Evaluating the effect of dietary changes and management strategies to feedlot cattle during the adaptation and finishing phase on performance, carcass characteristics and the rumen microbiome

The primary goal of feedlot cattle nutrition is to provide the most appropriate feeds for cattle to gain body weight efficiently while minimizing cost. Typical strategies utilize diets containing high concentrations of starch to maximize energy intake. An adaptation period is necessary to allow the microbial community in the rumen to adjust from forage-based diets to high starch diets. These adaptation diets have greater inclusions of forages and are usually managed by a series of step-up diets. Once cattle are consuming the finishing diet, different processing methods of grains such as grinding and ensiling have been utilized to change the availability of starch to be digested. Different processing methods can also alter the site of starch digestion to some extent. The high starch concentrations in finishing diets can cause acidosis and liver abscesses that can decrease cattle efficiency and profitability. Although grains are the primary source of energy in most diets, producers commonly incorporate coproduct feeds that are high in easily degradable fiber to mitigate metabolic disorders while maintaining energy concentrations in the diet. Because of the minimal starch yet high energy present in coproducts, an experiment was conducted to examine feeding coproducts as an alternative to forage in adaptation diets and its interaction with different adaptation protocols. Simmental × Angus and Angus steers (N = 140; 237 ± 28 kg) were blocked by initial BW and allotted to 20 pens. Steers were fed a common receiving diet for the first 9 d after weaning. Pens were randomly assigned to 1 of 2 dietary treatments: 1) decreasing coproduct inclusion (soybean hulls and modified wet distillers grains) while increasing dry-rolled corn over 36 d or 2) decreasing forage inclusion (alfalfa and grass hay) while increasing dry-rolled corn over 36 d. Pens were fed either 5 or 2 step-up diets for each dietary treatment during the 36 d adaptation period in a 2 × 2 factorial design. Steers were fed a common finishing diet for the remainder of the trial (160 d). Rumen fluid was collected on d 35 and 70 to determine the populations of bacteria. At d 36, steers fed the coproduct-based diets had a greater body weight (BW; 319 vs. 313 kg; P = 0.01) and average daily gain (ADG; 2.29 vs. 2.12 kg; P = 0.02) compared with steers fed the forage-based diets. Steers fed the coproduct-based diets with 2 step-up diets had the greatest overall ADG (P ≤ 0.05) and final BW (P ≤ 0.05) compared with all other treatments at d 196. Within the rumen microbiome, alpha diversity was greatest (P < 0.01) for steers fed forage over 2 steps at d 35, but only individual taxa changed from d 35 to 70. Hot carcass weight tended (P = 0.05) to be greater for steers fed the coproduct-based diets compared with forage-based diets (387 vs. 380 kg, respectively). Yield grade was greater (P = 0.01) for steers fed two step-up diets due to increased (P = 0.02) backfat thickness and smaller (P = 0.02) longissimus muscle areas. A second experiment was conducted to compare the effect of corn type on growth performance and indicators of gut permeability when fed with or without tylosin. Angus and Angus × Simmental heifers (N = 120; 188 ± 2.6 kg) were blocked by initial BW and alloted to 8 pens. Pens were randomly assigned to either be fed tylosin (T+) or no tylosin (T-) on d 0. Heifers were adapted to a common finishing diet over 35 d. On d 35, pens were then assigned to a high moisture corn-based diet (HMC) or a dry-rolled corn-based diet (DRC). The treatments were arranged in a 2 × 2 factorial design. Body weight was recorded, and blood was collected on d 0, 35, and approximately every 28 d following. Rumen fluid and fecal grab samples were collected on a subset (n = 48) every 56 d. Treatment did not affect (P ≥ 0.14) BW, DMI or ADG throughout the trial. On d 252, Heifers fed HMC had greater (P = 0.03) ruminal pH compared with DRC. A diet × tylosin × time interaction indicated that cholesterol was lowest (P ≤ 0.04) for HMC T+ heifers on d 175 and lower (P ≤ 0.01) than HMC T- and DRC T- on d 211. Heifers fed HMC had a larger (P = 0.04) longissimus muscle area than DRC. Heifers fed HMC T+ had the most (P = 0.04) desirable yield grade. In summary, utilizing coproducts during the adaptation period was an effective substitute for forage and increased BW and ADG through the finishing period regardless of number of steps utilized. Although adaptation strategy can alter the composition of the rumen microbiome, all treatments appeared to be well adapted to the finishing diet by d 35 without major differences at d 70. During the finishing period of the second experiment differences in ruminal starch digestion had little effect on performance while affecting ruminal pH and gut permeability. Overall, maximizing energy availability while reducing the risk of metabolic disorders improved cattle performance and liver function