Past summer reveals effectiveness of strong pasture competition

Droughts, in all probability, weakened the competitiveness of perennial pastures, the ones most likely to best compete with weeds like fleabane. This was often the case with native as well as introduced perennial pastures like lucerne, temperate species like Phalaris and tropicals like Premier digit.

Our property has been no exception, with an onset of fleabane from 2020 onwards in some paddocks following the 2017-2019 drought. An upgraded herbicide program, especially during the summer fallow, has also been essential for fallow weed control.

This past summer has been a good example of the effectiveness of strong pasture competition that has largely outmuscled fleabane for soil moisture. In contrast to the two previous summers, 2022/23 summer rain for us, and for much of central, northern and western NSW, was way below average (for us, nearly 50 per cent below average).

However, strong tropical grass pastures, and to a degree, strong native pastures with summer growing perennials, outcompete fleabane for soil moisture, especially during the later part of summer, mid-January through to the end of February (and into mid-March). A large percentage of fleabane plants, that were strong coming into mid-summer, ran out of moisture, failed to seed and died.

This was especially the situation where relatively new tropical grass pastures were still building their competitiveness. Fleabane had been an issue in some of them in the previous two summers. Older tropical grass pastures have been able to largely keep fleabane, as well as most summer weeds, out right through the range of dry to wet summers. Most of our older tropical grass pastures have not required any herbicide weed control treatments over the last 14 years. This is mainly because of their competitiveness.

READ MORE: Bob Freebairn explains the advantages of planting an early winter grazing crop

Routine fallow spraying with glyphosate, plus sometimes 2,4-D or similar products, have progressively failed with control of fleabane in fallows. Follow-up treatment, seven days later, with paraquat, has, however, generally resulted in very good weed kills (double knock technology). While adding to costs, double knock, as shown by research, is a valuable tool to not only kill some difficult weeds, like fleabane but also to conserve more fallow soil moisture.

For any pasture to be competitive against weeds like fleabane, perennial species and variety choice are only part of the story. These pastures need to be at their optimum productivity ability, which correlates closely with their competitiveness. Soil fertility is one of these key aspects. If grasses are deficient in nutrients like phosphorus and nitrogen, their productivity (and quality) and competitiveness are badly compromised.

Legumes are a vital part of many pastures for ensuring the grass component is well supplied by nitrogen. For example, in a tropical or temperate perennial grass pasture, winter annual legumes are commonly the companion species to not only supply some of the winter feed but also to build soil nitrogen. In our case, on commonly acidic soils, sub clover, serradella, and biserrula are the main species.

Legumes are able to build soil nitrogen for around 20 kilograms per hectare for every tonne per hectare drymatter they produce. In a good season, they, therefore, can build oil nitrogen by about 80kg/ha.

Grazing management is also important for a pasture's ability to be strong and competitive against weeds like fleabane. For example, constant and heavy grazing commonly means perennial plants have reduced root reserves. A degree of rotational grazing helps give plants opportunities to rebuild root reserves and better cope when seasonal soil moisture variability. Ensuring reasonable groundcover levels is also important, regardless of the grazing system.

Next week: Holbrook Landcare part of assessing the role of tropical grasses