Thursday, June 16, 2011

Broadleaf Dock

A recent visit to a farm, (that rears dairy calves) in South Gippsland has prompted me to talk about dock. Several paddocks on the farm were thick with broadleaf dock (Rumex obtusifolius), whilst out on a paddock the farmer asked "why is it growing with such vigour?" and "How can I get it under control without relying on herbicide?"

 

To address the first question one needs to try and understand what that plant is telling us. Docks are a perennial herb with a thick taproot. Plants with taproots are typically found in areas of poor drainage- the plant is trying to correct this problem by penetrating deep into the subsoil to improve drainage and aeration. Why might drainage be poor you ask- In pastures, calcium is a mineral removed in great quantities with the least return in the manure. Calcium is the element which, among other things, causes soil particles to move apart for good aeration and drainage. Soils deficient in calcium are in effect prone to water logging. Therefore even before doing a soil assessment or test we can assume that calcium may be deficient and that drainage is poor. High stocking rates may be another factor for compaction and subsequent poor drainage however the farmer indicated that stocking rates were fairly low and that they haven't even been stocked recently given the amount of water on the place.

Delving deeper, (quite literally!) a soil core sample was taken so that a quick visual assessment of the soils biotic and physical qualities could be made. It was obvious from the sample taken that the soil lacked diversity of microfauna with only earth worms present. A cockchafer grub was also found in the sample. Cockchafers are common pasture pests in Gippsland, affecting shallow rooted pasture species, having minimal impact on deep-rooted plants such as lucerne, cocksfoot, phalaris and dock for that matter. The extent and severity of damage caused by cockchafers varies markedly from year to year and from one property to the next. The main indicators of their presence can be found during a dry spell after the autumn break, when dead pasture is found among areas of green. Dead pasture, unless re-sowed, provides a perfect home for a new dock plant or another broadleaf plant such as capeweed. Perhaps this is another factor contributing to the growth of dock throughout the paddock. Looking at the grass in the sample, (namely the roots) it's clear that they are confined to the top few cm's of soil- yet another indicator that oxygen is poor lower down.

Following the visual assessment a *soil test was taken for chemical analysis. Results of the test found that, as suspected, the available calcium was slightly low. It also revealed that the available potash exceeded the available phosphate- this is characteristic of soil favoured by broadleaf weeds. Available nitrogen was low, (6.1ppm, desired level=26ppm). Nitrogen is the one nutrient that can be hard to manage as it's readily lost through leaching. Soil bacteria play a critical role in converting the nitrogen into amino acids- in this form it is not likely to leach.  The soil test didn't test for the present of soil bacteria yet if it did there would be affair chance they would be in insufficient numbers.

With all the above in mind – finding an answer to the second question can seem overwhelming! I'm not an agronomist- and I don't intent to give specific advice as such- rather I've put together a list of useful resources below that can help you build a picture of ways to correct your specific weed problems.

Remember- you can spray pasture weeds with herbicide until the cows come home yet correcting the health of the soil should be the aim of the game.

McCaman J.L. 1994. Weeds and Why They Grow. Acres Australia www.acresaustralia.com.au

 Department of Primary Industries, Victoria information on soil relevant to agriculture; http://new.dpi.vic.gov.au/agriculture/farming-management/soil-water

Department of Primary Industries, Victoria information on the Red headed cockchafer

http://new.dpi.vic.gov.au/agriculture/pests-diseases-and-weeds/pest-insects/ag1358-the-redheaded-pasture-cockchafer

Primary Industries, NSW information on fertilisers and soil improvement http://www.dpi.nsw.gov.au/agriculture/resources/soils/improvement

*It should be said that a plant tissue test is the preferred method for diagnosing trace element status of a pasture rather than a soil test but for the purposes of this exercise a soil test was taken.