Young Steers Research Articles

Meta-analysis of carcass and meat characteristics of young or super young steers and of different sexual conditions finished in confinement

The objective was to evaluate the carcass and meat characteristics of feedlot steers based on sexual condition and age at the beginning of the finishing phase, through a systematic review and meta-analysis. The selection of articles was carried out by two reviewers, who selected articles from the Google Scholar and Scielo platforms. The criteria for choosing articles followed the PICO tool. After searching for articles, they were subjected to analysis under the methodological conditions of the research. After this phase, 61 research articles carried out in Brazil remained, from which information was extracted from the material, methods and results. The data were compared according to the age of the steers: young (between 20 and 26 months of age) and super young (between 10 and 16 months of age); and depending on sexual condition: castrated or not castrated. Young steers have higher slaughter weights and higher hot and cold carcass weights. Meat from super young steers presented a higher score for tenderness when evaluated by the panel of evaluators. Uncastrated steers had higher slaughter weight, hot and cold carcass weight and rib eye area, while castrated steers demonstrated greater thickness of subcutaneous fat in the carcass and meat with higher marbling score. Slaughtering super young and castrated steers improves the quality aspects of the carcass and meat, however it reduces the amount of meat.

Skeletal muscle gene expression and meat quality of F1 Angus–Nellore young steers and bulls feedlot finished

This study investigated the effects of castration on skeletal muscle gene expression and beef quality. Six-hundred and forty F1 Angus–Nellore steers and bulls (initial body weight of 299 and 297 ± 4.1 kg, respectively) feedlot finished were used. Gene expression revealed that steers showed up-regulation (P<0.05) of acetyl-CoA carboxylase alpha, fatty acid-binding protein, fatty acid synthase, peroxisome proliferator activated receptor (PPAR)-gamma and stearoyl-coa desaturase, while bulls shown higher expression of PPAR-alpha and sterol regulatory element-binding protein 1. Consequently, steers had greater (P<0.01) intramuscular fat (IMF) in longissimus muscle compared to bulls (5.59 vs. 2.07%). Greater concentration of monounsaturated fatty acids was observed in beef loins of steers (P<0.01), while bulls trend to shown higher (P = 0.08) polyunsaturated fatty acids and higher elongase activity (P<0.05). Thus, differences in gene expression were associated with differences in IMF deposition (marbling). Even after 180 days on feed, crossbred young bulls does not produce meat with adequate IMF and better fatty acid profile than steers.

Proteomics Unveils Post-Mortem Changes in Beef Muscle Proteins and Provides Insight into Variations in Meat Quality Traits of Crossbred Young Steers and Heifers Raised in Feedlot.

Proteomics has been widely used to study muscle biology and meat quality traits from different species including beef. Beef proteomics studies allow a better understanding of the biological processes related to meat quality trait determination. This study aimed to decipher by means of two-dimensional electrophoresis (2D-PAGE), mass spectrometry and bioinformatics the changes in post-mortem muscle with a focus on proteins differentially expressed in the Longissimus thoracis (LT) muscle of immunocastrated young heifers and steers. Carcass traits, chemical composition, pH, instrumental color (L*, a*, b*), cooking loss and Warner-Bratzler shear force (WBSF) of meat from F1 Montana-Nellore cattle were also evaluated. Backfat thickness (BFT) and intramuscular fat content (IMF) were 46.8% and 63.6% higher in heifers (p < 0.05), respectively, while evaporation losses (EL) were 10.22% lower compared to steers. No differences (p > 0.05) were observed for tenderness evaluated by WBSF (3, 10, and 17 days post-mortem), pH, and color traits (L*, a* and b*) between the experimental groups. The study revealed several proteins to be differentially expressed proteins in heifers compared steers (p < 0.05). In heifers, proteins involved in nutrient transport (TF, ALB, and MB), energy metabolism (ALDOA, GAPDH, and PKM), and oxidative stress and response to stress (HSPA8 and CA3) were associated with a greater BFT and IMF deposition. The higher expression of these proteins indicated greater oxidative capacity and lower glycolytic activity in the LT muscle of heifers. In steers, there was greater abundance of protein expression related to muscle contraction and proteins of structure (ACTA1, TPM2 and TNNT3), energy metabolism (ENO1, ENO3, PYGM, PGM1 and TPI1) and ATP metabolism (ATP5F1B, PEBP1 and AK1), indicating greater glycogenolysis in LT muscle, suggesting a shift in the glycolytic/oxidative fibers of steers.

