The feasibility of growing grain sorghum this summer is very much dependent on sub soil moisture levels up to the end of the planting window, around late December to mid-January (depends on environment, i.e. north or south, east or west).
NSW Department of Primary Industries (DPI) leader for northern dryland cropping systems, Loretta Serafin, who spoke at a recent Agquip seminar in Gunnedah, noted that for many paddocks, even those long fallowed from a winter crop last year, and with good fallow management, plant available soil water (PAW) is still quite low.
The Soilwater app is a good guide to checking likely plant available soil water levels with estimates based on nearest Bureau of Meterology recording centres, or you can insert your own rainfall data. For example I checked a Walgett site that indicated PAW in a deep cracking clay following a winter crop in 2016 was only 74 millimetres. Paddocks not cropped last year, but well fallowed since April 2016, however, have an estimated 272mm, a different cropping risk entirely.
Ms Serafin pointed out the risk of sowing on limited moisture can be checked via various apps (e.g. CliMate) that assess probability of receiving a given amount of rain in the growing season. For western areas, risk of not receiving sufficient rain for a feasible crop this season is high and even in better rainfall eastern cropping areas risk level is no better than moderate.
Research (involving 15 sites across five years in northern NSW) comparing sowing time (early or late to avoid highest probability of extreme heat), led by Loretta Serafin and colleagues, funded by Grains Research and Development Corporation and NSW DPI, shows overall similar yields given comparable starting sub soil moisture levels. However, this season for many, the focus will be on late timing should good rains fall before the close of the sorghum sowing window.
Ms Serafin noted one needed to also consider other aspects associated with late sowing such as risk of harvest delays because of cool moist autumn conditions, changed rotation sequence, possible disruption to weed control and work clash commitments (e.g. sowing the winter crop).
Agronomy issues associated with sowing late and/or on limited stored soil moisture also need consideration. Ms Serafin noted research had shown solid row configuration, one metre spacing, generally gave the greatest return in above average seasons despite interest in skip row (single or double skip) sowing in recent years. However, it was also a higher risk proposition. If the season turns out good, for example with solid plant yields of 5t/ha, single skip row yields can often be 1t/ha or less, and double skip even less yield.
However, if yields are low (e.g. 2t/ha or less), yield difference between row configurations is much smaller and typically shows better results from skip sowing. While skip configurations are safer and help avoid crop failures in dry years, they can be at the expense of missing a big yield advantage should the season turn out well.
Research has also shown that, perhaps surprisingly, established plant populations from 30,000 to 50,000 plants a hectare have yielded similarly when yields are between 2t/ha and 4t/ha. Only in very low yield situations (around 1t/ha) 15,000 to 30,000 plants/ha has yielded better. At very high yield levels, 70,000 plants/ha have proved best, but were not statistically better than 50,000 plants/ha.
Hybrids with at least a moderate level of tillering and mid- season maturity have commonly yielded best in the research. Tillering ability allows crops to build yield potential should seasonal conditions be favourable.
Next week: Pasture legumes fix enough nitrogen for next three crops.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
