Grain Sorghum Weed Control Guide, written for Pacific Seeds by nationally recognised weed authority Andrew Summervaille, is a comprehensive and outstanding publication dealing with all control aspects. These include herbicides, with lots of insightful comment, fair but often acknowledging limitation of specific products, as well as the important contribution of agronomic aspects to help combat weeds' effect on yield.
Contributed by Qld Department of Agriculture and Fisheries research agronomist Michael Widderick, is an important section covering weed suppression by growing a competitive sorghum crop. Research over two years has shown that growing a competitive sorghum crop with increased density and reduced row spacing can significantly suppress growth and seed production of weeds like barnyard grass and Feathertop Rhodes grass.
While trial results were not always consistent, crops sown in 0.5 m rows generally suppressed weeds better than in the more traditional 0.75 and 1.0m row spacing. Increasing sorghum plant density from more traditional 5.0 plants sq/m to 10 plants sq/m also generally contributed to a more competitive crop against weeds.
Different varieties (of those tested) had no impact on suppressing weed growth, suggesting cultivar choice will have a lesser impact on sorghum competitiveness than agronomy. However, the researchers note that impact of cultivar may differ across seasons and locations. Also especially noteworthy, was that at least in favourably growing conditions sorghum at narrow row spacing and increased density, did not have any negative impact on sorghum yield.
Therefore, gains in competitiveness and reduction in weed growth can be achieved without reducing yield. Again the researchers note that rarely will a sorghum crop be grown without herbicides, whether they be residual or knockdown, or a combination of both. Integrating a competitive sorghum crop with herbicides should provide an additive effect on reducing in-crop weed pressures, growth and seed production. Over time, this strategy should deplete the weed seed banks, and reduce their impact on sorghum production.
A further valuable part of the publication is discussion of the role of Imidazolinone technology in sorghum, developed by Advanta Seeds. Sorghum has well and truly joined the list of crops with varieties that provide tolerance to Imidazolinone (IMI herbicides). Note this is not GMO technology. This technology allows the application of a new range of registered herbicides at recommended rates without causing crop damage.
Intervix (imazamox + imazapyr) is an example of an IMI herbicide. IMI products have broad spectrum activity with variation in the activity of individual herbicides for pre-emergence and post-emergence control. Control of broadleaf weeds post-emergence is normally limited to small weeds and relies to a measure on the effectiveness of crop competition occurring subsequent to application particularly for less susceptible species. While IMI herbicides like Intervix control a wide range of broadleaf and grass weeds it does, like most herbicides, have its limitations like not controlling fleabane or Feathertop Rhodes grass.
Excellent tables are presented in the publication that covers aspects like effect of various herbicides on specific weeds. These are detailed in tables for pre-emergent and post emergent. Tables also detail aspects like plant back intervals, application timing, rates per ha, rainfall requirement and the like.
Especially valuable is Andrew Summervaille's discussions about various herbicide products. He highlights advantages and disadvantages of the various herbicides. Planning for control of difficult weeds, like fleabane, Feathertop Rhodes grass, and even well known weeds like barnyard grass and liver-seed grass that have or are developing resistance to some herbicides, requires carful choice of herbicide and their application.
Further details obtain the booklet via www.pacificseeds.com.au/wp-content/uploads/2021/07/Pacific-Seeds-Grain-sorghum-weed-control-guide-_Low-Res.pdf
Next week: Ensuring legumes are a vital part of the pasture mix.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
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