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Effect of organic and integrated soil cultivation practices on the weed population in a Sauvignon blanc vineyard situated in the Paarl wine district
Johan Fourie1 & Pieter Raath2
1. ARC Infruitec-Nietvoorbij, Stellenbosch.
Key words: grapevine, organic production, integrated production, weed control, weed spectrum
Summary
Introduction
According to Cousens & Mortimer (1995), the ability of weeds to increase rapidly in abundance after introduction into a habitat, and tolerate a wide range of habitat conditions, commands human attention. Uncontrolled weeds may reduce crop yield by as much as 80%. Effective and sustainable weed control is, therefore, essential. Producers are, however, being subjected to mounting pressure to reduce their usage of herbicides. Reasons include the widespread appearance of herbicide-resistant weeds (Darmency & Gasquez, 1990; LeBaron & McFarland, 1990; LeBaron, 1991; Henkes, 1997; Powles et al., 1997), the risk of environmental contamination (Carter et al., 1991), and negative public perceptions of agrochemicals (Major, 1992). A reduction in herbicide use, nevertheless place the economic viability of crop production at increased risk (Zoschke, 1994).
Soil cultivation practices act as a selective force in the development of a weed flora, because certain species are adapted to survive intermittent habitat disturbances (Smith, 1970) and will quickly fill the vacated niches created by such practices (Putnam, 1990). The species composition of a weed community is, therefore, linked to the agronomic practices applied (Buhler et al., 1997). In consequence, situations where the weed population is dominated by a small number of species are indicative of a weed management system that provides opportunities that these species can exploit (Cousens & Mortimer, 1995).
This study was conducted to determine the effect of three minimum soil cultivation practices on cover crop performance, weed control efficiency and weed spectrum in a vineyard over a six year period. The aim was to improve our understanding of long-term weed population dynamics in perennial crops.
Materials and methods
Trial layout
The trial was carried out over a period of six years (April 2000 to March 2006) in a four year old non-irrigated Sauvignon blanc/99 Richter vineyard trained on a four strand extended Peroldt trellis system (Booysen et al., 1992). The grapevines were established on a sandy clay loam soil (Table 1) at Plaisir de Merle farm situated near Paarl (33°44’S, 18°57’E), Coastal wine grape region, Western Cape. Mean annual rainfall during this period was 1033 mm, of which approximately 75% fell during autumn and winter. Vines were spaced 1 m in the row and 2.5 m between rows.
Three soil management practices were applied as described in Table 2. Treatments were replicated four times in a randomised block design. Each plot (replication) consisted of 21 grapevines and the two adjacent work rows, resulting in a surface area of 105 m2. From seasons 2000/2001 to 2004/2005, rye (Secale cereale L. v. Henog) was used as cover crop in the IP and Organic treatments. In 2005/2006 the cover crop was ‘Pallinup’ oats (Avena sativa L. v. Pallinup). The cover crop was sown annually (seeding dates varying between 14 April and 10 May) at 100 kg/ha (Fourie et al., 2001). Seedbed preparation was carried out using a disc harrow approximately six weeks before the seeding date. After sowing (broadcasted by hand), the seeds were covered with a disc harrow. In the treatments managed according to integrated production of wine (IP) guidelines (Anonymous, 2006), the cover crops received limestone ammonium nitrate (LAN) at a rate of 100 kg/ha (28 kg/ha of N) at the two- to four-leaf development phases as proposed by Van Huyssteen & Van Zyl (1984).
Measurements
Dry matter production (DMP) by both the cover crops and the associated weeds was determined at the end of August, according to the procedure described by Fourie et al. (2001). To determine the weed species in the work row, an area of 14 m2 was evaluated visually. The total surface area covered by weeds was estimated visually by only one person to avoid the inherent variance of multiple assessors (Olmstead et al., 2004). Percentage surface area covered by a weed species was estimated visually and expressed as a percentage of the total surface area covered by weeds.
Results and discussion
Cover crop performance and weed control efficacy
Dry matter production by the cover crops as measured at the end of August was, with the exception of 2004, significantly higher in the IP than in the Organic treatment (Table 3). The result achieved in 2001 illustrated the importance of nitrogenous fertiliser, when applied at the two to four leaf growing stage, in maximising the DMP of a grain species. The cover crops in the IP treatment and the treatment in which no chemicals were applied (Organic) suppressed the winter growing weeds significantly compared to that of the IP treatment in which no cover crop was sown (Conventional) during 2001 and 2002, while only the IP treatment suppressed the winter growing weeds significantly compared to the Conventional treatment in 2003. Thereafter no significant differences could be detected between treatments. Although not significant, the stand of winter growing weeds in the Organic treatment exceeded that of the Conventional treatment in which no cover crop was sown during 2005. This was attributed to poor cover crop growth as a result of the cover crops being established as late as 10 May, as well as the weeds producing seeds as a result of no chemical control being applied during the grapevine growing season.
