Experiment 2: animal performance

One hundred and sixty-four B. indicus Nellore bulls (initial shrunk BW = 299 ± 2.5; age = 23 ± 3.0 mo) were assigned to pens in a randomized complete block design according to their shrunk BW (after 14 hr of feed and water restriction). The experimental period lasted 140 d, divided into 5 periods of 28 d each. Bulls were kept in a covered feedlot (10 pens per treatment; 4 to 5 bulls per pen; 3 × 6 m) with a concrete floor, feed bunk, mineral bunk, and waterer. Within blocks (n = 10), animals were randomly assigned to the same treatments as in experiment 1.

Throughout the experimental period (days 0 to 140), bulls were offered Tifton-85 haylage (Cynodon dactylon spp.) which was chopped daily utilizing a vertical mixer (Mixer VM8B, DeLaval International AB). Haylage average particle length distribution was 46.7 ± 3.1 % > 19 mm, 28.1 ± 2.1% > 8 mm, 15.2 ± 2.0 % > 4 mm, and 10 ± 3.8% on bottom sieve according to Penn State Particle Separator procedures (Heinrichs, 1996; Kononoff et al., 2003), whereas ground corn was used as a delivery vehicle for feed additives treatments (NAR, SAL, and FLA). Additionally, animals from CON group also received ground corn with no inclusion of feed additives. Feed additives were mixed into ground corn (200 g/pen for each 5 kg of haylage DMI; as-fed basis) and offered to each pen individually. Bulls promptly consumed the supplement within 30 min after feeding and then the haylage was offered. Treatments were offered daily prior to haylage feeding to avoid that the small amount of concentrate would be mixed with the hay and compromise the intake of feed additives. The nutritional profile of the forage used in the present experiment is described in Table 2.

Nutritional profile of the Tifton-85 (Cynodon dactylon spp.) haylage and GC used in experiment 2¹

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

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

³Calculations were performed according to the equations proposed by Weiss et al. (1992).

⁴Calculated composition using tabular values from NASEM (2016).

From days 0 to 140, animals were fed the treatments (ground corn with or without feed additives) once daily at 0730 hours and had ad libitum access to haylage (0800 hours), mineral–vitamin mix, and fresh water. Mineral mix (Premiphós 80; Premix) used herein was the same as in experiment 1 and was offered separately in feed bunk from haylage and treatments. The initial inclusion of additives in the ground corn was based on a 5.0 kg of forage DMI. Hence, for animals consuming 5.0 kg of forage, the ground corn would contain 65, 100, and 15 ppm of narasin, salinomycin, and flavomycin, for NAR, SAL, and FLA, respectively. The doses of NAR, SAL, and FLA used herein were according to manufacturer’s recommendation. Throughout the experimental period (days 0 to 140), additives dosage offered to the animals was based on the previous day total DMI.

At the beginning (day 0) of the experimental period, individual shrunk BW was recorded after 14 hr of feed and water withdrawal to determine animal initial BW and to perform the randomization of the animals into blocks and treatments. To calculate average daily gain (ADG) and feed efficiency (G:F), bulls were individually weighed on days 0, 28, 56, 84, 112, and 140 (final days of each period) after 14 hr of feed and water restriction. DMI was evaluated daily from each pen within each period by collecting and weighing nonconsumed feed weekly. Hay and total DMI of each pen were divided by the number of bulls within each pen and expressed as kilogram per bull/day. Within each pen, total BW gain and total DMI of each period were used for bull G:F calculation. Samples of feed and orts were collected weekly, pooled across all weeks within each period, and analyzed for nutrient profile as aforementioned for experiment 1.