반추위 삽입 센서를 활용한 육성기 홀스타인 거세우의 생리 상태 변화 평가

In the last decade, the wireless biosensor technologies for cattle have been widely spread over the world. The objective of this study was to evaluate the changes of physiological state in Holstein growing steers using the rumen bolus sensor. A total of thirty-two Holstein young steers (219.48 ± 16.61 kg, 7.25 ± 0.40 months of age) were used and the rumen bolus was orally administered to each steer before conducting this experiment. The rumen bolus used in this study (Smart alyac®, Hankook Iot Corp, Gimcheon, Korea) can measure not only body temperature (BT) but also moving behavior which was expressed as indices (moving index, MI). All steers equipped with the bolus sensor were housed in pens (4 animals per pen) and the sensor data were collected for 29 days. During experimental period, the average MI for the day (day: 0700–1900, night: 2000–0600) was significantly higher than that for the night (day: 111.10, night: 98.53, p &lt; 0.0001), but the result of average BT was opposite (day: 39.04°C, night: 39.62°C, p &lt; 0.0001). The average MI in animals having low average daily gain (ADG) was higher than those having high ADG (low ADG: 105.46, high ADG: 103.66, p &lt; 0.0001) and average BT was the same (low ADG: 39.41°C, high ADG: 39.33°C, p &lt; 0.0001). When the steers exposed to several stresses, the average MI was significantly lower than that in normal condition (normal: 117.40, stress: 109.88, p &lt; 0.01). Stressed steers also showed increased BT compared to steers in normal condition (normal: 38.78°C, stress: 39.50°C, p &lt; 0.0001). In conclusion, rumen bolus sensors, which generally used in cows to detect estrus, can be applied even to steers to detect the physiological changes rapidly in stressful environment.

Modelling Shows the Negative Impact of Age Dependent Pharmacokinetics on the Efficacy of Oxytetracycline in Young Steers.

The effect of age dependent pharmacokinetics (PK) on the clinical efficacy of oxytetracycline (OTC) against Bovine Respiratory Disease (BRD) in beef cattle was studied, using a Physiologically Based Pharmacokinetic (PBPK) model. The model includes a bodyweight dependent renal clearance. To mimic/reproduce the long terminal half-live a bone forming tissue compartment was considered. Data for the development, calibration and validation of the model were obtained from public literature. To integrate the PK with the pharmacodynamics (PD) of OTC, Monte Carlo simulations were performed using this PBPK model to predict time-concentration curves for two commonly used dosing regimens of short-acting and long-acting injectable OTC formulations in virtual populations of 5,000 steer calves of 100 kg and 400 kg. These curves were then used to calculate the value of the PKPD index for OTC, which is the ratio of the area under the concentration-time curve for 24 h (AUC24h) over the minimum inhibitory concentration (MIC) of the target pathogen (AUC24h/MIC). The MIC values were for Mannheimia haemolytica, the dose-limiting pathogen for BRD. This integration of PBPK and PD for OTC used for the treatment of BRD in calves indicated that the Probability of Target Attainment (PTA) was sufficient for efficacy in calves of 400 kg, but insufficient for calves of 100 kg, when using a long acting dosing regimen of 20 mg/kg BW, twice, with a 48-h interval. The use of a dosing regimen of 10 mg/kg BW/day for 4 days predicted sufficient PTAs in both age groups.