Changes in the spectrum of winter growing weeds
The different soil management practices impacted on the species composition of the winter growing weeds (Tables 4 & 5). The percentage perennial weeds in the total winter weed spectrum declined in the Conventional treatment over the five year period, while it increased in the IP and Organic treatments (Table 4). The decrease in perennial species observed in the Conventional treatment was due to a sharp decline in the percentage yellow sorrel, while the increase in the IP treatment was due to yellow sorrel becoming one of the two dominant winter growing species. This increase is not of economic importance, because yellow sorrel is not difficult to control chemically. The increase in the perennial winter-growing weeds observed in the Organic treatment was due to an increase in both yellow sorrel (Oxalis pes-caprae L.) and narrow-leaf ribwort (Plantago lanceolata L.). This latter species is not readily controlled chemically and may, in consequence, come to have a negative economic impact in the long-term. Despite the increase in the percentage perennial species observed in the IP and Organic treatments, the annual species nevertheless remained dominant in these treatments (Table 5).
The weed population observed in the Conventional treatment changed from yellow sorrel and wild radish (Raphanus raphanistrum L.) being the dominant species (Table 4) to bur clover (Medicago polymorpha L.) becoming the dominant species (Table 5). Bristly ox-tongue (Picris echioides L.) appeared in the Conventional treatment at some point after 2000 and by 2005 had become one of the three prominent species. Bur clover and bristly ox-tongue are more difficult to control chemically than yellow sorrel and wild radish and may, therefore, have a negative economic impact in the long-term. The ‘other’ winter growing species observed in the Conventional treatment during 2005 consisted of ryegrass (Lolium species), flax-leaf fleabane (Conyza bonariensis (L.) Cronq) and Cape marigold (Arctotheca calendula L.) (Table 5). Inclusion of these species increased the biodiversity of the Conventional treatment to 11 winter growing species (Tables 4 & 5). Unfortunately one of the two indigenous species, yellow sorrel, diminished from being one of the two dominant species to only 5% of the total weed stand. The Conventional soil management practice, therefore, promotes invasion by exotic species over a period of approximately five years.
In the IP treatment, wild radish gave way as dominant species to yellow sorrel and wild radish, both of which are easy to control chemically. Hairy wild lettuce (Hypochperis radicata L.), a species that is more difficult to control chemically than yellow sorrel and wild radish, was not observed in this treatment after five years. This may be beneficial in the long-term. The ‘other’ winter growing species observed in the IP treatment during 2001 were musk herons bill (Erodium moschatum (L.) L’Herit. ex Ait) and chickweed (Stellaria media (L.) Vill.). These species disappeared so that by 2005, the only species present in the ‘other’ category was ryegrass. The exotic species present in the IP treatment decreased by one species, to give a total of eight winter growing species (Tables 4 & 5), while the only indigenous species, yellow sorrel, became one of the two dominant species. This soil management practice, therefore, creates conditions that favour the species endemic to the region.
The weed population observed in the Organic treatment changed from one in which wild radish was dominant to one in which broadleaf purple vetch (Vicia sativa L.), wild radish, yellow sorrel, and narrow-leaf ribwort were all prominent. Since broadleaf purple vetch and narrow-leaf ribwort are more difficult to control chemically than wild radish, a forced reversion to chemical weed control could have negative long-term economic implications. The ‘other’ winter growing species observed in the Organic treatment during 2001 consisted of musk herons bill and chickweed. By 2005 the chickweed was replaced by flax-leaf fleabane. The winter growing weed biodiversity of this treatment did not change and remained a total of nine winter growing species (Tables 4 & 5). This soil cultivation practice has the advantage that it creates conditions that favour the development of species that are endemic to the region.
Changes in the spectrum of summer growing weeds
The different soil management practices affected the species composition of the summer growing weeds (Tables 6 & 7). The percentage of perennial weeds in the total summer weed spectrum declined drastically over the five year period in the IP treatment, but remained much the same in the Conventional and Organic treatments (Table 6). This decline in the perennial species observed in the IP treatment was attributed to hairy wild lettuce and narrow-leaf ribwort being prevented from producing seeds, despite the fact that chemical weed control was applied late (mid-October). Flax-leaf fleabane, a very hardy species that is difficult to control chemically, nevertheless filled the vacated niche and became the dominant species in the IP treatment (Tables 6 & 7). Despite a decline in the percentage of hairy wild lettuce in the Conventional treatment, the percentage of perennials present in the weed population remained the same (Table 6). This lack of change was attributed to an increase in the percentage wild lettuce (Lactuca serriola L.). The same phenomenon was observed in the Organic treatment with narrow-leaf ribwort and wild lettuce filling the niche vacated by hairy wild lettuce. As far as the annuals in the Conventional treatment are concerned, a definite population shift occurred, with Spanish black jack (Bidens bipinnata L.) replacing bristly ox-tongue as the dominant species (Table 7). The ‘other’ summer growing species observed in the Organic treatment during 2001 consisted of sowthistle (Sonchus oleraceus L.) and wild radish, while that observed in the IP treatment during 2006 was sowthistle. The exotic species present in the Organic treatment decreased by two over the five year period, while that in the IP treatment increased by one (Tables 6 & 7). The exotic species observed in the Conventional treatment was reduced by one over the five year period, while the only endemic summer growing species, namely common dubbeltjie (Tribulus terrestris L.), also disappeared.
