Diet quality, liveweight change and responses to N supplements by cattle grazing

Experiments during 4 years examined the diets selected, growth, and responses to N supplements by Bos indicus-cross steers grazing summer-rainfall semi-arid C4 Astrebla spp. (Mitchell grass) rangelands at a site in north-western Queensland, Australia. Paddock groups of were not supplemented (T-NIL), or fed non-protein (T-NPN) cottonseed meal (T-CSM) supplement. In Experiment 1, young older measured late dry season (LDS) rainy (RS), while 2–4 through annual cycle. Because severe drought measurements 3 cycle limited T-NIL steers. Pasture availability species composition twice annually. Diet was 1–2 week intervals using near infrared spectroscopy faeces (F.NIRS). Annual rainfalls (1 July–30 June) 42–68% long-term average (471 mm), seasonal break ranged from 17 December March. There wide variation pasture, diet (crude protein (CP), DM digestibility (DMD), CP metabolisable energy (CP/ME) ratio) steer liveweight change (LWC) within between cycles. High quality (LW) gain RS declined progressively transition (TS) early (EDS), often first part LDS. Steers commenced losing LW as LDS progressed. 1 2 where forbs comprised ≤15 g/kg pasture sward, selected strongly for so that they 117–236 diet. However, substantial proportions (173–397 g/kg), there comparable (300–396 g/kg). With appropriate stocking rates gains acceptable (121–220 kg) despite low rainfall. The had no effect on TS EDS, but usually reduced loss 20–30 kg Thus rainfall Mitchell grass pastures towards end when contained c. <58 g CP/kg <7.0 CP/MJ ME.