Eighty crossbred steers (approximately 25% Brahman with the remainder represented by Hereford, Angus, Shorthorn and Charolais breeds in various proportions) with an average weight of 239±15 kg were used in a 229-d experiment to evaluate the effects of EHY cell wall (TruMax, Vi-COR, Mason City, IA, USA) supplementation on growth performance, dietary net energy, and carcass characteristics of feedlot cattle. Upon arrival, steers were vaccinated for bovine rhinotracheitis-parainfluenza (Cattle Master Gold FP 5 L5, Zoetis, New York, NY, USA), clostridials (Ultrabac-7, Zoetis, USA), treated for parasites (Dectomax Injectable, Zoetis, USA), injected subcutaneously with 500,000 IU vitamin A (Vital E-A + D3, Stuart Products, Bedford, TX, USA), and 1,200 mg ceftiofur (Excede, Zoetis, USA), branded, ear-tagged, and implanted with Revalor-IS (Intervet, Millsboro, DE, USA). Bull calves were castrated and horns, if present, were tipped. Steers were blocked by weight and randomly assigned within weight groupings to 16 pens (4 pens per treatment; 5 steers per pen). Pens were 43 m² with 22 m² of overhead shade, automatic waterers, and 2.4 fence-line feed bunks. Treatments consisted of steam-flaked corn-based diets supplemented to provide 0, 1, 2, or 3 g EHY/hd/d. Ingredient and nutrient composition of diets are shown in Table 1. Diets were prepared at weekly intervals and stored in plywood boxes located in front of each pen. Steers were allowed ad libitum access to their experimental diets. Fresh feed was provided twice daily. Individual steers were weighed upon initiation and at periods of 28-d until completion of the 229-d trial. In the calculation of steer performance live weights were reduced 4% to adjust for digestive tract fill. Estimates of steer performance were based on pen means. Readings of daily ambient temperature and humidity during the course of the study were obtained from the California Department of Water Resources Information and Management System (CIMIS) weather station located roughly 100 meters distance from the feedlot.

Energy gain (EG) was calculated by the equation: EG = ADG^1.097 0.0557 W^0.75, where EG is the daily-energy-deposited (Mcal/d), W is the mean shrunk body weight (BW) (kg; NRC, 1984). Maintenance energy (EM) was calculated by the equation: EM = 0.077W^0.75 (NRC, 1996). Dietary net energy of gain (NEg) was derived from net energy of maintenance (NEm) by the equation: NEg = 0.877 NEm−0.41 (Zinn and Shen, 1998). Dry matter intake (DMI) is related to energy requirements and dietary NEm according to the equation: DMI = EM/NEm+EG/(0.877 NEm −0.41), and can be resolved for estimation of dietary NEm by means of the quadratic formula: x = (−b±[b²−4ac]^0.5)/2a, where x = NEm, a = −0.877DMI, b = 0.877 EM+0.41 DMI+EG, and c = −0.42 EM (Zinn and Shen, 1998).

Hot carcass weights (HCW) were obtained at time of slaughter. After carcasses chilled for 48 h, the following measurements were obtained: LM area (cm²) by direct grid reading of the Longissimus muscle (LM) at the 12th rib; subcutaneous fat (cm) over the LM at the 12th rib taken at a location 3/4 the lateral length from the chine bone end (adjusted by eye for unusual fat distribution); kidney, pelvic and heart fat (KPH), as a percentage of HCW; marbling score (USDA 1997; using 3.0 as minimum slight, 4.0 as minimum small, 5.0 as minimum modest, 6.0 as minimum moderate, etc.), and estimated retail yield of boneless, closely trimmed retail cuts from the round, loin, rib and chuck (% of HCW; Murphey et al., 1960) = 52.56 – 1.95×subcutaneous fat – 1.06×KPH+0.106×LM area – 0.018×HCW.

For calculating steer performance, initial BW is the arrival off-truck shrunk weight. Interim and final LW was reduced 4% to account for digestive tract fill. Final shrunk LW was adjusted for HCW by dividing HCW by the decimal fraction of the average dressing percentage (0.64). Pens were used as experimental units. The experimental data were analyzed as a randomized complete block design experiment according to the following statistical model:

Where μ is the common experimental effect, B_i represents initial weight group effect (df = 3), T_j represents dietary treatment effect (df = 3), and E_ij represents the residual error (df = 9). Treatments effects were tested using the following contrasts: 0 vs EHY, and linear and quadratic polynomials (Stastix 9, Analytical Software, Tallahassee, FL, USA).

