MUCH is written and spoken about reducing grass seed contamination in sheep, especially lambs, but rarely have I seen tropical grasses raised as one of the best options to avoid the problem.

Costs of seed penetration include reduced weight gains and carcase damage

Logic suggests tropical grasses that are very competitive against weeds like wire grass (Arisida species) and spear grasses (Stipa species), weeds that supply damaging seeds from spring through to autumn. Even against winter weeds like barley grass, tropical grasses can be quite competitive. Options include pasture topping, crash grazing.

Part of the reason tropical grasses are not detailed as an important control aspect of weed seed contaminant is because of lack of appreciation about how hardy they are and can thrive in most parts of NSW. A research led by NSW DPI, with MLA funding, indicates tropical grasses are doing extremely well in environments at Cowra, Orange, Condobolin, Trangie, northern slopes, plains and even tablelands.

Two neighbours, the Sanson's (Dave, Lyn and Brad) and Sue McGoldrick are respectively sheep and cattle and sheep producers. Both have substantial areas of tropical grasses for some 25 years stress that tropical grasses provide them with pastures that if well managed are free of summer grass seed problems from species like wire grass, and tropical grass seeds provide no contamination problems.

Lucerne is also a good pasture option, especially in early years of a stand's life, provided its density is good. It provides options to spray out grasses like barley grass. Carefully chosen tropical grasses unlike lucerne, will thrive on many soils not that suited to lucerne, especially acid soils. Plus, reasonably well managed tropicals will gradually thicken up if initial density is not perfect.

For tropical grasses to be effective at outgrowing plants like wire grass, they need to be established after these weeds have been removed from a previous native or other pasture. Normally a crop cycle of three years prior to their sowing will suffice and weeds are not allowed to seed in the fallow, as well as good in-crop control of weeds like barley grass occurs. This helps ensure a new tropical grass sowing gets off to a good start. Once well-established tropical grasses are very good at keeping out most undesirable weeds.

A criticism levelled at tropical grasses is their lower quality than species like phalaris or winter legumes. Other than lucerne, quality can be higher than native grasses and other herbage growing from mid-spring, summer, through to the end of autumn. Livestock, including sheep, commonly perform well on them.

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Feed quality of all grasses is closely related to soil fertility. One aspect, nitrogen, is provided by legumes growing in conjunction with tropicals. In our area this past year, while October was a good rainfall month, July to September was below average. Yet winter legumes of sub clover, on less acid country, and serradella and biserrula on more acid soils, have grown feed of over 4t/ha drymatter in tropical grass paddocks. Not only does that supply a lot of high quality feed, but also a lot of soil nitrogen.

That amount of legume growth, provided well nodulated, can build soil nitrogen by over 100kg/ha. Extra nitrogen helps ensure high quality tropical grass. In seasons where rainfall is well below average during the legume phase, like 2017, 2018 and 2019, legumes, either in tropical, native, or temperate perennials, all commonly grew poorly and contributed little nitrogen. When legumes have not grown well the alternative option is to add nitrogen via fertiliser ahead of spring or summer rains. We have profitably done that in those years and has assured high quality with extra feed.

Tropical grasses grow quickly in hotter parts of summer and can head within a few weeks post rain. While they may look poorer in quality, in a tall rank tropical grass pasture there will generally be a lot of later tillers highly palatable and high in quality. Finally, choose varieties carefully. Premier digit for example, our choice in a medium central west environment and on lighter soils, is proving to be high in quality, long lasting and compatible with winter legumes. Stock growth can be outstanding with few weed issues.

Next week: Another way to control saffron thistle.