GroundCover™ Supplement Issue: 135 July-August 2018 | Author: Tony Swan, Dr John Kirkegaard, Phil Bowden
While grazing is just one of the many post-harvest strategies for stubble management, it can ultimately increase the profitability of the whole system for mixed farms.
Stubble provides a valuable feed resource that allows pasture paddocks to be spelled during summer, while grazing helps the cropping program by reducing the amount of stubble in preparation for sowing and increasing nutrient availability to crops by speeding up the breakdown process and reducing nitrogen tie-up.
Grazing of stubble should be carefully managed so that sufficient stubble quantities are maintained to protect the soil surface from wind and water erosion, and maximise water infiltration and water storage. It is widely accepted that a minimum of 70 per cent ground cover is required to minimise erosion and maximise water infiltration and stored soil water. Retain at least two to three tonnes per hectare of stubble to maximise the yield of the following crop (see Managing stubble in summer protects soils and profits).
However, what about the effect of livestock on soil compaction and the cropping system’s nutrient balance? These were investigated as part of a long-term CSIRO and FarmLink Research study at Temora, New South Wales, from 2009 to 2017, supported through the GRDC’s water use efficiency and stubble initiatives.
Overgrazing can reduce the amount of water that can infiltrate the soil surface, but this is due to removal of plant cover, not compaction. In the ungrazed treatments at Temora with no grazing or wheel traffic for nine years, the steady-state infiltration rate of water in March 2017 was 28 millimetres per hour, compared with an infiltration rate of 23mm/hour where canola or wheat stubble was grazed post-harvest every year.
Although the water infiltration rate was slightly lower in the grazed treatment, there are few rainfall events with an intensity greater than 23mm per hour over the summer fallow. This explains why careful grazing had little impact on stored soil water and subsequent yield.
Interestingly, when comparing the effect of establishing a crop with either a disc or tyne seeder, there was no difference in the steady-state infiltration rate across all treatments, whether stubble was grazed or not. In a no-till, controlled-traffic system the full crop potential can be reached when carefully grazing crop stubbles sown with either a disc or tyne seeder.
The nutritional value of stubble to livestock has been much studied, but what about the benefit to the cropping system’s nutrition?
Livestock will redistribute nutrients across the paddock through urine and manure, and increase stubble breakdown by trampling residues, improving contact between the straw and soil.
In addition, by removing some of the carbon-rich stubble, they also reduce the potential nitrogen tie-up that can reduce nitrogen availability to the young crop. Together these processes can improve the availability of nutrients to crops grown in grazed stubble.
This was demonstrated by the long-term study at Temora, where grazing stubbles in a canola/wheat/wheat sequence improved the yield of crops through improved nitrogen availability. For instance, grazing canola stubble increased soil mineral nitrogen for the first wheat crop by 13 kilograms nitrogen/ha, while grazing the stubble of the first wheat crop increased mineral nitrogen for the second wheat crop by 33kg N/ha. When stubble is broken down in a grazing animal’s gut there is no tie-up of nitrogen as there would be if stubble were left on the soil surface to break down (see Why do stubble-retained systems need more nitrogen).
GRDC Research Codes CSP00174, EPF00001, CWF00018
Tony Swan, CSIRO
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