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.

Diets selected and growth of steers grazing buffel grass (Cenchrus ciliaris cv. Gayndah)–Centro (Centrosema brasilianum cv. Oolloo) pastures in a seasonally dry tropical environment

Context Liveweight (LW) gain of grazing cattle in the seasonally dry tropics is usually moderate during the wet season (WS) and declines to slow growth or LW loss during the dry season (DS). Cattle growth can often be improved by inclusion of herbaceous legumes into pastures to improve their nutritional quality. Aims A study examined the quality of the diet selected and the growth of young cattle grazing a buffel grass (Cenchrus ciliaris)–Centro (Centrosema brasilianum) pasture in a high-rainfall, seasonally dry, tropical environment in northern Australia to relate the diet selected to cattle growth. Methods During three annual cycles, young steers grazed a grass–Centro legume pasture at moderate stocking rate. LW was measured monthly, and diet attributes (legume content, DM digestibility (DMD) and crude protein concentration) were measured fortnightly by near-infrared reflectance spectroscopy analyses of faeces. Pasture available and species were measured twice annually. Key results The annual LW gain and diet attributes followed a consistent profile through the annual cycles. Following the seasonal break, the diet DMD and crude protein concentration increased abruptly to maxima (means 732 and 184 g/kg respectively), and then declined approximately linearly during the remainder of the WS and the wet–dry transition season (TS); DMD decreased by 0.49, 0.74 and 0.88 g/kg units per day. DMD and crude protein averaged 561 and 61 g/kg respectively during the DS. Centro comprised 86–291 g/kg of the pasture on offer, and averaged 283 and 205 g/kg of the diet during the TS and DS, respectively, but only 58 g/kg during the WS. Cattle selected for Centro during the TS and the DS, but not during the WS. Cattle LW gain reflected diet quality averaging 0.86, 0.59 and 0.12 kg/day during the WS, TS and DS respectively. Conclusions The Centro legume contributed substantially to the diet of growing cattle during the TS and DS, but not during the WS. The LW gains of cattle were moderate during the WS and TS, and low during the DS. Implications Centro in a buffel grass pasture contributed substantially to the diet, but nevertheless annual LW gain was only modest (mean 179, range 159–209 kg/annum).

The efficacy of a cultured Synergistes jonesii inoculum to control hydroxypyridone toxicity in Bos indicus steers fed leucaena/grass diets

An experiment was conducted to investigate the efficacy of a cultured Synergistes jonesii inoculum in degrading the Leucaena leucocephala (leucaena) toxins: 3-hydroxy-4(1H)-pyridone and 3-hydroxy-2(1H)-pyridone (3,4- and 2,3-DHP). Sixteen stall-housed Bos indicus steers naïve to leucaena were fed varying combinations of forage-harvested leucaena and Chloris gayana (Rhodes grass). Dietary treatments, offered at 25 g dry matter/kg LW.day, were: 25% leucaena; 50% leucaena; 100% leucaena; and 50% leucaena, switched to 50% Medicago sativa (lucerne) after 6 weeks at time of inoculation. The experiment was 10 weeks in duration, consisting of a 6-week pre-inoculation period, followed by inoculation with cultured S. jonesii, and a 4-week post-inoculation period. Mean daily dry matter intake was recorded. Twenty-four-hour urine collections and rumen fluid samples were obtained weekly for estimation of total urinary DHP, and detection of S. jonesii using nested polymerase chain reaction analysis including presence of single nucleotide polymorphisms (SNP), respectively. In the pre-inoculation period, total urinary DHP increased quickly to high levels, then gradually declined after Week 3 with 2,3-DHP the dominant isomer through to Week 6. Indigenous strains of S. jonesii were sporadically detected by PCR analysis, indicating S. jonesii was present before inoculation but at the lower limits of detection. After inoculation there was no change in the rate of total DHP degradation or the frequency of detection of S. jonesii, although there was increased rate of degradation of 2,3-DHP. SNP indicated the presence of different strains of S. jonesii in both indigenous and cultured S. jonesii. DMI was low, especially in the 100% treatment, due in part to the high stem content of the forage-harvested leucaena and probable DHP toxicosis. It was concluded that the cultured S. jonesii inoculum did not fully protect animals against leucaena toxicity.

The concordance between greenhouse gas emissions, livestock production and profitability of extensive beef farming systems

