Animals, housing, and diets

Thirty-two rumen-cannulated B. indicus Nellore steers [initial body weight (BW) = 220 ± 12.6 kg; age = 20 ± 1.0 mo] were assigned to individual pens (concrete-surface; 2 × 2 m, with a feed bunk and waterer) in a randomized complete block design according to their initial shrunk BW. Within block (n = 8), animals were randomly assigned to 1 of 4 treatments: (1) forage-based diet without feed additives (CON; n = 8), (2) CON diet plus 13 ppm of narasin (Zimprova; Elanco Animal Health, São Paulo, Brazil; NAR; n = 8), (3) CON diet plus 20 ppm of salinomycin (Posistac; Phibro Animal Health Corporation, Guarulhos, São Paulo, Brazil; SAL; n = 8), or (4) CON diet plus 3 ppm of flavomycin (Flavomycin 80, Huvepharma, Porto Alegre, Rio Grande do Sul, Brazil; FLA; n =8). The administration rates of NAR, SAL, and FLA used herein were according to manufacturer’s recommendation. The experimental period lasted 140 d and was divided into 5 periods of 28 d each (0, 28, 56, 84, 112, and 140 d).

Throughout the experimental period (days 0 to 140), steers were offered Tifton-85 haylage (Cynodon dactylon spp.), which was chopped daily with a vertical mixer (Mixer VM8B, DeLaval International AB, Tumba, Sweden). Haylage average particle length distribution was 50.3 ± 2.5% > 19 mm; 25.8 ± 3.2% >8 mm; 16.1 ± 1.8 % >4 mm; and 7.8 ± 2.0% on bottom sieve according to Penn State Particle Separator procedures (Heinrichs, 1996; Kononoff et al., 2003). Feed additives (NAR, SAL, and FLA) were separately mixed with a 50:50 mixture of ground citrus pulp:ground corn (CI:C; 25 g of each ingredient used as a delivery vehicle; as-fed basis). The initial inclusion of feed additive treatment in the 50:50 CI:C mixture was based on a 5.0 kg of forage DMI. Hence, for steers consuming 5.0 kg of forage, the CI:C mixture would contain 65, 100 and 15 ppm of narasin, salinomycin and flavomycin for NAR, SAL and FLA, respectively. Steers from CON group also received the CI:C supplement without the inclusion of feed additives.

Treatments (NAR, SAL, FLA, and CON) were offered to each pen individually and daily prior to haylage feeding to avoid that small amount of supplement would be mixed with hay and compromise intake of the feed additives treatments. Treatment amount were calculated daily based on the previous day individual total forage dry matter intake (DMI). From days 0 to 140, animals were fed treatments once daily (0800 hours) and had ad libitum access to haylage (0830 hours), mineral-mixed (offered in separately feed bunk from the haylage and treatments), and fresh water. Steers promptly consumed treatments within 30 min after feeding. The mineral mix (Premiphós 80; Premix; Ribeirão Preto, SP, Brazil) used herein contained 150 g/kg Ca, 80 g/kg P, 12 g/kg S, 134 g/kg Na, 4,500 mg/kg Zn 1,600 mg/kg, 1,400 mg/kg Mn, 800 mg/kg F, 210 mg/kg Co, 180 mg/kg I, and 27 mg/kg Se. The nutritional profile of the haylage and supplement used herein is described in Table 1.

Nutritional profile of the Tifton-85 (Cynodon dactylon spp.) haylage and ground citrus pulp and ground corn (GC) mixed used in experiment 1¹

¹Based on nutritional profile of each ingredient, which were analyzed via wet chemistry procedures (AOAC, 1990).

²The experimental period lasted 140 d and was divided into 5 periods of 28 d each.

³CI:C: 50% ground citrus pulp dry and 50% GC.

⁴Calculations were performed according to the equations proposed by Weiss et al. (1992).

⁵Calculated composition using tabular values from NASEM (2016).