Bioequivalence of a Preparation of Tulathromycin in Cattle

Testing the quality of antibacterial pharmaceutical preparations that can be regarded as generic is of utmost importance to ensure sustainable medical practices. Hence, comparative pharmacokinetics studies become necessary. In this trial, a new pharmaceutical preparation of tulathromycin (TTM) was submitted to a bioequivalence test, taking the reference preparation of this macrolide derivative as control i.e., the commercial preparation of TTM containing 100 mg of the drug Pisadrax ® , manufactured by PiSA Agropacuaria S.A. de C.V. and the reference brand of TTM Draxxin ® (Zoetis, Mexico). Twenty-four young steers Charolais/Brahman randomly divided into two groups of twelve animals each, received a single injection of 2.5 mg/kg subcutaneously of either preparation. A validated HPLC-masses analytical technique of plasma concentrations of tulathromycin was used to determine TTM plasma concentrations at fixed intervals for 240 h. The key pharmacokinetic parameters were obtained by compartmental and non-compartmental analysis. Results show that the experimental preparation of tulathromycin (Pisadrax ® ) can be regarded as bioequivalent to the reference one (Draxxin ® ) in steers, given that AUC0-240, MRT, and K½el values from both preparations resulted statistically indistinguishable and with confidence interval &gt; 95%.

Effect of feeding sorbent and enzymes on heavy metal toxicity in young steers

Effect of feeding sorbent and enzymes on heavy metal toxicity in young steers

241 Skeletal Muscle Mitochondrial Capacities Are Impacted by Breed and Temperament in Young Angus and Brahman Steers

Abstract Temperament impacts skeletal muscle mitochondria in Brahman heifers, but this has not been investigated in steers or between cattle breeds. We hypothesized mitochondrial measures would be greater in Angus than Brahman, temperamental than calm steers, and the trapezius (TRAP) than the longissimus thoracis (LT) muscle. Samples from calm (n = 13 per breed), intermediate (n = 12 per breed), and temperamental (n=13 per breed) Angus and Brahman steers (mean±SD 10.0±0.8 mo) were evaluated for mitochondrial enzyme activities via colorimetry. Calm and temperamental LT samples were evaluated for oxidative phosphorylation (P) and electron transfer (E) capacities by high-resolution respirometry. Data were analyzed using linear models with fixed effects of breed, muscle, temperament, and all interactions. Brahman tended to have greater mitochondrial volume density (citrate synthase activity; CS) than Angus (P = 0.08), while intrinsic (relative to CS) mitochondrial function (cytochrome c oxidase activity) was greater in Angus than Brahman (P = 0.001) and greater in TRAP than LT (P = 0.008). Angus exhibited greater integrative (per mg tissue) and intrinsic P with complex I (PCI), P with complexes I+II (PCI+II), maximum noncoupled E, and E with complex II (ECII; P ≤ 0.04) and tended to have greater intrinsic leak (P = 0.1) than Brahman. Contribution of PCI to total E was greater in Angus than Brahman (P = 0.01), while contribution of ECII to total E was greater in Brahman than Angus (P = 0.05). A trend for the interaction of breed and temperament (P = 0.07) indicated calm Angus had the greatest intrinsic ECII (P ≤ 0.03) while intrinsic ECII was similar between temperamental Angus and calm and temperamental Brahman. Integrative PCI+II and ECII, and the contribution of PCI and PCI+II to overall E tended to be greater in temperamental than calm steers (P ≤ 0.09), while intrinsic ECII tended to be greater in calm than temperamental steers (P = 0.07). The impact of these mitochondrial differences on meat quality measures remains to be determined.