Here we examine the concordance among emissions, production and gross margins of extensive beef farming systems by modelling a range of scenarios for herd management, animal genotype and pasture nutritive quality. We based our simulations on a case-study farm in central Queensland, Australia, and studied the influence of interventions designed for emissions mitigation, increasing productivity, or increasing gross margin. Interventions included replacing urea supplementation with nitrate, finishing cattle on the perennial forage leucaena (L), herd structure optimisation (HO), higher female fecundity (HF), and a leucaena finishing enterprise that had net farm emissions equal to the baseline (leucaena equal emissions; LEE). The HO intervention reduced the ratio of breeding cows relative to steers and unmated heifers, and lowered the ratio of costs to net cattle sales. Gross margin of the baseline, nitrate, L, LEE, HO and HF scenarios were AU$146 000, AU$91 000, AU$153 000, AU$170 000, AU$204 000 and AU$216 000, respectively. Enterprises with early joining of maiden heifers as well as HO and HF further increased gross margin (AU$323 000), while systems incorporating all compatible interventions (HO, HF, early joining, LEE) had a gross margin of AU$315 000. We showed that interventions that increase liveweight turnoff while maintaining net farm emissions resulted in higher gross margins than did interventions that maintained liveweight production and reduced net emissions. A key insight of this work was that the relationship between emissions intensity (emissions per unit liveweight production) or liveweight turnoff with gross margin were negative and positive, respectively, but only when combinations of (compatible) interventions were included in the dataset. For example, herd optimisation by reducing the number of breeding cows and increasing the number of sale animals increased gross margin by 40%, but this intervention had little effect on liveweight turnoff and emissions intensity. However, when herd optimisation was combined with other interventions that increased production, gross margins increased and emissions intensity declined. This is a fortuitous outcome, since it implies that imposing more interventions with the potential to profitably enhance liveweight turnoff allows a greater reduction in emissions intensity, but only when each intervention works synergistically with those already in place.

A meta-analysis of nutrient intake, feed efficiency and performance in cattle grazing on tropical grasslands

It is essential to quantify the potential of tropical grasslands to allow significant feed efficiency for grazing livestock in controlled conditions such as at pasture. We conducted a quantitative analysis of published studies reporting the experimental results of average daily gains (ADG) and diet characteristics obtained specifically under grazing conditions (17 publications and 41 experiments), which have been less studied compared with controlled conditions in stalls. The database was analyzed to determine the average and range of values obtained for ADG (g/kg BW), dry matter digestibility, intake (DMI) and digestible DMI (DDMI, g/kg BW) and feed conversion efficiencies (FCE), as well as to predict the response of these parameters to the main strategies investigated in the literature - that is, mainly the stocking rate (SR) and the concentrate intake (CI). The ADG reached 1.2 kg BW per day and was directly linked to DDMI (ADG=-1.63+0.42 DDMI -0.0084 DDMI2, n=90, r.m.s.e=0.584, R 2=0.93). The DDMI, which was representative of the nutrient input, was driven mainly by DMI rather than dry matter digestibility, whereas these two parameters did not correlate (r=0.068, P=0.56). The average global FCE (0.11 g ADG/g DDMI) showed a greater association with the metabolic FCE (0.17 g ADG/g DMI) than the digestive FCE (0.62). The CI (g DM/kg BW) increased ADG (ADG=2376+CI 56.1, n=16, r.m.s.e.=441, R 2=0.95). The SR expressed as kg BW/ha decreased the individual ADG by 1.19 g/kg BW per additional ton of BW/ha, whereas the global ADG calculated per ha increased by 0.57 per additional ton BW/ha. When the SR was expressed as kg BW/ton DM and per ha rather than as kg BW/ha, the impact on the individual ADG decreased by 0.18 or 0.86 g per additional ton BW/ha, depending on the initial BW of the cattle. These results provide a better view of the potential performance and feeding of cattle in tropical grasslands. The results provide an improved quantification of the relationships between diet and performance, as well as the overall quantitative impact of SR and supplementation.

Prediction of diet quality for sheep from faecal characteristics: comparison of near-infrared spectroscopy and conventional chemistry predictive models

This study evaluated the ability of equations developed from the analysis of faecal material by conventional chemistry (F.CHEM), and by near-infrared spectroscopy (F.NIRS), to predict intake and digestibility of forages fed with or without supplements. In vivo datasets were obtained using 30 sheep and 25 diets to provide 124 diet–faecal pairs, with each sheep fed four or five of the diets. The diets were five forages fed alone or with urea, molasses, cottonseed meal or sorghum grain supplements. Ninety-nine diet–faecal pairs were selected at random, but ensuring that all diets were represented and both the F.CHEM and F.NIRS prediction equations were developed from this dataset. The remaining 25 diet–faecal pairs were used as a validation dataset. Regressions for F.CHEM were developed by stepwise regression, and F.NIRS prediction equations were developed by partial least-squares regression. Prediction equations based solely on faecal analyte concentrations (F.CHEMc) had poor predictive ability, and models incorporating faecal constituent excretion rates (F.CHEMe) were the best at predicting feed constituent intakes. These models had slightly lower standard errors of prediction (SEP) for organic matter (OM) intake and digestible OM intake compared with the F.NIRS models that did not include faecal excretion rates. However, F.NIRS models had lower SEP for protein intake and OM digestibility. Good agreement between the F.CHEMe and F.NIRS methods was evident (according to the 95% limits-of-agreement test), and both predicted the reference values precisely and with small bias. Equations derived from a dataset that included representatives of all diets used in the experiment gave much better prediction of diet characteristics than those developed from a dataset constructed entirely at random. Equations for F.NIRS developed in this way successfully predicted the characteristics of diets that included forages fed alone and with the type of supplements used in tropical Australia.