Persistence, productivity, nutrient composition, and aphid tolerance of Cullen spp.

The persistence, productivity, and nutrient content of accessions of Cullen australasicum, C. cinereum, C. tenax, and C. parvum were compared with lucerne (Medicago sativa) in grazed swards for 3 years on an acid soil in southern New South Wales. All Cullen spp. established satisfactorily from seed sown into a cultivated seed bed, but C. australasicum accessions were more persistent than C. cinereum, C. parvum, and C. tenax, which declined to a frequency of <5% by the start of the fourth year under rotational grazing. Cullen australasicum was the most productive of the 4 Cullen spp. with herbage yields similar to the lucerne cv. Sceptre. Leaves and stems of the Cullen spp. had significantly lower S, K, B, and Na contents than lucerne. The herbage P content of all Cullen spp., except C. tenax, was lower than in lucerne. Calcium and Mn levels were higher in C. australasicum than in all other species. Sheep often avoided grazing all Cullen spp. when other herbage was available, suggesting that these species had lower palatability. The dry matter digestibility (DMD) of lucerne herbage (76%) in September was higher than in C. australasicum (74.3%), C. cinereum (71.3%), and C. tenax (69.7%) (P < 0.001). The metabolisable energy content of the herbage of C. australasicum was similar to lucerne (10.7-10.9 MJ/kg DM) but was lower in C. cinereum and C. tenax (9.9-10.3 MJ/kg DM). The crude protein content of the 4 Cullen spp. was similar (21.4-22.3%) but significantly lower than for lucerne (33.6%). Glasshouse studies found C. australasicum, C. cinereum, C. pallidum, C. parvum, and C. tenax to be moderately to highly susceptible to bluegreen aphids (BGA) (Acyrthosiphon kondoi Shinji), but one C. australasicum accession was highly tolerant, suggesting that aphid susceptibility can be overcome by selection. All 5 Cullen species proved highly resistant to spotted alfalfa aphids (Therioaphis trifolii Monell). Cullen australasicum was found to be susceptible to Alfalfa mosaic virus, which resulted in stunting of growth of some plants in the field experiment. Of the 4 Cullen spp. examined in the field experiment, C. australasicum demonstrated the most potential as a forage plant for low-rainfall regions with superior persistence and productivity under grazing and the highest resistance to BGA.