Animals, Housing, and Diets

From d −7 to −1 of the study, all animals were housed in group-pens with ad libitum access to water and Brachiaria brizantha cv. Marandu hay, in order to acclimate animals to the facilities prior to the beginning of the experiment.

On d 0 of the study, 383 Nellore (Bos taurus indicus) bulls were ranked by initial shrunk body weight (BW; after 16 hours of feed and water restriction; 384 ± 29.2 kg; initial age = 24 ± 2 mo) and randomly assigned to treatments in a randomized complete block design. Within blocks (n = 15), animals were randomly assigned into pens (n = 6 to 8 animals/pen) and pens were randomly assigned to receive 1 of 4 treatments: 1) 14 d adaptation with 2 step-up diets and transition to a high-concentrate finishing diet on d 15 (CONT; n = 15), 2) CONT plus oral administration of 20 mL of Lactipro-NXT (M. elsdenii NCIMB 41125, 1 × 10¹⁰ CFU/head; MS Biotec, Wamego, KS) on d 0 of the study (MEG-14; n = 15), 3) Six d of adaptation with 2 step-up diets plus oral administration of 20 mL of Lactipro-NXT on d 0 of the study (MEG-6; n = 15), and 4) Finishing diet and oral administration of 20 mL of Lactipro-NXT on d 0 of the study (MEG-0; n = 15). The mixing of the MEG with the saline solution and oral administration of Lactipro-NXT was performed individually following manufacturer´s recommendations. Regardless of treatment, all animals received the same step-up diets during the adaptation period (if applicable) and the same finishing diet during the experimental period, which lasted 119 d. For CONT and MEG-14, the adaptation diet was offered for 14 d and consisted of 2 step-up dies (7 d each), whereas MEG-6 also had 2 step-up diets in a shorter period of time (6 d). The roughage:concentrate ratio for the adaptation diets was 25:75 and 20:80, respectively, whereas the finishing diet had an 8.5:91.5 ratio. The entire composition and nutritional profile of the diets are reported in Table 1. Corn was processed through a hammer mill (Indústria e Comercial Lucato, Limeira, SP, Brazil) to achieve a mean particle size of 1.93 mm (Table 2), according to procedures described by Yu et al. (1998), using sieves with 6.0, 3.5, 2.0, and 1.25-mm square pores (Produtest T model; Telastem Peneiras para Análises Ltda., São Paulo, SP, Brazil). Diets were formulated using NASEM (2016) to provide an average daily gain (ADG) of 1.5 kg during the experimental period.

Composition and nutritional profile of the diets offered during the experimental period^1,2

¹Experimental period lasted 119 d.

²ADAP-1: adaptation diet (step-up 1) offered for 7 d to CONT and MEG-14 and for 3 d to MEG-6; ADAP-2: adaptation diet (step-up 2) offered for 7 d to CONT and MEG-14 and for 3 d to MEG-6. FIN: finishing diet offered for all animals following or not an adaptation diet.

³Mineral-vintamin mix (MCassab Comércio e Indústria; São Paulo, SP, Brazil) contained 21.0% Ca, 2.0% Mg, 15.0% Na, 23.1% Cl, 750 ppm Cu, 2,000 ppm Mn, 3,000 ppm Zn, 16.7 ppm Co, 30 ppm I, 5 ppm Se, 115 IU Vitamin A, 14 IU Vitamin D3, 180 IU Vitamin E, and 1,500 ppm sodium monensin (Rumensin; Elanco Animal Health, São Paulo, SP, Brazil).

⁴Calculated according to Weiss et al. (1992) and chemical composition of ingredients.

⁵Estimated with the equations proposed by NRC (1996; Level 1) with the addition of ionophore (sodium monensin) and using the TDN values, which had been calculated with equation proposed by Weiss et al. (1992).

Corn grain particle size distribution

¹Corn retained on the 6 mm screen was determined in 20 randomly particles using a digital caliper. The residue retained in the bottom was assumed to have a mean particle size of 0.625 mm. Based on Yu et al. (1998).

On d 0, all bulls were individually identified with ear tags, vaccinated against clostridial (Covexin-9; MSD, São Paulo, Brazil) pathogens and dewormed with 1 mL/50 kg BW of an antiparasitic (Evol; Ouro Fino Saúde Animal, Cravinhos, SP, Brazil). Throughout the experimental period, diets were supplied once daily as a total mixed ration using a feed wagon (Rotormix-40; Casale Equipamentos, São Carlos, SP, Brazil) with an electronic scale (ez3400VL; Digi Star, Fort Atkinson) and offered to ensure ad libitum intake and result in 3% orts. Additionally, all animals had full access to water and were maintained into open-sided paved pens with a coverall in the feed bunk (4.0 m of linear feed bunk for pens housing 6 animals and 5.0 m of linear feed bunk for pens housing 8 animals).