This trial was conducted at the Faculdade de Agronomia e Zootecnia, Universidade Federal de Mato Grosso (UFMT; Cuiabá, Mato Grosso, Brazil), following humane animal care and handling procedures, according to the UFMT guidelines (Protocol 23108.060964/13 − 6).
Experimental design and treatments
The experiment was conducted in the rainy-dry transition season at the beef cattle facility of the Sector Nutrition of Beef Cattle at Pasture, Universidade Federal de Mato Grosso - UFMT, from March 2016 to June 2016. The climate is classified as tropical (Aw in the Köppen international system), the average maximum temperature is 32.8°C, and the average minimum temperature is 19.7°C.
Sixty-four young bulls (Nelore) with an average age of 15 months and 305 ± 17.81 kg of body weight (BW) were used in the experiment to evaluate the effects of varying the ground corn size in their supplements on intake and nutrient digestibility, performance and economics. The experiment was structured in a completely randomized design with 4 treatments. The experiment lasted 90 days, divided into 3 experimental periods of 30 days.
Initially, the animals were weighed, identified, treated against ecto- and endoparasites by administration of ivermectin (Ivomec; Merial, Paulínea, Brazil), and allocated into 16 paddocks of 0.81 ha each, with 4 young bulls randomly allocated to 1 of 4 treatments. The paddocks were seeded with Brachiaria brizantha cv. Marandu forage and were fitted with smooth wire fencing, waters, and feed bunks. The digestibility trial was conducted with one animal in each paddock between days 41 and 50. To evaluate intake and nutrient digestibility, 7 d were allowed for adaptation to external indicators, and 3 d were allowed for fecal sampling.
The effects of different ground corn sizes in the supplements were evaluated using the following treatments: 1-Whole grain (geometric mean particle size, dgw, 6.262 µm, WG); 2-Coarse ground grain (dgw, 2.882 µm, CG); 3-Medium ground grain (dgw, 1.011 µm, MG); and 4-Fine ground grain (dgw, 0.621 µm, FG) (Table 1). The corn size was determined according to the methodology proposed by Yu et al. (1998).
The supplement was provided at 10 g/kg BW daily at 10:00 am. The supplements were formulated to be isonitrogenous, with CP contents of approximately 22%. The proportions of the ingredients and the chemical composition of the forage and supplements are showed in Table 1.
The average sward height was randomly measured by reading 100 sampling points in each paddock with a measuring stick graduating in centimeters (Barthram, 1985). The forage mass in each paddock was estimated for each period (30 d). Three forage samples were collected by clipping all forage within a 0.25 m2 frame in each paddock at each sampling to a 5.0 cm stubble height with hand shears, according to the average sward height. The clipped samples were dried to a constant weight under forced air at 55°C for 72 h. The estimate forage mass and potentially digestible dry matter, according to Detmann et al. (2016), for multiplied by the paddock area by dry weights of these clippings.
The paddocks were managed through continuous stocking (Allen et al., 2011). Forage samples to address the herbage chemical composition were collected using a hand plucking methodology
(Johnson, 1978) to mimic forage selected by grazing bulls. Samples were collected from each pasture during each period and dried to a constant weight at 55°C under forced air, after, subsequent analysis of their chemical composition in to the laboratory.
Chemical composition analysis
Supplement ingredient samples, forage samples, and feces were dried at 55°C for 72 h. Samples were then ground in a Wiley mill (Thomas Scientific, Swedesboro, NJ) to pass through a 2 mm screen for indigestible neutral detergent fiber (iNDF) analysis (Valente et al. 2011). A subportion of 20 g of each sample was ground to pass through a 1 mm screen for analyses of dry matter (DM), ash, crude protein (CP), and neutral detergent fiber (NDF). Samples of forage, supplement ingredient samples and feces were analyzed following the procedures described by Detmann et al. (2012) for DM (index INCT-CA G-003/1), CP (index INCT-CA N-001/1), ash (index INCTCA M-001/1), neutral detergent fiber corrected for contaminant ash, and protein (NDFom(n); index INCT-CA F-002/1, INCT-CA M-002/1, and INCT-CA N-004/1).
The pdDM was estimated using the second pasture sample collected in each period, as described previously, using the following equation (Detmann et al. 2016):
pdDM (%; dry matter basis) = 0,98* (100-NDF) + (NDF-iNDF);
where 0.98 is the true digestibility coefficient of the intracellular content; NDF is the forage content of neutral detergent fiber corrected for the residual ash and nitrogen; and iNDF is the forage content of indigestible neutral detergent fiber.
Intake Estimation
Intake and nutrient digestibility were estimated just once during the period between the 41st day and the 50th day on the bulls in each paddock (16 animals, 4 bulls for each treatment) using the marker method. Chromium oxide, titanium dioxide and indigestible NDF (iNDF) were used to estimate the excretion of fecal matter (as dry weight), supplement intake and forage intake, respectively.
