Six Holstein steers (539±15 kg) with cannulas in the rumen (3.8 cm internal diameter) and proximal duodenum were used in a replicated 3×3 Latin square experiment to study treatment effects on characteristics of digestion. Dietary treatments consisted of a steam-flaked corn-based growing-finishing diet supplemented (diet DM basis) with 0%, 1%, or 2% of kaolinite clay (Ione Minerals Inc., Ione, CA, USA). Composition of experimental diets is shown in Table 1. Chromic oxide (3.0 g/kg of diet air-dry basis) was used as an indigestible marker to estimate nutrient flow and digestibility. Steers were maintained in individual pens (4 m²) with automatic waterers. Diets were fed at 0800 and 2000 h daily. In order to avoid feed refusals, dry matter intake (DMI) was restricted to 10.3 kg/d (equivalent to 1.9% of live weight [LW]). Experimental periods consisted of a 10-d diet adjustment period followed by a 4-d collection period. Between each experimental period, steers were allowed a 7-d recovery period during which all steers were fed the control diet. During collection, duodenal and fecal samples were taken twice daily as follows: d 1, 0750 and 1350 h; d 2, 0900 and 1500 h; d 3, 1050 and 1650 h, and d 4, 1200 and 1800 h. Individual samples consisted of approximately 700 mL of duodenal chime 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% (wt/vol) 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 acids (VFA) analysis by gas chromatograph. 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, duodenal and fecal samples were subject to the following analysis: DM (oven drying at 105°C until no further weight loss; method 930.15; AOAC, 2000); ash (method 942.05; AOAC, 2000), 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 Technology, Macedon, NY, USA) at 1 mL per 100 mL of NDF solution (Midland Scientific, Omaha, NE, USA)]; chromic oxide (Hill and Anderson, 1958), and starch (Zinn, 1990). Microbial organic matter (OM) and nitrogen leaving the abomasum were calculated using purines as a microbial marker (Zinn and Owens, 1986). The OM fermented in the rumen was considered equal to OM intake minus the difference between the amount of total OM reaching the duodenum and microbial OM reaching the duodenum. Feed N escape to the small intestine was considered equal to total N leaving the abomasum minus ammonia-N, microbial N (MN) and endogenous N, assuming endogenous N is equivalent to 0.195 W^0.75 (Orskov et al., 1986). Ruminal microbial efficiency was estimated as duodenal MN, g/kg OM fermented in the rumen and N efficiency represent the duodenal non-ammonia N, g/g N intake. Methane production (mol/mol glucose equivalent fermented) was estimated based on the theoretical fermentation balance for observed molar distribution of VFA (Wolin, 1960).

Treatment effects on characteristics of digestion in cattle were analyzed as a replicated 3×3 Latin square design using the MIXED procedure (SAS Inst. Inc., Cary, NC, USA). Treatments effects on digestion and fermentation variables were tested by means of polynomial contrasts (SAS Inst.; Version 9.3). Contrasts were considered significant when the p-value was ≤0.05, and tendencies were identified when the p-value was >0.05 and ≤0.10.

One hundred eight Holstein steers (132.4±5.6 kg) were used in a 308-d experiment to evaluate the influence of kaolinite clay supplementation on growth performance, dietary net energy (NE), and carcass characteristics. Cattle originated from Tulare, California, and were received at the University of California Desert Research Center, El Centro, on April 29, 2014. Upon arrival, steers were treated for parasites (Dectomax Injectable, Zoetis, New York, NY, USA), and injected subcutaneously with tuluthramycin (Draxxin, Pfizer, New York, NY, USA), and 500,000 IU vitamin A (Vital E-A+D, Stuart Products, Bedford, TX, USA). Steers were balanced by weight and assigned within weight groupings to 18 pens (6 steers/pen). Pens were 78 m² with 33 m² of overhead shade, automatic waterers, and fence-line feed bunks. Dietary treatments were the same as in Trial 1. Composition of experimental diets is 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. On day 112 and 224, all steers were injected subcutaneously with 500,000 IU vitamin A (Vital E-A+D, Stuart Products, USA) and implanted with Revalor-S (Intervet, Millsboro, DE, USA).

For calculating steer performance measures of live weight were reduced 4% to account for digestive tract fill. Final shrunk LW was carcass-adjusted by dividing hot carcass weights (HCW) by the decimal fraction of the average dressing percentage (0.618). Daily energy gain (EG; Mcal/d) was calculated by the equation: EG = ADG^1.097 0.0557W^0.75, where W is the mean shrunk weight (kg; NRC, 1984). Maintenance energy (EM) was calculated by the equation: EM = 0.084W^0.75 (Garrett, 1971). Dietary net energy for gain (NE_g) was derived from net energy for maintenance (NE_m) by the equation: NE_g = 0.877 NE_m − 0.41 (Zinn et al., 2008). DMI is related to energy requirements and dietary NE_m according to the equation: DMI = EG/(0.877NE_m−0.41), and can be resolved for estimation of dietary NE by means of the quadratic formula: x = (−b−√ b² − 4ac)/2c, where: x = NE_m, a = −0.42EM, b = −0.887EM+0.41DMI+EG, and c = −0.887DMI (Zinn et al., 2008).

The HCW were obtained at time of slaughter. After carcasses chilled for 48 h, the following measurements were obtained: Longissimus (LM) area (cm²) by direct grid reading of the muscle 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.

Performance (gain, gain efficiency, and dietary energetics) and carcass data were analysed as a randomised complete block design. The experimental unit was the pen. The MIXED procedure of SAS (SAS Institute, 2004) was used to analyse the variables. The fixed effect consisted of treatment, and pen as the random component. Treatment effects were tested by means of orthogonal polynomials (SAS Inst.; Version 9.3). Contrasts were considered significant when the p-value was ≤0.05, and tendencies were identified when the p-value was >0.05 and ≤0.10.