Seasonal variations in the herbage mass, crude protein and in-vitro digestibility of native perennial grasses on the north-west slopes of New South Wales.

Abstract

Seasonal variations in herbage mass, crude protein content and in-vitro organic matter digestibility of both the warm-season (summer-growing) native perennial grasses, Aristida mrnosa R.Br., Bothriochloa macra (Steud) S.T. Blake, Dichanthium sericeum R.Br. (Camus), Sporobolus elongatus RBI., Chloris truncata R.Br. and Eragrostis leptostachya Steud, and the year-long-green perennial grasses Stipa variabilis Hughes and Danthonia linkii Kunth were investigated at three sites on the North-West Slopes of New South Wales. These preliminary data were used to assess the relative grazing value of the dominant grasses in the region, although these initial rankings can only be confirmed by diet selection and animal production studies. The eight grasses tested were classified into three groups with respect to their green leaf mass per unit basal area throughout the year: (i) Aristida ramosa, Bothriochloa macra, Dichanthium sericeum and Sporobolus elongatus had low amounts of green leaf in winter and their green leaf mass in summer was related to rainfall. (ii) Chloris truncata and Eragrostis leptostachya showed considerably less seasonal variation in the amount of green leaf and had a higher percentage of green leaf than (i). (iii) Stipa variabilis and Danthonia linkii had the lowest variation in green leaf, and in winter and spring they generally had higher green leaf masses than the other groups. Among the warm-season and year-long-green grasses the green leaf crude protein content varied between 5 and 15% throughout summer and autumn and from 6 to 17% in winter and spring. In all seasons dead leaf crude protein contents were around 5% and dead stem crude protein contents were consistently less than 6%. Irrespective of species the digestibilities of the green leaf portions were higher in spring and summer (60 to 80%) than in autumn and winter (50 to 60%). At each of the ~ites the digestibility ofDanthonia linkii green leaf material was generally higher than 60% in all seasons, but that ofStipa variabilis green leaf was always less than 60%. Estimates of green stem digestibility ranged from as low as 25% for Avistida ramosa in autumn to around 60% for most other species in spring. Digestibilities of dead leaf ranged from 30 to 55%, those of dead stem ranged from 20 to 40%. These results indicate that Danthonia linkii is potentially valuable for grazing and that Aristida ramosa is an undesirable pasture species. Of the other grasses tested Chloris truncata and Eragrostis leptostachya may produce some green forage in autumn and winter but at these times the green forage production of Aristida ramosa, Bothriochloa macra, Dichanthium sericeum and Sporobolus elon- gatus is limited by low temperatures.