Fecal samples were collected on d 48, 49, and 50, directly from the rectum, at 16:00, 12:00, and 08:00 h on the first, second, and third days of collection, respectively. The fecal samples were dried at 55°C for 72 h and proportionately composited throughout the day for each animal based on the fecal dry weights.
To estimate DM fecal excretion, 15 g of chromium oxide was provided for 9 d by esophageal infusion; the first 7 d were used to stabilize fecal excretion of the marker, and the last 3 d were used to collect the samples (Titgemeyer et al., 2001). Chromium was determined in the feces by atomic absorption spectrophotometry (model 2380 spectrophotometer; PerkinElmer, Bois d’Arcy, France) at a wavelength of 357.9 nm with an air/acetylene flame directly on the supernatant obtained by centrifugation (5,000 × g for 15 min at room temperature; Michalet-Doreau and Doreau, 2001).
Fecal DM excretion was estimated based on the ratio between the amount of marker supplied and its concentration in the feces, according to the equation described by Smith and Reid (1955):
FE = (CrO2 provided/CrO2 concentration feces) x 100
in which FE = fecal excretion (g/d), CrO2 provided = amount of chromium oxide provided (g/d), and CrO2 concentration feces = chromium concentration in the feces (g/kg).
Dry matter voluntary intake (DMI) was estimated by using iNDF as an internal marker (Valente et al. 2011) by the equation:
DMI (kg/day) = {[FE × fecal iNDF) – supplement iNDF] ÷ forage iNDF} + SDMI
in which fecal iNDF = iNDF in the feces (%); supplement iNDF = iNDF in the supplement (kg/day); forage iNDF = iNDF in the forage (kg/kg) and SDMI = supplement dry matter intake (kg/day).
For the determination of individual consumption of the supplement (ICS), titanium dioxide was used at an average quantity of 20 g/animal and mixed with the supplement immediately before supplying it, according to the procedure described by Valadares Filho et al. (2006). For analysis, titanium dioxide content in the feces was determined according to the procedure described in Holleman and White (1989). The ICS was performed by following the same scheme of fecal collection as for chromium oxide, through the equation:
ICS (g/day) = (FE × feces TiO) ÷ supplement TiO
where fecal TiO and supplemental TiO are the titanium dioxide concentrations in the feces and in the supplement, respectively.
For determination of iNDF, 0.5 g samples of feces, forage, and supplement were placed in 5 by 5 cm polypropylene bags (nonwoven fabric, weight 100 g/m2). The samples were weighed to allow for 20 mg DM/cm2 of surface area (Nocek, 1988) and incubated in the rumen of a cannulated Nellore bull for a period of 288 h (Valente et al. 2011).
The fecal samples collected for the determination of DMI, ICS and forage intake in the digestibility trial were used (50 mg) for the determination of starch excretion in the feces according to the methodology proposed by Hall (2009), and the supplemental sample was analyzed for the determination of starch intake.
Performance and Economic Analysis
After 14 h of solid fasting, young bulls were weighed at the beginning and end of the experiment for daily weight gain and total weight gain. The economic analysis was determined by the nutrition costs (supplementation and land occupation), operation, acquisition of the animals (US$ 1.34/kg of BW), gross revenue, net profit, area production, and profitability during the feeding period.
The prices of the ingredients grain corn, soybean meal, mineral supplement and urea were 0.14, 0.25, 0.37 and US$ 0.49/kg, respectively. For the price of grinding corn based on the cost of the grinding machine with a 7 horse power motor, a price value of US$ 673.85, and consumption of 5.59 kW/h, flow rates for coarse ground corn (2.882 µm), medium ground corn (1.011 µm), and fine ground corn (0.621 µm) of 1,300, 1,000 and 700 kg/h, respectively, were calculated. The price of power consumption, taking into account the mean year in Brazil, had a value of US$ 0.25/kW.
Statistical Analysis
Nutrient intake and digestibility, fecal starch concentration, and performance data (initial weight; final weight, daily weight gain, total weight gain) were analyzed using SAS/STAT software (SAS Inst. Inc., Cary, NC) as a randomized complete design with 4 treatments (4 corn grinding particle size). Only performance data considering the average of each paddock were considered 1 experimental unit, and another variable considered only each animal paddock. The model included the fixed effects of the ground corn particle size.
When no significant interactions (P > 0.05) were detected, the main effects of the ground corn particle size were examined. Differences between ground corn particle sizes were tested by an ANOVA F-test using orthogonal contrasts, where we compared: 1-WC vs GC (whole grain versus ground corn); 2-CG vs MG (coarse ground grain versus medium ground grain); and 3-CG vs FG (coarse ground grain versus fine ground corn).