Methodology to identify candidate genes from beef carcass traits at weaning: A pilot study

The goal of the work reported herein was to investigate if a new methodology could provide gene expression profiles of brachiocephalicus muscle tissue from young steers and to compare this genetic expression with carcass traits determined at slaughter to identify potential candidate genes. Tissue samples were collected from steers using a fine needle biopsy from muscle in the neck region at weaning. Muscle tissue was removed using the Dunn Biopsy method, and RNA was extracted for sequencing and transcriptomic analysis. Animal groups for data analysis were designated using marbling score codes (MSC) established from carcass grade at slaughter, which identified DNER, PCBD1 and BGN genes associated with adipose; muscle genes included MAP7, PDE1B, and ADAMTS2; tenderness genes identified were FMO2 and ZKSCAN2; and marbling genes RPTN, DNER, and CACNA2D2. These results indicate muscle biopsies may yield complementary information associated with carcass traits to the current industry standards. Application of this technique may provide insight to the identification of candidate genes that could improve production decisions, increase accuracy of prediction from transcriptomic profiling, and ultimately speed genetic progress.

The Effects of Age at Weaning and Length of Lipid Supplementation on Growth, Metabolites, and Marbling of Young Steers.

Simple SummaryConsumers value quality beef and producers are starting to look at the ways production decisions affect the long-term performance of the animals. Early weaning is a production option in many beef cattle production environments. We are looking at the addition of rumen by-pass lipids in addition to early weaning to increase the marbling of steers. The supplementation of rumen protected lipids’ increased plasma concentrations of fatty acids. Supplementation of rumen-protected lipids improved carcass quality of young steers by increasing marbling scores and lipid concentration of steaks without negatively impacting dressing percentage. Therefore, a combination of early weaning and rumen by-pass lipid supplementation can be used as management practices to meet current consumer demands.The objective of this study was to determine how weaning age, days on supplements, and lipid supplementation affected the growth and marbling deposition of steers. Steers from a single sire were early weaned (n = 24) at 150 ± 11 days of age or traditionally weaned (n = 24) at 210 ± 11 days of age. Steers were assigned to control (n = 12/weaning group) or an isocaloric, isonitrogenous rumen by-pass lipid (RBL, n = 12/weaning group) for either 45 (n = 6/treatment) or 90 (n=6/treatment) days then harvested. Steer body weight (BW) was recorded on days −14 and −7, then BW and blood samples were collected on days 0, 22, 45, 66, and 90. The right rib section of each animal was collected for proximate analysis. Longissimus dorsi from RBL steers had increased lipids compared with control steers (3.6 ± 0.2 vs. 2.4 ± 0.2% on a wet basis; p < 0.0001). Steers fed for 90 days had greater (p = 0.02) concentrations of Longissimus dorsi lipid (3.3 ± 0.2%) than those fed for 45 days (2.7 ± 0.2%). There was a weaning age by treatment by days on feed interaction for intramuscular adipocyte diameter (p = 0.02) in which early weaned RBL fed for 90 days steers had an increased adipocyte diameter compared to the early weaned control fed for 90 and early weaned fed for 45 days steers with all other treatment groups as intermediates. Supplementation of RBL increased concentrations of C18:2, C20:4, and total fatty acids on days 45 and 90 (p ≤ 0.05). Data show that RBL supplementation increased the marbling content of the Longissimus dorsi. Furthermore, a longer period of supplementation resulted in increased adipose diameter.

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).

Locusta migratoria extruded meal in young steers diet: evaluation of growth performance, blood indices and meat traits of Calves Kasakh white-headed breed

ABSTRACT The article presented the data that confirm the efficiency of the extruded Locusta migratoria (ELM) meal used in feeding steers. The experiment was based on collecting locust, feeding steers and calculation of economic efficiency. One hundred steers of Kasakh white-headed breed were equally divided to 2 treatment groups, and fed either a basal ration (BR, Control group), or a ration containing complete feed supplemented with ELM at 10% (Test group). In the population under study, the live weight dynamics was observed, and the physiological state and haematological status were controlled. The steers’ meat productivity and meat quality were studied on the results of the control slaughter. Partial replacement of the feed with locust reduced the feed costs and contributed to an increase in profits per animal in Test group compared to the control animals, as well as in the beef production profitability.

