to yield losses of 8.5%-11%, though other data suggest that much higher infestation levels are needed to cause significant yield loss.
Of the broadleaved weeds, the Macaulay Institute Land Use report10 on biological invasions stated that common field-speedwell (Veronica persica) is one of the common grain contaminants. The reported annual control costs, combined with those for wild oat, amounted to £100 million. Common field-speedwell has been recorded throughout cultivated land and gardens and is indeed one of the commonest annual weeds in the UK (Salisbury 1962). Bond et al. (2007) reported an average 3% incidence in cereal crops and oil seed rape in England and Scotland.
Defra’s pesticide usage surveys11 record the area of crops sprayed, taking into account multiple applications, and provide information about the main weeds that are being controlled. These surveys also provided information for the main horticultural crops in the country. Based on this information and that provided in Lutman et al. (2009), the percentage of weeds controlled that are non-native was estimated. Knowledge of which species were non-native was taken from the species list compiled for this research project. Herbicide costs, including application costs, for different crops were taken from The John Nix Farm Management Pocketbook (2009). The results of the calculations are presented in Table 5.1.
Table 5.1. Estimated cost of non-native weed control
England Wales Scotland GB
Wheat £46,479,000 £471,000 £2,734,000 £49,684,000
Winter barley £6,886,000 £134,000 £1,147,000 £8,167,000 Spring barley £6,216,000 £288,000 £5,112,000 £11,616,000
Oats £1,703,000 £69,000 £347,000 £2,119,000
Rye - - - -
Triticale £168,000 - £11,000 £179,000
Oilseeds £6,049,000 £33,000 £226,000 £6,308,000
Potatoes £3,768,000 £82,000 £1,015,000 £4,865,000
Fodder crops £5,291,000 £9,000 £45,000 £5,345,000
Total Arable £76,560,000 £1,086,000 £10,637,000 £88,283,000
England Wales Scotland GB
Orchards £189,000 £10,000 - £199,000
Soft fruit £81,000 £2,000 £16,000 £99,000
Outdoor vegetables £788,000 £3,000 £83,000 £874,000
Outdoor bulbs & flowers £41,000 - - £41,000
Fodder & grassland £585,000 £114,000 £170,000 £869,000 Hardy nursery stock £261,000 £14,000 £14,000 £289,000 Total Horticulture £1,945,000 £143,000 £283,000 £2,371,000
Total Herbicides £78,505,000 £1,229,000 £10,920,000 £90,654,000
Although herbicides control the majority of weeds, competition between the crop and remaining weeds does lead to some yield loss from the crop. There is a paucity of data on the yield loss experienced by crops subsequent to herbicide use. Much information available stated that weed growth was reduced to ‘a commercially acceptable level’ or that weed control was total. Oerke et al. (1994) state that potential yield losses in wheat and barley without weed control could amount to 17% but with control the losses are reduced to 5%. In potatoes the potential losses could be up to a 23% yield loss, but again with control this is reduced to 5%. Therefore, using this information and that from May (2003) and a Scottish Agricultural College Technical Note12
, a yield loss of 5% was assumed for all cereal crops and potatoes, 4% for oilseed crops and commercial vegetables and 3% for sugar beet and fodder crops. It was also assumed that weed control would be totally effective in preventing yield loss in plants and flowers, hardy nursery stock and fresh fruit (as the
herbicides are primarily used for inter-row weeds) and therefore no additional cost due to competition with INNS was attributed for these crop groups. The yield loss figures were adjusted to account for the proportion of weeds affecting the crops that are non-native (using the same percentages used for the weed control estimates above) and the cost of the estimated losses calculated using the yield of crops grown and the market value, taken from Defra statistics13.
Market prices for crops can vary between 1% to 30% between years depending on amongst other things, global and local output and demand, weather conditions in the growing season and the area of a crop planted. Crops that are traded on international markets are especially vulnerable to price fluctuations resulting from changing global supply and demand, e.g. the price of wheat has decreased from US$277/ton to US$179/ton within one year14. Due to this variability of market prices and all the factors that cause price fluctuations at any one time, including those that are demand driven, an assumption has to be made as to the effect of a small increase in crop yields in Great Britain on overall trading prices. Therefore, it is assumed that an average of a 2% decrease in market prices would occur across all crops due to the increased production in the absence of any non-native weeds. The price differential between the increased production value due to increased output at a lower unit cost, and the cost of the yield loss caused by non-native weeds at the lower unit cost is taken as the cost of yield loss attributable to the presence of non-native weeds post control.
These are estimated at £102,716,995 (England £87,730,752, Scotland £1,134,194, Wales
£13,852,009; Table 5.2).
Table 5.2 Yield loss due to non-native weeds.
England Scotland Wales GB
Cereal Crops £54,100,286 £842,090 £8,686,024 £61,628,400
Potato £11,352,077 £209,668 £2,962,568 £14,524,313
Oilseeds £1,283,048 £6,765 £88,029 £1,377,842
Sugar Beet £3,828,500 - - £3,828,500
Peas & beans (fodder) £222,854 £879 £5,977 £229,710 Commercial Vegetables £18,943,987 £74,792 £2,109,411 £21,128,190 Total £89,730,752 £1,134,194 £13,852,009 £104,716,955