Survey of the productivity, composition and estimated inputs of fixed nitrogen by pastures in central-western New South Wales
A. M. Bowman A B D , W. Smith A , M. B. Peoples C and J. Brockwell CA NSW Agriculture, Agricultural Research Centre, PMB No. 19, Trangie, NSW 2823, Australia.
B Present address: NSW Department of Primary Industries, Agricultural Institute, PMB, Pine Gully Road, Wagga Wagga, NSW 2650, Australia.
C CSIRO Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia.
D Corresponding author. Email: alison.bowman@agric.nsw.gov.au
Australian Journal of Experimental Agriculture 44(12) 1165-1175 https://doi.org/10.1071/EA03118
Submitted: 12 June 2003 Accepted: 7 May 2004 Published: 24 January 2005
Abstract
Total productivity and legume nitrogen fixation (N2 fixation) in dryland pastures were examined in a 2 year study (1999–2001) on 118 farms in central-western New South Wales. Pasture exclosure cages, placed at 217 on-farm sites, were harvested on 7 occasions and the foliage hand-sorted according to species in order to measure shoot dry matter (DM). The separated legume shoot material collected in spring 1999 (52 different legume samples) and 2000 (76 different legume samples) from a subset of representative pastures (41 cages on 28 different farms in 1999, 32 cages on 25 different farms in 2000) was also analysed for concentration of nitrogen (%N) and 15N natural abundance. These data were subsequently used to calculate the proportion of the legume shoot N derived from atmospheric N (%Ndfa), comparative measures of the relative efficiency of N2 fixation (kg N fixed/t DM accumulated) and the amounts of shoot N fixed (kg N/ha). The survey encompassed 8 common pasture types, and 5 others that were less common, ranging from native perennial grass pastures with little legume content to lucerne (Medicago sativa L.) pastures with and without companion clovers. Fifteen legume species were found in the pastures, some only occasionally. Lucerne and white clover (Trifolium repens L.) were the only perennials.
Mean spring estimates of %Ndfa were similar in 1999 and 2000 for lucerne (72 and 81%, respectively), rose clover (T. hirtum All., 82 and 77%) and annual medics (Medicago spp., 89 and 86%). For the remaining 12 legume species, measures of %Ndfa ranged from 64 to 95% and averaged 83%. Shoot %N contents were greater for lucerne than for the other 14 legumes and this was reflected in the comparative measures of N2 fixation which ranged from 14.5 kg N/t DM for rose clover to 25.7 kg N/t DM for lucerne in 2000. The most productive pasture type comprised lucerne plus balansa clover [T. michelianum Savi var. balansae (Boiss.) Azn.], white clover or arrowleaf clover (T. vesiculosum Savi), but all pasture types that contained lucerne were highly productive. Spring was the most productive season and summer the least. Lucerne was overwhelmingly the most productive legume and was responsible for >83% of the fixed N in those pastures that contained both lucerne and other legumes. Lucerne productivity was approximately uniform throughout the year whereas, for other pastures, especially those based on rose clover or subterranean clover (T. subterraneum L.), there were sharp peaks in spring and little or no dry matter production over summer. The presence of lucerne in pastures significantly (P<0.05) reduced broadleaf weeds. It was concluded that, where there are requirements in central-western New South Wales agriculture for uniform forage production throughout the year and a high input of fixed N, lucerne is substantially superior to other species.
Additional keywords: alfalfa, dryland pastures, lucerne, Medicago sativa, nitrogen fixation, Trifolium spp.
Acknowledgments
We are grateful to the many farmers in central-western New South Wales who gave us access to their properties and collaborated in the maintenance of the experimental sites. We acknowledge Frank Syrch for harvesting. We thank Ian Toole, Jayne Jenkins, Rob Pither and David MacMillan for long hours spent hand-sorting the foliage, and Gayle Williams for mass spectrometer analyses. The project was partly funded by a PIRD grant from AWRAP to Central West Farming Systems.
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