The objective of the study was to determine the effect of different weed management treatments on the stand of the ryegrasses (one of the species that dominated the spectrum in the experiment vineyard) over the medium term.
A group of Lolium species, more commonly known as ryegrass, has become a problem weed in South Africa and, more specifically, in the vineyards of the Western Cape. These are loosely to densely tufted grasses (Figure 1a). During grapevine bud break (late August/early September), the tussocks are approximately 30 cm in height with the inflorescence (Figure 1b) not yet developed in most cases. At this growth stage, the glabrous leaves have a waxy shine. Ryegrasses invade vineyards from the surrounding border areas or could be brought in by seed contaminated bales of hay used as a mulch in the vine row. Ryegrasses also have the ability to become resistant to glyphosate and, more recently, paraquat. It is, therefore, of prime importance that the species should be controlled effectively before resistance to herbicides develops.
FIGURE 1. An example of the ryegrasses (Lolium species) (a) habitus, and
(b) Inflorescence and leaves.
Materials and methods
Twelve treatments (Table 1) were applied from April 2009 to August 2014 in a full-bearing, seven-year-old drip-irrigated Shiraz/101-14 Mgt vineyard established on a sandy to sandy clay loam soil (33°58’S, 18°50’E) near Stellenbosch. Five cover crop species and a treatment in which no winter growing cover crop was established (weeds) were evaluated. Two management practices were applied to each of the six winter treatments during grapevine bud break. One consisted of full surface, post-emergence weed control with 1.8 kg/ha glyphosate (CC) and the other consisted of cover crops/weeds slashed and immediately thereafter incorporated to a depth of 200 mm with a disc harrow (MC). In the last-mentioned practice, 0.6 kg/ha glyphosate was applied to the vine row. In both practices, 1.8 kg/ha glyphosate was applied during the first week of December. During May 2012, 1 kg/ha of fluasifop-butyl was applied to all treatments, except the two Pallinup oats (Avena sativa cv. Pallinup) treatments. The seedbed in the work row of the cover crop treatments was prepared with a disc harrow (two passes in opposite directions) to a depth of approximately 150 mm. A shallow cultivation (approximately 30 mm deep) with a rotary harrow covered the seeds. The cover crops were sown annually, with the seeding dates varying between 4 and 10 May from 2009 to 2012. However, in 2013 the cover crops were established on 23 May, due to the late onset of winter rain.
The dry matter production (DMP) of the ryegrasses was determined by harvesting the above-ground growth of five 0.5 m2 grids placed diagonally across the work row and 0.7 m apart. This was done just before grapevine bud break (end of August), during grapevine berry set (end of November) and post-harvest (early April).
Results and discussion
As expected, no ryegrasses occurred in the April evaluations. Therefore, the following discussion focuses on the August and November evaluations.
Weeds, CC and MC
The ryegrass stand in the CC treatment measured in August (winter growth) declined continuously from 2009 to 2012 (Figure 2a). The chemical control of the ryegrasses during grapevine bud break, followed by another herbicide application in November, seemed to prevent seed production or ripening of the seeds. In the CC treatment, the lowest winter ryegrass stand (August) realised in 2012. This was attributed to the grass specific herbicide applied in May 2012 for the control of the early germinating ryegrasses. During 2009, the stand of the ryegrasses increased by 34% from August to November. However, total control was achieved in November 2010 and 2011. The higher ryegrass stands measured in November 2012 and August 2013, when compared to the stand in August 2012, may indicate glyphosate resistance and the importance of a grass specific herbicide application during May. The stand of the ryegrasses in August nearly doubled in the MC treatment from 2009 to 2010 and remained high in 2011. The grass specific herbicide applied in May 2012 more than halved the ryegrass stand by August 2012. Although the ryegrass stand in November increased fourfold from 2009 to 2010, it declined again to reach a level in 2012 that was similar to that observed in 2009. The ryegrass stand measured in the CC treatment was, with the exception of November 2009, lower than that of the MC treatment from August 2009 to August 2012. Although not significant, this trend continued up to the end of the trial.
Pallinup oats, CC and MC
During the first two winters (August 2009 and 2010), Pallinup oats (Figure 2b) suppressed the ryegrasses significantly (least significant difference (LSD) August 2009 = 4.98, LSD August 2010 = 4.20) compared to the two weeds treatments (Figure 2a). Pallinup oats (CC) and Pallinup oats (MC) gave total control of the ryegrasses from November 2010 and November 2011 respectively, excluding August 2012 (Figure 2b). The ryegrass stand in the CC treatment tended to be lower than that of the MC treatment from August 2009 to August 2011, while the ryegrass stand was still meaningful. This supports the trend observed for the two weeds treatments (Figure 2a).