Conclusions
A grain cover crop established annually, combined with either chemical weed control or slashing during the grapevine growing season prevents an increase in the percentage non-indigenous species. The absence of a cover crop in the working row during winter creates conditions which promote an increased variety of exotic weeds. This is not desirable and should be avoided at all cost. The importance of nitrogenous fertiliser (LAN, 28% N), applied at the two to four leaf growing stage, in maximising the DMP of a grain species was illustrated.
All soil cultivation practices cause greater or lesser changes in the weed spectrum, species dominance shifting with time. This is an aspect of weed control that should be studied more extensively in future, since it may be a useful way of determining whether a soil cultivation practice will promote biodiversity and be sustainable in the long term. Such studies also create a better understanding of the weed population dynamics in the vineyards of the Western Cape, and enable guidelines regarding future soil management decisions to be formulated with greater confidence.
Literature cited
Anonymous, 2001. Integrated Production of Wine. South African guidelines for the integrated production of wine, P.O Box 2176, Dennesig, 7600 Stellenbosch, South Africa.
Booysen, J.H., Steenkamp, J. & Archer, E., 1992. Names of vertical trellising systems (with abbreviations). Wynboer September, 15.
Buhler, D.D., Hartzler, R.G. & Forcella, F., 1997. Implications of weed seedbank dynamics to weed management. Weed Sci. 50, 329-336.
Carter, A.D., Hollis, J.M., Thompson, T.R.E., Oakes, D.B. & Binneyu, R., 1991. Pesticide contamination of water sources: current policies for protection and a multi-disciplinary proposal to aid future planning. Brighton Crop Prot. Conf. Weeds 2, 491-498.
Cousins, R. & Mortimer, M., 1995 (1st ed). Dynamics of weed populations. Cambridge University Press, Cambridge.
Darmency, H. & Gasquez, J.,1990. The fate of herbicide resistant genes in weeds. In: M.B. Green, H.M. LeBaron & W.K. Moberg (eds). Managing resistance to agrochemicals: From fundamental research to practical strategies. Washington DC: American Chemical Society.
Fourie, J.C., Louw, P.J.E. & Agenbag, G.A., 2001. Effect of seeding date on the performance of grasses and broadleaf species evaluated for cover crop management in two winegrape regions of South Africa. S. Afr. J. Plant Soil 18, 118-127.
Henkes, R., 1997. Handling herbicide resistance. The furrow 102, 8-11.
LeBaron, H.M., 1991. Distribution and seriousness of herbicide-resistant weed infestations worldwide. In: Caseley, J.C., Cussans, G.W. & Atkin, R.K. (eds). Herbicide resistance in weeds and crops. Butterworth-Heinemann, Boston. pp. 27-43.
LeBaron, H.M. & McFarland, J.E., 1990. Herbicide resistance in weeds and crops: an overview and prognosis. In: M.B. Green, H.M. LeBaron & W.K. Moberg (eds). Managing resistance to agrochemicals: From fundamental research to practical strategies. Washington DC: American Chemical Society.
Major, C.S., 1992. Addressing public fears over pesticides. Weed Technol. 6, 471-472.
Olmstead, M.A., Wample, R., Greene, S. & Tarara, J., 2004. Nondestructive measurement of vegetative cover using digital image analysis. HortSci. 39, 55-59.
Powles, S.B., Preston, C., Bryan, I.B. & Jutsum, A.R., 1997. Herbicide resistance impact and management. Adv. Agron. 58, 57-93.
Putnam, A.R., 1990. Vegetable weed control with minimal herbicide inputs. HortSci. 25, 155-159.
Smith, A.G., 1970. The influence of Mesolithic and Neolithic man on British vegetation: a discussion. In: D. Walker & R.G. West (eds). Studies in the vegetational history of the British Isles. Cambridge University Press, Cambridge. pp 81-96.
Van Huyssteen, L. & Van Zyl, J.L., 1984. Mulching in vineyards. Farming in South Africa E.12.
Zoschke, A., 1994. Toward reduced herbicide rates and adapted weed management. Weed Technol. 8, 376-386.
Acknowledgements
Financial support by Winetech and ARC, technical support by Isabella Van Huyssteen, Karen Freitag and the staff of Soil Science ARC Infruitec-Nietvoorbij.
For further information contact: Dr Johan Fourie, tel. (021) 809 3043, E-mail Fouriej@arc.agric.za.
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