Four Holstein steers (264±5 kg) with cannulas in the rumen (3.8 cm internal diameter) and proximal duodenum (Zinn and Plascencia, 1993) were used in a 4×4 Latin square experiment to evaluate the influence of EHY (TruMax, Vi-COR, USA) supplementation-level in finishing diets for steers based on steam-flaked corn and distillers dried gains plus solubles on characteristics of rumen and total tract digestion. Dietary treatments were the same as indicated for the finishing diet used in Trial 1 (Table 1) plus the inclusion of chromic oxide (2.5 g/kg) as a digesta marker. Steers were maintained in individual pens (5.6 m²) with automatic waterers. Diets were fed at 08:00 and 20:00 h daily. In order to avoid the complications of feed refusals, DMI was restricted to 6.02 kg/d (2.3% of BW). Experimental periods were 14 d, with 10 d for dietary treatment adjustment, 4 d for collection. During collection, duodenal and fecal samples were taken twice daily as follows: day 1, 0750 and 1350 h; day 2, 0900 and 1500 h; day 3, 1050 and 1650 h, and day 4, 1200 and 1800 h. Individual samples consisted of approximately 700 mL of duodenal chyme and 200 g (wet basis) of fecal material. Samples from each steer within each collection period were composited for analysis. During the final day of each collection period, ruminal samples were obtained from each steer via ruminal cannula 4 h after feeding. Ruminal fluid pH was determined on fresh samples. Samples were strained through 4 layers of cheesecloth. Two milliliters of freshly prepared (25 g/100 mL) meta-phosphoric acid was added to 8 mL of strained ruminal fluid. Samples were then centrifuged (17,000×g for 10 min), and supernatant fluid was stored at −20°C for volatile fatty acid (VFA) analysis (gas chromatography; Zinn, 1988). Upon completion of the experiment, ruminal fluid was obtained via the ruminal cannula from all steers and composited for isolation of ruminal bacteria by differential centrifugation (Bergen et al., 1968).

Feed and fecal samples were subjected to the following analysis: DM (oven drying at 105°C until no further weight loss); ash (method 942.05, AOAC, 1986), Kjeldahl N (method 984.13, AOAC, 2000); aNDFom (Van Soest et al., 1991), corrected for neutral detergent fiber (NDF)-ash, incorporating heat stable α-amylase (Ankom FAA, Ankom Technology, Macedon, NY, USA) at 1 mL per 100 mL of NDF solution); chromic oxide (Hill and Anderson, 1958); and starch (Zinn, 1990). Duodenal samples were subjected the following analysis: DM (oven drying at 105°C until no further weight loss); ash (method 942.05, AOAC, 1986), Kjeldahl N (method 984.13, AOAC, 2000), ammonia N (method 941.04, AOAC, 2000); aNDFom (Van Soest et al., 1991), corrected for NDF-ash, incorporating heat stable α-amylase (Ankom FAA, Ankom Technology, USA) at 1 mL per 100 mL of NDF solution); purines (Zinn and Owens, 1986); chromic oxide (Hill and Anderson, 1958); and starch (Zinn, 1990). Duodenal flow and fecal excretion of DM were calculated based on marker ratio, using chromic oxide. Microbial organic matter (MOM) and N (MN) leaving the abomasum was calculated using purines as a microbial marker (Zinn and Owens, 1986). Organic matter (OM) fermented in the rumen was considered equal to OM intake minus the difference between the amount of total OM reaching the duodenum and MOM reaching the duodenum. Feed N escape to the small intestine was considered equal to total N leaving the abomasum minus ammonia-N, MN, and endogenous N (0.195×BW^0.75, Ørskov et al., 1986). Methane production (mol/mol glucose equivalent fermented) was estimated based on the theoretical fermentation balance for observed molar distribution of VFA (Wolin, 1960).

The effects of EHY cell wall level (0, 1, 2, or 3 g EHY/hd/d) on characteristics of digestion in cattle were analyzed as a balanced 4×4 Latin square design experiment:

Where, Y_ijk is the response variable, μ is the common experimental effect, S_i is the steer effect, P_j is the period effect, T_k is the treatment effect and E_ijk is the residual error. Treatment effects were tested using the following contrasts: 0 vs EHY, and linear and quadratic polynomials (Stastix 9, Analytical Software, USA).

Treatment effects on growth performance are shown in Table 2. There were no effects on growth performance during the initial 139-d period. However, from d-139 to harvest, when 24-h temperature humidity index averaged 80, EHY increased DMI (linear effect, p<0.01) and ADG (linear effect, p = 0.01). This improvement in ADG was largely due to increased DMI, as gain efficiency and estimated dietary NE were not affected by EHY supplementation (Table 2). Comparable studies evaluating effects of EHY on feedlot cattle growth-performance are limited. In a 56-d feeding trial, Finck et al. (2010) observed increased ADG associated with increased DMI in feedlot steers fed a receiving diet supplemented to provide 5 g/d yeast cell wall. In a 50-d feeding trial, Lei et al. (2013) observed increased ADG and gain efficiency in feedlot steers fed 2 g d of a yeast cell wall product.

