Fiber inclusion in feedlot diets

Do we need less starch later in the finishing period and how important is the fiber source?

By Anna Kobza, Daniel Young, Ty Lawrence, John Richeson and Kendall SamuelsonRoughage is a proportionately small but important component of feedlot cattle finishing diets, as it is essential for healthy rumen function. Although it is beneficial to ruminal health, roughage is typically fed at low concentrations in feedlot diets for economic and logistical reasons. Greater roughage concentrations are also commonly associated with decreased dietary energy concentrations that lead to reduced growth performance despite increased dry matter intake (DMI; Galyean and Defoor, 2003; Jennings et al. 2020). The roughage contribution of fibrous co-products such as wet distillers grains with solubles (WDGS), wet corn gluten feed, and Sweet Bran (Cargill Corn Milling; Blair, Nebraska) is not well defined for feedlot cattle and can be difficult to interpret because of differences in the nutrient concentrations and physical characteristics of these feedstuffs. Fibrous co-products contain greater energy concentrations than traditional roughage sources, less starch than processed grains, and may further stimulate DMI because of differences in particle size and/or fermentability (Galyean and Hubbert, 2014). Therefore, there may be opportunity to use fibrous co-products in place of a traditional roughage source to reduce dietary starch while maintaining the cost of gain, particularly at the end of the feeding period when feed efficiency declines and use of management practices that may influence intake patterns are common. This study evaluated the effects of starch dilution with different fiber sources from terminal implant to slaughter on growth performance and carcass characteristics of feedlot steers.Materials and methods
In November 2021, 416 crossbred steers were received at the West Texas A&M University Research Feedlot (Canyon, Texas), weighed individually over two consecutive days, and stratified into blocks based on body weight (BW). All steers received a growth-promoting implant (Revalor XS; 40 mg estradiol and 200 mg trenbolone acetate, Merck Animal Health, Summit, New Jersey) before study initiation. On d 0 (initial), steers were sorted into 48 treatment pens in 12 blocks with 8 or 9 steers per pen. Pens of cattle were randomly assigned to receive one of four dietary treatments (12 pens per treatment; Table 1). Treatments were steam-flaked corn-based feedlot diets:

CON; 7.50 % corn stalks on a DM basis fed for the entire feeding period

CS;14.75% corn stalks on a DM basis from terminal implant to slaughter

WD; 9.50% WDGS and 7.50% corn stalks on a DM basis from terminal implant to slaughter

NR; 19.00% WDGS, and no corn stalks on a DM basis from terminal implant to slaughter

The CS, WD and NR diets supplied less total starch than CON. The CS, WD and NR diets contained similar total starch to evaluate the impacts of altering fiber source and concentration on a starch equivalent basis. All four diets were formulated to contain similar concentrations of net energy for maintenance (NEm) and gain (NEg) using corn oil as a source of added fat. Dietary treatments were assigned after steers were individually weighed on d-1 and d 0 (initial BW), but all steers received a common finishing diet for the first 103 days of the study. After this initial feeding period, steers were weighed individually (transition BW), then transitioned to CS, WD or NR over 6 d using a two-ration system. At the end of the dietary transition (100% of treatment diet fed), steers were administered a terminal growth-promoting implant (Revalor-200; 200 mg of trenbolone acetate and 20 mg of estradiol, Merck Animal Health) and withheld from feed and water in sorting pens for 2 h. Steers assigned to receive CON consumed the same dietary treatment from d 0 until slaughter (initial to final). Feed bunks were managed using a “slick bunk” system and steers were fed once per day. Daily feed offered was recorded and diet samples were collected twice per week for analysis of dry matter (DM) and nutrient concentrations. Feed refusals were collected as needed, analyzed for DM concentration, and used to calculate DMI. Hot carcass weights (HCW) and liver scores were recorded at slaughter and individual carcass measurements were recorded after a 24 to 48 h chill. Treatment differences were considered statistically significant when P ≤ 0.10 and a tendency when 0.10 > P ≤ 0.15.Results
Before dietary treatments were fed, no differences were observed in transition BW (P = 0.17) or DMI (P = 0.93) and average daily gain (ADG; P = 0.34) from initial to transition (Table 2). Final BW did not differ among treatments (P = 0.21). However, from transition to final, both DMI (P < 0.01) and metabolizable energy (ME) intake (P < 0.01) were greatest for steers fed CS, intermediate for WD and CON, and least for NR. When evaluated over the entire feeding period (initial to final), DMI and ME intake were greater (P < 0.01) for CON, CS, and WD than NR. The average daily gain (ADG) from transition to final did not differ (P = 0.18) among steers fed CON, CS, WD, and NR; however, feed:gain (F:G) was greatest for CON and CS, intermediate for WD, and least for NR (P = 0.10). From initial to final, ADG was not different (P = 0.18) among treatments, whereas F:G tended (P ≥ 0.14) to be greater for CON, CS, and WD than NR. From transition to final, the feed cost of gain was greatest for CS, intermediate for CON, and least for WD and NR (P < 0.01). From initial to final, the feed cost of gain was greater for CON, CS, and WD than NR (P < 0.01). No differences (P ≥ 0.23) were observed among treatments for HCW, dressed yield, marbling score, or ribeye area (Table 3). The 12^th rib s.c. fat thickness was greater (P = 0.08) for CS than CON, WD and NR. Calculated yield grade, quality grade, or the proportion of abscessed livers did not differ among steers receiving CON, CS, WD and NR (P ≥ 0.26). Implications
These results suggest that in general, decreasing the dietary starch concentration post-terminal implant does not affect feedlot cattle performance. However, the concentration and source of dietary fiber plays an important role in control of DMI, growth performance and is an important economic consideration for feedlot cattle diets. In the current study, using WDGS as the primary fiber source resulted in improved F:G and the lowest feed cost of gain. However, additional research is needed to determine how using different fiber sources to decrease dietary starch influences feedlot cattle performance when fed over the entire feeding period and interactions with rumen health. Acknowledgements
This research was supported by Cargill Branded Feeds (Blair, Nebraska). References
Galyean, M.L. and P. J. Defoor. 2003. Effects of roughage source and level on intake by feedlot cattle. J. Anim. Sci. 81:E8–E16. Galyean, M. L. and M. E. Hubbert. 2014. Traditional and alternative sources of fiber-roughage values, effectiveness, and levels in starting and finishing diets. Prof. Anim. Sci. 30:571- 584. Jennings, J., C. Lockard, L. Tedeschi, and T. Lawrence. 2020. Effects of corn stalk inclusion rate of rumination and ruminal pH in finishing beef steers. Appl. Anim. Sci. 36:377-388. NASEM (National Academies of Science, Engineering, and Medicine). 2000. Nutrient Requirements of Beef Cattle. 7^th rev. ed. Natl. Acad. Press, Washington, D. C

Kobza and Young are graduate students, Lawrence and Richeson are professors, and Samuelson is an associate professor, all with West Texas A&M University.