Productive performance of Nellore steers on rotational grazing on Marandu grass and Convert HD 364 grass

The objective of this study was to evaluate the effect of Brachiaria spp. Cv. Mulato II (Convert) on performance, meat quality and carcass characteristics of castrated Nellore steers in the growing and finishing phases, in rotational grazing system, compared to Brachiaria brizantha cv. Marandu, under the same conditions. The experimental area was divided into two treatments: Marandu and Convert with 20 hectares each, splited in four replications per treatment, composed of five paddocks of one hectare each. Animals were managed based on the availability of leaf blade dry matter (6.19% body weight) in rotational grazing with 7 days of occupation and 28 days of rest. Ten animals were used in each replicate (testers), as well as regulatory animals whenever necessary. Animals were managed based on the availability of leaf blade dry matter (6.19% body weight) in rotational grazing with 7 days of occupation and 28 days of rest. Ten animals were used in each replicate (testers), as well as regulatory animals whenever necessary. For performance analysis, 10 animals of each replicate were used, while for carcass and meat analyses, only six of each replicate. Production and chemial characteristics of the two grasses were evaluated. The experimental design was completely randomized with two treatments and four replicates; data were tested by analysis of variance using the R software. Animals fed on Convert grass were superior than those fed on Marandu grass, with higher average daily gain (0.682 kg vs. 0.605 kg), slaughter weight (470.45 kg vs. 451.43 kg), hot carcass weight (239.93 kg vs. 232.36 kg). The study also showed the possibility of finishing castrated young steers (up to 30 months) and with subcutaneous fat thickness required by industry (3 mm) in both pastures.

Current situation and future trends for beef production in the United States of America - A review.

USA beef production is characterized by a diversity of climates, environmental conditions, animal phenotypes, management systems, and a multiplicity of nutritional inputs. The USA beef herd consists of more than 80 breeds of cattle and crosses thereof, and the industry is divided into distinct, but ofttimes overlapping sectors, including seedstock production, cow-calf production, stocker/backgrounding, and feedlot. Exception for male dairy calves, production is predominantly pastoral-based, with young stock spending relatively brief portions of their life in feedlots. The beef industry is very technology driven, utilizing reproductive management strategies, genetic improvement technologies, exogenous growth promoting compounds, vaccines, antibiotics, and feed processing strategies, focusing on improvements in efficiency and cost of production. Young steers and heifers are grain-based diets fed for an average of 5 months, mostly in feedlots of 1,000 head capacity or more, and typically are slaughtered at 15 to 28 months of age to produce tender, well-marbled beef. Per capita beef consumption is nearly 26 kg annually, over half of which is consumed in the form of ground products. Beef exports, which are increasingly important, consist primarily of high value cuts and variety meats, depending on destination. In recent years, adverse climatic conditions (i.e., draught), a shrinking agricultural workforce, emergence of food-borne pathogens, concerns over development of antimicrobial resistance, animal welfare/well-being, environmental impact, consumer perceptions of healthfulness of beef, consumer perceptions of food animal production practices, and alternative uses of traditional feed grains have become increasingly important with respect to their impact on both beef production and demand for beef products. Similarly, changing consumer demographics and globalization of beef markets have dictated changes in the types of products demanded by consumers of USA beef, both domestically and abroad. The industry is highly adaptive, however, and responds quickly to evolving economic signals.

