Abstract

The cost of irrigation or simply a water limitation are major constraints to the production of forage and grain crops. This study examined species differences in biomass production under both irrigated and water limiting conditions over several seasons. Specifically, cultivars of soybean, lablab, mungbean, cowpea, sorghum and rice were examined. Differences in biomass production were examined in relation to water extraction patterns and water use. Sorghum produced significantly higher biomass than legume species in each experiment and within the legume species soybean and cowpea had higher biomass than mungbean and lablab. Sorghum had a higher water use efficiency and slightly lower water use than legumes.

To maintain milk production, high quality forage is required to supplement the relatively poor quality pasture feed produced during summer and autumn. However, water limitation is a major constraint to biomass production of forage. Genotypic differences in water use efficiency (WUE) have been identified (eg.1,2) and therefore species and cultivar differences in WUE can contribute to improving the efficiency of milk production.

Materials and methods

The biomass production of 6 species (Glycine max cultivarvs (cvs.) Nitrobean 60, Manta, PKN 101-1, Manark and Warrigal, Vigna unguiculata cv. Meringa , Lablab purpureus cvs. Highworth, Koala, Rongai, 76996, 103821, 24-1 and 29398, Vigna radiata cv. Berken , Sorghum bicolor cv. MR31 and Oryza sativa cvs. Lemont and Rikuto Norin 12) was examined in four experiments in Southeast Queensland. Rice, a drought susceptible species, was introduced as a contrast to other crops. Three experiments (Exp. 1, 3 and 4) were conducted at Redland Bay (27^o37?S 153^o 19?E) Research Station (Exp.1 1995/96 Irrigated trial (50mm/week); Exp. 3 1996/97 Irrigated trial (50mm/week) and Exp.4 1996/97 Water stress trial (50 DAS rainout shelter was used to impose terminal water stress), while Expt 2 was conducted at Mutdapilly (27^o49?S 152^o 36?E) Research Station, (1995/96 Zero till, dryland trial (470mm rainfall). In each experiment, 10 cultivars were used, therefore the species were each represented by 1 to 3 cultivars. At the Mutdapilly site, only the legume species were examined. Biomass was sampled at 60, 67 and 76 days after sowing (DAS) for Exp.1, Exp.2 and Exps.3 and 4, respectively, from areas of 0.3 m²-1.8 m² depending on the trial, and expressed at 0% moisture. Water extraction patterns and water use were calculated from neutron moisture meter readings conducted between 50-76 DAS in Exp.4. In Exp. 4, biomass was also harvested 50 DAS and this data was used to calculate WUE (ie. Dry weight increase/water use). Statistical results are based on the 10 cultivars used in each experiment.

Results and discussion

Sorghum produced the highest biomass in each experiment (Table 1). Of the legume species soybean produced the highest biomass in all trials, however this did not differ significantly from that of cowpea. Cultivar differences existed within species, particularly in Exp. 2 under zero till conditions, but this was not related to plant density (r²=0.06). In Exp.2 Rongai (lablab) produced 270 g/m² more biomas than cv. 29398, and Nitrobean 60 (Soybean) produced 200 g/m² more than PKN 101-1. Biomass production was associated with leaf area index (r²=0.46**).

As expected, rice used the least amount of water below 70 cm, whereas cowpea appeared to extract water to a depth of 150 cm (Fig. 1). The water extraction pattern was similar among legume species, but soybean extracted consistently more water, especially between 30 and 90 cm depth which resulted in higher water use. Within species, water use differences of 20 mm existed. Soybean cv.Nitrobean 60 produced a similar biomass to Manta however used 20mm less water. While rice cv. Rikuto Norin 12 used 28mm less water and produced less biomass than Lemont. Sorghum had the highest WUE and among legume species soybean had the highest.

Table 1: Total dry matter (g/m²) of 6 species in 4 experiments and the mean across experiments, and water use (WU, mm) and water use efficiency (WUE, g/kg) from Exp.4. Range of values indicates 2 or 3 cultivars were used for the species. *Mungbean matured by the harvest date and WUE could not be estimated.

Figure 1: Water use of cowpea, rice, soybean and lablab from 50 to 76 days after sowing.

Conclusion

Sorghum (C4 species) has higher biomass production because of its higher WUE. Sorghum (maize) - soybean rotation should be examined further to improve efficiency of overall water use in the dairy industry.

Acknowledgement

The co-operation of QDPI Mutaplilly staff is acknowledged

Reference

1. White, D.S., Wright, G.C. and Bell, M.J. 1995. ACIAR Food Legumes Newsletter 23, 1-3.

2. Lawn, R.J. 1982. Aust. J. Agric. Res. 33, 511-521.

• "Agronomy, growing a greener future?". Edited by DL Michalk and JE Pratley. Proceedings of the 9th Australian Agronomy Conference, 20-23 July 1998, Charles Sturt University, Wagga Wagga, NSW.