A series of rotation experiments at five sites over four years has explored the environmental and agronomic implications of growing herbicide tolerant oilseed rape and sugar beet. This paper reports on the population dynamics of volunteer rape (Brassica napus). The experiments compared four winter oilseed rape (WOSR) cultivars: a conventional cultivar (Apex) and three developmental cultivars either genetically modified (GM) to be tolerant to glyphosate or glufosinate, or conventionally bred to be tolerant to herbicides of the imidazolinone group. Seed losses at harvest averaged 3575 seeds m K2 but ranged from less than 2000 up to more than 10 000 seeds m K2. There was a rapid decline in seed numbers during the first few months after harvest, resulting in a mean loss of seeds of 60%. In subsequent seasons, the seedbank declined much more slowly at four of the five sites (ca 20% per year) and the models predicted 95% seed loss after approximately 9 years. Seed decline was much faster at the fifth site. There were no clear differences between the four cultivars in either the numbers of seeds shed at harvest or in their subsequent persistence. The importance of the persistence of GM rape seeds, in the context of the coexistence of GM and non-GM crops and the role of good management practices that minimize seed persistence, are discussed.
Summary Two winter oilseed rape (Brassica napus) cultivars, tolerant to glyphosate and glufosinate, were compared with a conventional cultivar at three sites over 4 years, in 3‐year crop rotations in the UK. The winter oilseed rape was grown in Years 1 and 4, with winter cereals, which received uniform herbicide treatments, in the intervening years. The second winter oilseed rape treatments were applied to randomised sub‐plots of the original plots. Weed densities were recorded in autumn and spring and weed biomass was measured in summer. At most sites, there was only one application of glufosinate or glyphosate, whereas two products were often used on the conventional variety. The timing of glyphosate and glufosinate application was, on average, 34 days later than that of the conventional broad‐leaved weed control treatments. Overall weed control, across all sites and years, was not statistically different between the conventional, glyphosate and glufosinate treatments. However, glyphosate achieved higher control of individual weed species more frequently than the other treatments. Glufosinate and the conventional treatments were similar in performance. The treatments in Year 1 sometimes affected weed populations in the subsequent cereal crops and, in rare instances, those in the rape in Year 4. Carry‐over effects were small after most treatments. In general, weed survival was greater in the oilseed rape crops, irrespective of the treatment, than it was in the intervening cereal crops.
The influence of sward height at the time of slurry application on sward responses to slurry was investigated using perennial ryegrass (Lolium perenne)/white clover (Trifolium repens) mini‐swards under greenhouse conditions. Pig slurry, cattle slurry and a fertilizer control were applied either to: swards cut 1 d before slurry application to heights of 2, 4 or 8 cm (CH2, CH4 or CH8); or to swards cut to 4 cm with regrowth intervals of 1, 4 or 8 d before slurry application (RI1, RI4 and RI8). Scorch, smother and growth of marked clover stolons and grass tillers were monitored after slurry application. Dry‐matter yields of both species were recorded over two harvests. Electrolyte leakage from leaves was used to assess leaf damage. Both slurries increased leakage, which was greatest from clover leaves, with biggest increases caused by pig slurry. Yield responses to slurry application varied with plant species and sward height. Reduced grass growth after slurry application to long swards (CH8, RI4, RI8) was probably caused by greater smother. In short swards (CH2), grass leaf growth was not depressed following slurry application but grass yields were reduced. Ammonia volatilization losses may have been greater in both long and short swards, contributing to yield reductions. Yield responses to slurry were greatest in swards cut to 4 cm, 1 d before application.
Perennial ryegras,s/white clover mini-swards were used 10 determine whether sward responses to slurry were modified by subsequent simulated rainfall (SR). Combinations of two SR volume rates (6-5 and 13 mm), three SR timings (3, 24 and 48 b after slurry application) and two slurry types (cattle, pig) were compared with a fertilizer control (no SR). Scorch, .smother and growth of marked stolons and tillers were monitored after slurry application, and yields of both species were recorded. Slurry coverage of plastic squares indicated that smother may be greater on horizontally orientated leaves (clover). Reduced grass leaf growth and numbers of clover growing points in the week following slurry application were attributed mainly to smother, and possibly other phytotoxic effects of slurry. Reasons for the superior clover growth with cattle slurry are unknown, although its higher P and K content may have contributed. Timing of rainfall is important in influencing sward responses to slurry application. Results suggest that application of slurry may be most beneficial when it immediately precedes rainfall and that longer intervals between slurry application and rainfall reduce grass and clover growth through either greater ammonia loss or slurry negative effects.
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