Bodyweight performance, estimated carcass traits and methane emissions of beef-cattle categories grazing Andropogon gayanus, Melinis minutiflora and Stylosanthes capitata mixed swards and Brachiaria humidicola pasture

Cow–calf operations constitute the main land-use system in the 18 million ha of well drained tropical savannas of Colombia located in the Orinoco basin. There, numerous sex and age animal categories are present, ranging from suckling calves to old cull cows and steers, which in variable proportions are stocked at one animal unit (AU)/5–10 ha, depending on the ranch and distance from markets. In parallel, early-on farm observations showed that when sown pastures are introduced, graziers recur to opportunistic partial fattening of cull animals for a rapid economic return. The paper reports animal weight gains of cull cows, old steers, and young heifers and yearlings over seven consecutive years grazing on a low fertiliser-input well managed Andropogon gayanus-based pasture and four contemporary years on Brachiaria humidicola. The first 2 years involved three stocking rates (SR; 1.38, 1.85 and 2.32 animal units/ha), whereas a single SR of 1.33 animal units/ha was used for the remainder. A large database that included chemical, and in vitro and in vivo digestibility, values for all forages involved, was used as an input to a mathematical model to estimate methane (CH4) emissions. Estimations were compared with weight-based predictions derived from chamber measurements recorded recently in tropical Australia. Faecal mineral concentration varied little (not significantly) among SR and animal categories. Weight gains of old steers were 4–24% larger than those of cull cows, which, in turn, were larger than those of young steers and heifers (P &amp;lt; 0.01). Excepting one year, the performance of cull cows and old steers did not differ between pastures, confirming earlier on-ranch observations of reasonable weight gains of adult cattle on both pastures. The predicted carcass composition of cull cows and old steers did not differ between pastures either. Over the length of the experiment, cull cows emitted significantly less CH4 than did old steers (129 vs 141 g/day) on A. gayanus, while emissions on B. humidicola amounted to 128 and 137 g/day respectively. Despite between-year differences in animal performance, they are indicative of well managed pasture sustainability. They also show some of the trade-offs involved in the use of contrasting animal categories and pastures in terms of weight performance and predicted carcass composition and CH4 production. The authors expect that the present results will contribute to a rational, science-based discussion of the role of beef systems vis-à-vis environment in a region with limited production alternatives.

COMPONENTES NÃO CARCAÇA DE NOVILHOS DE CORTE SUBMETIDOS À CASTRAÇÃO CIRÚRGICA OU IMUNOLÓGICA

Resumo Objetivou-se avaliar as características dos componentes do corpo vazio que não integram a carcaça de novilhos castrados cirurgicamente ou imunologicamente. Foram utilizados 48 bezerros Aberdeen Angus, monitorados a partir de idade média inicial de seis meses e peso médio inicial de 160 kg. Os animais foram distribuídos nos seguintes tratamentos: castrados cirurgicamente ao nascimento; castrados cirurgicamente à desmama; imunocastrados com três doses de vacina (6º, 9º e 14º meses de idade dos animais) e imunocastrados com quatro doses (6º, 7º, 10º e 15º meses de idade dos animais). O delineamento experimental foi o inteiramente casualizado. Durante o abate, todos os componentes internos e externos do corpo do animal foram separados e pesados individualmente. O total de órgãos internos expressos em percentual do peso de corpo vazio diferiu entre os dois protocolos de imunocastração, com superioridade quando aplicou-se quatro doses (3,61 vs. 3,39 kg). Novilhos castrados ao nascer depositaram mais gordura cardíaca, renal e pélvica, e gordura do trato gastrintestinal que imunocastrados com três doses, independentemente da forma como foi expressa. A castração cirúrgica ao nascimento promove maior deposição de gordura visceral, em novilhos abatidos em idade jovem, em relação à imunocastração com dosagem definitiva com nove meses de idade.

