After rainfall in March burning temperatures were reduced but long burning durations maintained a high potential for weed seed destruction
Much lower temperatures and shorter durations of windrow burning were recorded following rainfall in April resulting in reduced potential for weed seed destruction
This study aimed to determine the effect of large rainfall events (50 mm) in March and April on the burning temperatures of narrow windrows and subsequent potential for weed seed destruction.
Two trials were established in wheat stubble at Merredin in March 2007 by watering windrows in a simulated rainfall event. The first trial was established in March where simulated rain fall events of 50mm were applied on three occasions (Table 1) to three replicated 2m2 areas of windrow (i.e. 1m wide by 2m long) by spraying 100 L of water on these sections with a fire hose. These watered windrow sections along with three dry sections were burnt on the 27th of March. Temperature probes and a data logger were used to measure soil surface temperatures during burning of each windrow section. This experiment was repeated in April where windrow sections were treated with 50mm simulated rainfall events as described above. These sections were subsequently burnt on the 3rd May with burning temperatures recorded as described earlier. In each experiment soil and stubble samples were collected from each plot prior to burning for the determination of moisture levels. Collected samples were oven dried for two days at 70C before weighing.
Table 1. Simulated rainfall date and the corresponding number of days prior to burning rainfall treatments in experiment one and two.
Narrow windrow burning temperatures, and therefore, the potential for killing weed seeds were reduced following a significant rainfall event in both March. In March, the maximum temperatures recorded on the soil surface during windrow burning were generally lower for all treatments that received 50 mm of rainfall when compared with the no rain treatment (Figure 1A). Therefore, with lower maximum soil surface temperatures occurring beneath the windrows there is likely to be a reduced potential for weed seed destruction following a large rainfall event. However, the durations of high temperatures (> 300C) is still likely to have provided a high potential for weed seed destruction.
Large rainfall events in March did not result in large increases in low soil and stubble moisture levels and therefore, delaying burning treatments had no effect on burning temperatures. There were no real differences in recorded temperatures between windrows that received rain 29 days prior to burning or those that received rain 15 days prior to burning (Figure 2A). Therefore, although there was likely to be a slightly reduced potential for weed seed destruction delaying burning did not increase this potential.
Figure 1. Effect of time between 50mm rainfall and burning on the soil surface temperatures during burning of 23 t/ha wheat stubble in narrow windrows in (a) March and (b) April. April burning
In April the potential for weed seed destruction was reduced for all treatments with the timing between rain fall events and burning having a large impact on the potential for weed seed destruction. There were generally lower burning temperatures for all treatments and particularly following the last two rainfall treatments (Figure 1B). For these treatments low maximum burning temperatures (< 400 C) were recorded and the duration of these high temperatures was very short. Therefore, the potential for weed seed destruction for these treatments was very low.
Higher stubble and soil moisture levels in April were responsible for the lower burning temperatures recorded for all windrow burning treatments. As well as the simulated rainfall there was approximately 14 mm of rain fall received across the site during April resulting in much higher stubble and soil moisture levels at the time of burning (Figure 2B). In particular stubble moisture levels were markedly higher in April than March and were consequently the primary reason for the generally lower burning temperatures recorded.
Figure 2. Stubble and soil moisture levels recorded at burning following 50mm rainfall in (a) March and (b) April Conclusions The potential for weed seed destruction form windrow burning remains high for early season burning despite the occurrence of significant rainfall events. Although maximum burning temperatures are reduced by rain the long duration of burning is likely to maintain a high potential for weed seed destruction. However when burning is delayed until April then rain fall will reduce both maximum burning temperatures and durations resulting in much lower potentials for weed seed destruction.
The authors wish to thank the WAHRI staff for their contributions and support in the conduct of this research. This research was financially supported by the GRDC. This research comprises part of Natalie Maguire’s 4th year honours project for which she received a Weeds CRC scholarship.