White mustard (Sinapis alba cv. Braco), CC and MC
Both the white mustard treatments (Figure 2c) reduced the ryegrass stand significantly in August 2009 (LSD = 4.98) compared to the two weeds treatments (Figure 2a). This was achieved with white mustard (CC) in August 2010 (LSD = 4.20) as well. White mustard (CC) gave total ryegrass control in November 2011 (third year of application), as well as August 2013. The ryegrass stand in the MC treatment was lower than that of the CC treatment during 2009. However, the reverse trend was observed from August 2010 onwards, with the exception of August 2012 (Figure 2c). During 2010 and November 2012, the ryegrass stand was significantly lower in CC than in MC. This is similar to the trends observed for the weeds and Pallinup oats treatments (Figure 2a and 2b). The winter ryegrass stand (August) declined to insignificant levels in 2012 (Figure 2c), which was attributed to the grass specific herbicide applied in May 2012. The stand of the ryegrasses in the CC treatments being higher in November 2012 than in November 2010 and November 2011 may indicate glyphosate resistance.
Canola (Brassica napus cv. AVJade), CC and MC
Canola (CC) (Figure 2d) reduced the ryegrass stand significantly in August 2009 (LSD = 4.98) and August 2010 (LSD = 4.20) compared to the two weeds treatments (Figure 2a). The ryegrass stand in the CC treatment was lower than that of the MC treatment throughout the study (Figure 2d). The differences were significant, except during November 2009, November 2011, August 2012 and August 2013. This supports the trends observed for the weeds, Pallinup oats and white mustard treatments (Figure 2a – 2c). Canola (CC) suppressed the ryegrasses totally in August 2012 and controlled the ryegrasses totally in November 2010 and November 2011 (Figure 2d). The ryegrasses re-appeared in canola (CC) in November 2012, indicating that the seedbed was not depleted.
Caliente 199 (Brassica juncea cv. Caliente 199), CC and MC
Both the Caliente 199 treatments (Figure 2e) reduced the ryegrass stand significantly during August 2009 (LSD = 4.98) compared to the two weeds treatments (Figure 2a). This was achieved with Caliente (CC) in August 2010 (LSD = 4.20) as well. The ryegrass stand in the CC treatment was lower than that of the MC treatment throughout the study (Figure 2e). The differences were significant during 2010, August 2011 and November 2012. This supports the trends observed for the weeds, Pallinup oats, white mustard and canola treatments (Figure 2a – 2d). Caliente 199 (CC) suppressed the ryegrasses totally in November from 2010 (second year of application) onwards (Figure 2e).
Nemat (Eruca sativa cv. Nemat), CC and MC
Both the Nemat treatments (Figure 2f) reduced the ryegrass stand significantly in August 2009 (LSD = 4.98) compared to the two weeds treatments (Figure 2a). This was achieved with Nemat (CC) in August 2010 (LSD = 4.20) as well. The ryegrass stand in the CC treatment was lower than that of the MC treatment, except in November 2009 (Figure 2f). The differences were significant during 2010, 2011 and November 2012. This supports the trends observed for the weeds, oats, white mustard, canola and Caliente 199 treatments (Figure 2a – 2e). Nemat (CC) suppressed the ryegrasses totally from November 2010 (second year of application) onwards, with only a negligible ryegrass stand detected in August 2010 and August 2012 (Figure 2f).
FIGURE 2. The stand of ryegrass (Lolium species) at grapevine bud break (end of August) and grapevine berry set (end of November) measured from August 2009 to August 2013. Two management practices were applied, the one in which full surface chemical control was applied during bud break (CC) and the other in which mechanical weed control was applied in the work row and chemical weed control in the vine row during bud break (MC). These management practices were applied to the following six winter soil management practices, namely: (a) No cover crop (weeds), (b) Pallinup oats (Avena sativa cv. Pallinup), (c) white mustard (Sinapis alba cv. Braco), (d) canola (Brassica napus cv. AVJade), (e) Caliente 199 (Brassica juncea cv. Caliente 199) and (f) Nemat (Eruca sativa cv. Nemat). The values followed by different letters differ significantly at the 5% level for that specific time.
With the exception of Pallinup oats (Avena sativa cv. Pallinup), incorporating the cover crops mechanically during grapevine bud break allowed the ryegrasses (Lolium species) to maintain a significant stand during the study. This is an indication that: 1) the seedbed was possibly not exhausted over time, 2) mechanical removal of the above-ground growth created conditions favourable for the germination of the ryegrass seeds, or 3) that the mechanical cultivation did not control all the ryegrass, thus allowing them to produce seeds. The cover crops used in the study suppressed the ryegrasses to a greater or lesser extent when combined with full surface chemical control during bud break. To ensure total suppression as soon as possible, Nemat (Eruca sativa cv. Nemat) and Pallinup oats are the preferred cover crops.
Where broadleaf cover crops are sown, the application of a grass specific herbicide during May should be part of a ryegrass control strategy. A paraquat/terbuthylamine mixture should replace glyphosate as herbicide at the end of November as part of a resistance prevention strategy as well.
The author thanks the ARC, Winetech and Dried Fruit Technical Services for financial support, the staff of the Soil and Water Science Department at ARC Infruitec-Nietvoorbij for technical support and Blaauwklippen Wine Estate for supplying the trial site and farm support.
– For more information, contact Johan Fourie at FourieJ@arc.agric.za.