Pastures in a dry time

By Bill Manning, DPI Agronomist Gunnedah

The continuing dry conditions bring in to question the traditional approaches to pasture improvement.

In a mixed pasture situation pasture legumes either annual or perennial fix nitrogen which then becomes available for the grass component of pastures. Annual legumes such as clovers and medics for best production and persistence require autumn rains for germination and establishment and winter spring rain for production and seed set. Superphosphate has been used successfully to correct Phosphorus (P) and Sulphur (S) deficiencies, increase legume growth and nitrogen fixation. Extensive trials in the Coonabarabran, Tamworth and Narrabri areas in the 1990s showed massive responses to (P) and (S) application in mixed native grass/annual legume pastures. In autumn 2008 with assistance from Landmark we established 2 sites near Kelvin on mixed native grass/ annual legume pastures to test these responses in the Gunnedah area.

One site was deficient in (S) and the other in (P) and (S). The low Sulphur site contained Plains Grass, Queensland Bluegrass and burr medic. The other site contained Red grass and burr medic. Treatments on the low (S) site included 130 and 260kg/ha SF 45 and the site lacking in (P) and (S) 130kg/ha and 260kg/ha of Single Super. In both 2008 and 2009 low autumn winter rainfall resulted in a poor response by legumes to (P) and (S) fertilizer.

The above results show that during the drought production from winter legumes has been poor and as a result less nitrogen is fixed for the grass component of the pasture. This can result in summer grass pastures that are extremely N deficient. This is a particular problem for tropical grass pastures. Because their potential growth rates are so high their nitrogen requirement is also high and they respond to additional nitrogen more than native grasses do. For this reason a number of producers are now topdressing tropical pastures with fertilizer Nitrogen to improve production and groundcover.

A trial supported by the LPLM west of Boggabri in 2007-8 and again in 2008-9 on George and Maree Avendano’s property “Towri” looked at the potential for fertilizer nitrogen to improve tropical grass growth. In both 2007 and 2008 winter rainfall was low at the experimental site and at the beginning of summer in 2007 there was virtually no available nitrogen in the soil profile despite applications of Sulphate of Ammonia in previous years. The 12 year old pasture consisted of Premier Digit, Bambatsi Panic and a small portion of Consol Lovegrass. At the commencement of the experiment groundcover was close to 100% but pasture mass was low. The growth of winter legumes had been relatively poor due to dry seasons despite the presence of annual legumes and a history of superphosphate use.

Crystalline Sulphate of Ammonia was applied prior to rainfall in October each year at two rates 200 and 400 kg/ha with a nil fertilizer strip for comparison. In the December 2007 pasture sampling showed pasture production of 2800kg/ha on the nil fertilizer control strips and 5200kg/ha on the 200 kg/ha fertilizer strips. As well as this increase in pasture mass protein increased from 6.4 to 11.1%. The 400kg/ha treatment gave a further increase in production but to a lesser extent. Results in the summer of 2008-09 were similar.

What does this all mean? Annual legumes pasture species are no doubt a cost effective way to improve soil nitrogen and boost growth of summer grasses. In dry years however fertilizer Nitrogen may be a cost effective way to boost production from tropical grasses.

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