In mid-March a farmer contacted me that his stock were not doing well on a tropical grass pasture. Plenty of feed but poor quality.
A laboratory analysis indicated low protein (4 per cent) and low digestibility.
In that same period an analysis on the same species on a different property tested 20pc protein and 12.3 megajoules a kilogram of dry matter metabolizable energy, both high quality readings.
Key factors for big differences from the same pasture type on the same day are soil fertility and pasture management.
Legumes with their correct rhizobia bacteria are also important. In the example above of poor livestock performance, the owner had detailed next to no pasture fertiliser use. No superphosphate or other product had been used to correct soil phosphorus and sulphur deficiency. Legumes as part of the pasture were poor, low in population and had likely not contributed to nitrogen supply.
Legume component of a pasture, for example native grasses, temperate perennials, herbs like chicory or tropical grasses, can build soil nitrogen by around 20kg/ha per t/ha of legume dry-matter production. With a good early start for winter legumes in many areas this year, including on our own property, 4t/ha legume production is a real possibility even if from now on the season is only moderate. Soil nitrogen build up this year can be 80kg/ha or more, a great contributor to high feed quality.
Research undertaken by NSW DPI, on tropical grasses, has as an example shown that the difference in protein between high and low soil nitrogen can be around six to 10pc or more. For example 11pc verses 20pc for fresh regrowth. That's the difference between high weight gains and barely maintaining weight.
Soil testing from accredited Australian laboratories using methodologies assed by years of research to be the most accurate for Australian conditions, is important for devising fertiliser strategies. For example Colwell phosphorus and KCl40 sulphur tests are widely used across a range of soils and environments with good correlations for devising fertiliser types and rates. Note more difficulties occur from interpretation of soil tests than from test accuracy.
Our property is an example of the importance of soil testing. A lot of research has shown legumes will not thrive unless sulphur and phosphorus deficiency are corrected. Even relatively low fertiliser rates, like 100kg/ha single super, make a several fold increase in legume production and their ability to build soil nitrogen. In our case soil phosphorus and sulphur have built to higher levels from fertiliser use. Annual soil testing now indicates application every year is no longer needed. But when it is it is applied to avoid a drop in productivity and nitrogen building ability.
An example of how important soil fertility is to pastures is the vast areas of brigalow cleared and sown to buffel grass 50-60 years ago in mainly Queensland. Brigalow is a legume so soils started off high in nitrogen and feed production and quality was high.
However, without legumes in the pasture and without deficiencies like phosphorus and sulphur being addressed, feed production and quality gradually declined.
While fertiliser is expensive, for example costing us around $70/ha applied this year (compared to around $40/ha in previous years) it does not take much production to cover for that. For example 13kg/ha beef, at $5.50kg equates to this example.
The difference between even moderate to low and high soil fertility in our example is estimated to be well in excess of 100kg/ha beef production. Currently our conservative steer fattening grass based business is producing around 220kg/ha beef.
Legumes that work best for a given environment, combined with their appropriate rhizobia bacteria (essential for legume nitrogen fixation) is commonly an unappreciated pasture aspect. Past NSW DPI research, led by Dr Belinda Hackney and Janelle Jenkins, noted many paddocks sampled suffered from poor nodulation. Aspects to address this issue include sampling for detection, lime application for some species and in some soils, and reintroduction of appropriate rhizobia strains.
Next week: "Ideal grazing" often a compromise.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