Considering supplemental yeast, per se, Hinman et al. (1998) in a 115-d trial observed greater ADG and gain efficiency in feedlot steers supplemented with yeast. In contrast, Swyers et al. (2014) did not observed an effect of supplemental yeast on in a 125-d feedlot growth performance of yearling steers. Likewise, Baumann et al. (2004) observed no advantage of yeast supplementation on 126-d ADG and gain efficiency of growing-finishing feedlot steers.

Observed variation in growth-performance response to EHY supplementation as affected by periods of unfavorable ambient conditions, may be more particularly a function of immune status (Swyers et al., 2014). Heat stress alters endocrine profiles and energy metabolism in cattle (Rhoads et al., 2009). Supplemental EHY can modulate immune status (Nocek et al., 2011; Lei, et al., 2013; Sanchez et al., 2013; 2014). Sanchez et al. (2014) observed that in beef heifers newly-received into the feedlot, supplementation with yeast cell wall enhanced energy metabolism during an immune challenge. Ganner et al. (2010) observed that yeast derivatives (cell walls) had a selective effect against some pathogenic bacteria. Reisinger et al. (2012) observed that yeast cell wall supplementation increased jejunal goblet cell density, reducing the number of apoptotic enterocytes. Lei et al. (2013) observed that yeast cell wall can effectively bind lipopolysaccharides within the intestine, preventing translocation into the circulation.

Liu et al. (2014) observed that yeast supplementation improved milk yield and immune response of dairy cows under conditions of heat stress. Temperature–humidity index (THI = [0.8×ambient temperature]+[{% of relative humidity/100}×{ambient temperature−14.4}]+46.4), a measure of heat load, is coded as follows: normal, THI <74; alert, 75<THI<78; danger, 79 <THI<83; and emergency, THI >84 (Mader et al., 2006). In the present study, enhancements in DMI and ADG during the period of high THI may indicate a potential role of EHY in association with heat stress.

Consistent with overall treatment effects on ADG and final harvest weight, there were no treatment effects (p>0.10) on carcass characteristics (Table 3). Comparable studies involving yeast cell wall are limited. With regard to yeast supplementation, per se, Hinman et al. (1998) and Baumann et al. (2004) did not observe an effect of supplemental yeast on carcass characteristics of feedlot steers. Gomes et al. (2009) observed that supplemental yeast increased carcass dressing percentage, but did was without effect on other carcass measures. Swyers et al. (2014) observed that supplemental yeast increased the proportion of carcass that graded USDA Choice or better.

Treatment effects on characteristics of digestion are shown in Table 4. There were no treatment effects (p>0.10) on ruminal digestion of OM, starch, feed-N, microbial efficiency (g microbial N/kg OM fermented) and N efficiency (non-ammonia N entering the small intestine/N intake). Ruminal digestion of NDF tended to increase (linear effect; p = 0.08) with the increasing level of EHY, reflecting a stimulatory effect of EHY on cellulase activity (Kmet et al., 1992). There were no treatment effects (p>0.10) on total tract digestion of DM, OM, NDF, starch, and N.

Comparable studies evaluating effects of supplemental EHY on characteristics of digestion are limited. Lei et al. (2013) observed increased fiber digestion in steers supplemented with yeast cell walls. Indeed, enhanced fiber digestion has been a consistent response to yeast supplementation, per se, across a variety of diets and feeding practices (Dawson et al., 1990; Williams et al., 1991; Zinn and Borquez, 1993; Plata et al., 1994; López-Soto et al., 2013). Nevertheless, as fiber comprises a comparatively small component of the conventional finishing diets, effects of supplementation on total tract digestion were small and non-appreciable.

Treatment effects on characteristics of ruminal fermentation are shown in Table 5. There were no treatment effects (p>0.10) on ruminal pH, total VFA, or molar proportion of butyrate. Consistent with the present study, Baumann et al. (2004) and Lopez-Soto et al. (2013) did not observe an effect of supplemental yeast on ruminal pH. In contrast, Vyas et al. (2014) observed an increase in ruminal pH with yeast supplementation of feedlot diet. Although, as in the present study, yeast supplementation did not affect ruminal VFA concentration.

Supplemental EHY decreased ruminal molar proportion of acetate (p = 0.08), increased molar proportion of propionate (p = 0.09), and decreased acetate:propionate molar ratio (p = 0.07) and estimated ruminal methane production (p = 0.09). A similar effect on ruminal acetate:propionate supplementation has been observed with yeast supplementation, per se (Williams et al., 1991; Plata et al., 1994; Hinman et al., 1998).