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

Growth Performance and Plasma Metabolites of Grazing Beef Cattle Backgrounded on Buffel or Buffel-Desmanthus Mixed Pastures

Simple Summary

Pasture quality and digestibility decline during the dry season resulting in weight loss or marginal weight gains of grazing cattle in the seasonally dry subtropics of northern Australia. Oversowing grass with legume pastures has shown potential to improve pasture quality and cattle weight gain. This study aimed to evaluate the change in steers’ weight gain and plasma metabolites in response to grazing buffel grass pastures oversown with Desmanthus spp. (Desmanthus), a tropical legume adapted to cracking clay soils, compared to buffel-grass-only pastures. Results showed that Desmanthus at a low botanical composition had no effect on weight gain and plasma metabolites, although pasture yield and stocking rate were 443 kg/ha and 9.5% higher, respectively. Since the productivity of grazing systems depends on cattle annual weight gain and stocking rate, the practical implication of this study is that Desmanthus may improve the profitability of beef production in the dry tropics of northern Australia by improving pasture-carrying capacity with no adverse effect on cattle health status and growth performance.

Abstract

Dietary crude protein and dry matter digestibility are among the major factors limiting feed intake and weight gain of cattle grazing native and improved pastures in the subtropics of Northern Australia during the dry season. Incorporating a suitable legume into grasses improves pasture quality and cattle weight gain, but only a limited number of legume pastures can establish and persist in cracking clay soils. This study aimed to evaluate the effect of Desmanthus inclusion in buffel grass (Cenchrus ciliaris) pastures on the plasma metabolite profile and growth performance of grazing beef cattle during the dry season. We hypothesised that backgrounding steers on buffel grass-Desmanthus mixed pastures would elicit significant changes in plasma glucose, bilirubin, creatinine, non-esterified fatty acids and β-hydroxybutyrate, resulting in higher liveweight gains than in steers on buffel grass only pastures. Four hundred tropical composite steers were assigned to buffel grass only (n = 200) or buffel grass oversown with Desmanthus (11.5% initial sward dry matter) pastures (n = 200) and grazed for 147 days during the dry season. Desmanthus accounted for 6.2% sward dry matter at the end of grazing period. Plasma metabolites results showed that changes in β-hydroxybutyrate, creatinine, bilirubin, glucose and non-esterified fatty acids were within the expected normal range for all the steers, indicating that with or without Desmanthus inclusion in the diet of grazing steers, animal health status was not compromised. It was also evident that Desmanthus inclusion in buffel grass pastures had no impact on the plasma metabolite profile, liveweight and daily weight gain of grazing steers. Therefore, our tested hypothesis of higher changes in plasma metabolite profile and higher liveweight gains due to backgrounding on low-level buffel grass-Desmanthus mixed pastures does not hold.