Benefits of Retaining Stubble – Western Australia

Key Points

In the past stubble has been burnt to control weeds but retaining stubble has several advantages for soil fertility and productivity.

Retaining stubble can decrease soil erosion and increase soil water content.

Retaining stubble can increase labile carbon, which increases the biological fertility of soil.

If controlling weeds is the aim, burning windrows is more effective than burning standing stubble and also decreases erosion risk.

Background

Historically, stubble has been burnt because it improves weed control and creates easier passage for seeding equipment. However, the practice of burning stubble has recently declined due to concerns about soil erosion and loss of soil organic matter. Instead of being burnt, stubble is increasingly being retained which has several advantages for soil fertility and productivity (figure 1).

Figure 1: Inspecting stubble after sowing with a disc opener.

Reducing erosion risk

One of the main benefits of stubble retention is reduced soil erosion. Retaining stubble decreases erosion by lowering wind speed at the soil surface and decreasing run-off. To minimise erosion approximately 50 % ground cover is required and adequate stubble needs to be maintained for 6–8 weeks following seeding (Leonard, 1993). It is generally considered that 50 % ground cover is achieved by 1 t/ha of cereal stubble, 2 t/ha of lupin stubble and 3 t/ha of canola stubble.

Increase in soil water content

Another advantage of retaining stubble is that it increases soil water content by decreasing run-off and increasing infiltration (figure 2). However, the potential benefits of decreased run-off and increased infiltration will depend on the timing and intensity of rainfall as well as the quantity and orientation of stubble. Late summer-early autumn rains have more chance of improving the germination and establishment of the next crop. In addition, increased infiltration of water over summer can result in greater nitrogen mineralisation and availability for the subsequent crop.

Figure 2: The influence of different amounts of ground cover from retaining wheat stubble on water infiltration (orange) and time to runoff (blue) (Thomas et al., 2008).

Increase in biological fertility of soil

Retaining stubble increases the input of carbon to soil. Stubble is approximately 45 % carbon by weight and therefore represents a significant input of carbon to soil. It can take decades for retaining stubble to increase the amount of total organic carbon in soil. A field experiment in Merredin, Western Australia found that retaining stubble for 17 years only increased the total organic carbon content of the soil by 31 % (2.1 tonnes carbon per hectare, table 1).

However, retaining stubble can have a larger effect on the carbon in the microbial biomass in soil. When stubble is retained, the greater inputs of organic carbon to soil increase its biological fertility. Microorganisms in soil require organic carbon to obtain the C, nutrients and energy they need to live.

Labile carbon is a particularly important form of organic carbon for soil microorganisms. Management practices that increase inputs of organic carbon to soil, such as retaining stubble, can increase the number of microorganisms in soil and also cause them to be more active (table 1).

The Merredin field experiment found that 17 years of retaining stubble increased the carbon in the microbial biomass by 42 % (table 1).

Reasons to consider burning stubble

The main reason for burning stubble is to control weeds. Burning windrows in paddocks can be a useful way to control weeds, especially populations of weeds that are herbicide resistant. Burning chaff in windrows is more effective for controlling weeds than burning whole paddocks. This is because the weed seeds are exposed to higher temperatures for a longer period (Walsh and Newman, 2007). For example, 20 % of ryegrass seeds emerged when standing stubble was burnt but only 1 % emerged when windrows were burnt (Walsh and Newman, 2007). However, without a high temperature fire, viable weed seeds will be concentrated in strips.

Also, when chaff is collected in windrows, less of the field is burnt. This means more stubble is retained and the erosion risk is decreased. When chaff is collected in narrow windrows of 50–70 cm, only 10 % of the field is burnt.

Burning whole paddocks can decrease stubble borne diseases in several cereal crops including wheat (yellow spot – Pyrenophora tritici-repentis, septoria nodorum blotch – Phaeosphaeria nodorum, septoria tritici blotch – Mycosphaerella graminicola) and barley (net blotch – net-type Pyrenophora teres f.sp. teres, net blotch—spot-type Pyrenophora teres f.sp. maculata).

However, stubble-borne diseases can be managed without burning by use of resistant cultivars and crop rotation. In addition, burning whole paddocks has little effect on soil borne diseases.

Table 1: Estimated reduction in wheat or barley stubble cover from different tillage operations (reproduced from Measuring stubble cover: Photo standards for winter cereals).

SOIL PROPERTY
STUBBLE RETAINED
STUBBLE BURNT

Total organic carbon (t/ha)
8.7
6.6

Water content at field capacity (10 kPa, %)
23.2
19.7

Microbial biomass carbon (0-5 cm, kg/ha
203
143

Microbial respiration (kg carbon/ha/day
30.2
13.7

Fact Sheets Benefits of Retaining Stubble - WA