063 The effects of age at weaning and length of lipid supplementation on growth, metabolites, and carcass quality of young steers

063 The effects of age at weaning and length of lipid supplementation on growth, metabolites, and carcass quality of young steers

草地放牧における若令牛の栄養および肉生産 : 第1報 寒地型混播草地放牧における若令肥育牛の増体および養分摂取量

草地放牧における若令牛の栄養および肉生産 : 第1報 寒地型混播草地放牧における若令肥育牛の増体および養分摂取量

Responses to various protein and energy supplements by steers fed low-quality tropical hay. 2. Effect of stage of maturity of steers

Dose response curves to various supplements were established in two pen-feeding experiments (Exp1 and Exp2) with Bos indicus crossbred steers of two age groups (Young, 10–12 months; Old, 33–36 months) fed low-quality tropical grass hays ad libitum. Diets included supplements based on (Exp1) cottonseed meal (CSM; intake (as fed) 0–10 g/kg liveweight (W).day) and a barley mix (Bar; 0–20 g/kg W.day) and (Exp2) a molasses mix (MUP) and a Bar mix, both fed at 0–20 g/kg W.day. Urea was provided with the Bar mixes and urea/copra meal with the MUP mix. Growth rates of Young steers increased linearly with Bar and MUP supplements but asymptotically with CSM whereas those of Old steers increased asymptotically with all supplement types. With supplement intake expressed on a liveweight basis (g/kg W.day), responses were greater for both steer age groups with CSM compared with Bar (Young, P &lt; 0.001; Old, P &lt; 0.01) and Bar compared with MUP treatments (Young, P &lt; 0.01; Old, P &lt; 0.05). Furthermore, Old steers outperformed their Young counterparts with both CSM (P &lt; 0.05) and Bar (P &lt; 0.001) supplements fed in Exp1 and with Bar and MUP supplements (P &lt; 0.01) fed in Exp2. When supplement intake was expressed in absolute terms (kg/day), growth responses were not different between age groups for different supplements except that Old steers had a higher daily W gain on Bar than their Young counterparts (P &lt; 0.05). Intake of hay (W-corrected) was higher for Young compared with Old steers without supplement but was variably reduced for both steer groups with increasing supplement intake. The results of these experiments have implications for supplement formulation for steers at different stages of maturity grazing low-quality forages.

Responses to various protein and energy supplements by steers fed low-quality tropical hay. 1. Comparison of response surfaces for young steers

Response curves were established for different supplements, offered at intakes ranging from 0 to 20 g/kg liveweight (W).day to young Bos indicus crossbred steers fed low-quality Rhodes grass (Chloris gayana) hay ad libitum in two pen experiments. Supplements included protein meals of varying rumen-degradability (cottonseed meal (CSM) or fishmeal), as well as ‘energy sources’ comprising grains of high and low ruminal starch degradability (barley and sorghum) and a highly fermentable sugar source (molasses), with all diets adjusted for rumen-degradable nitrogen and mineral content. Unsupplemented steers gained 0.08 and 0.15 kg/day, in Experiments 1 and 2, respectively. Growth of steers increased linearly with intake of ‘energy source’ supplements in increasing order of molasses, sorghum and barley (all differences P &lt; 0.05). Steer growth rate also increased linearly with fishmeal, albeit over a narrow intake range (0–4.1 g/kg W.day), whereas the response with CSM was asymptotic, showing a steep response at low intake before levelling at ~1.2 kg/day. All supplement types were associated with a linear reduction in hay intake by the steers (energy substitution) where the reduction was greater (P &lt; 0.05) for barley and molasses (not different) than for sorghum (P &lt; 0.05), and for fishmeal compared with CSM (P &lt; 0.05). In concurrent metabolism studies with the same rations, organic matter digestibility of the total ration (561–578 g/kg DM, unsupplemented) was increased linearly by barley and molasses (both P &lt; 0.05) but was unaffected by CSM and sorghum supplements. The efficiency of microbial protein synthesis in steers increased linearly, from 91 g microbial crude protein/kg digestible organic matter (unsupplemented), in both molasses and CSM-supplemented steers, with the trend for a higher response to molasses (P = 0.05), and appeared most closely related to digestible organic matter intake. The response curves from these studies provide the practical framework upon which to formulate rations for cattle grazing low-quality forages.