3.1. Experiment 1
Initial BW (
p = 0.43;
Table 2) was not influenced by treatment, based on allocation. Intakes were impacted by treatment (
p < 0.01) with steers fed DODGS having the greatest DMI (10.8 kg) and all other treatments being similar in DMI. Dietary treatment tended to impact ADG (
p < 0.06), with DODGS and DODGS + Oil having the greatest ADG (1.68 and 1.65 kg, respectively); FFDGS-fed cattle had intermediate ADG (1.61 kg), while CON was lowest (1.52 kg). Jolly-Breithaupt et al. [
2] observed greater ADG for cattle fed de-oiled WDGS compared to full-fat WDGS (
p < 0.01). The results for DMI and ADG from the current experiment do not agree with Bremer [
17] who observed no difference in DMI or ADG between de-oiled and full-fat MDGS. Vander Pol et al. [
3] observed a decrease in DMI and ADG as supplemental corn oil was added to a corn-based diet. As a result of increased ADG, G:F was impacted by treatment (
p < 0.01), with DODGS + Oil steers having the greatest G:F, although not differing from the FFDGS treatment, and CON steers having the lowest G:F. Feed efficiency was improved by 1.2% for steers fed the FFDGS treatment compared to the DODGS treatment. Bremer [
17] reported that cattle fed full-fat MDGS at 30% of diet DM were 3.4% more efficient than steers fed 30% de-oiled MDGS. In the current experiment, when 2% corn oil was added back to de-oiled MDGS (DODGS + Oil), there was a 4.9% improvement in G:F compared to DODGS. The feeding value of DODGS was calculated to be 115% of corn and FFDGS was 119% the feeding value of corn. These results differ from Jolly-Breithaupt et al. [
2] who reported a feeding value of 130% of corn for both de-oiled and normal-fat MDGS at 40% diet DM inclusion. Bremer [
17] observed a feeding value of 117% of corn for de-oiled MDGS at 30% diet DM inclusion, which is similar to the current experiment; however, they observed 129% as the feeding value of corn for full-fat MDGS, which is greater than the current experiment. Dietary treatment impacted NEm and NEg (
p < 0.01), with DODGS + Oil having the greatest NE values and CON having the lowest (
Table 2). Bremer [
17] also reported tendencies for NEm and NEg to be greater for full-fat MDGS compared to de-oiled MDGS at 30% diet DM inclusion level. However, the values in the current experiment were higher than what Bremer [
17] observed. There was no difference between the NEm value for DODGS + Oil when compared to FFDGS, but DODGS + Oil had greater NEg than all other treatments.
Hot carcass weight (HCW) differed between treatments (
p = 0.05) with steers on DODGS treatment having the heaviest HCW (406 kg) though not much different from DODGS + Oil (404 kg) or FFDGS (401 kg), and steers in the CON treatment having the lightest HCW (
Table 2). Dietary treatment did not impact marbling score (
p = 0.64) or LM area (
p = 0.52), with steers averaging a marbling score of 459, qualifying for USDA choice quality grade, and LM area of 87.8 cm
2. Dietary treatment impacted 12th rib fat thickness (
p = 0.01) and calculated YG (
p = 0.02), where the three treatments that included MDGS (DODGS; DODGS + Oil; FFDGS) had similar fat thickness and calculated YG and were greater than that of the corn CON. The increase in calculated YG for all three treatments that included MDGS in the diet was observed due to the increased 12th rib fat thickness and heavier HCW while no difference was observed in LM area compared to CON treatment. Jolly-Breithaupt et al. [
2] and Bremer [
17] reported that carcass characteristics were not impacted by the removal of oil from distillers grains compared to full-fat distillers grains plus solubles.
3.2. Experiment 2
Dry matter intake was not impacted by dietary treatment (
p = 0.94;
Table 3) with intakes ranging from 8.9 kg/d for CON to 9.4 kg/d for FFDGS. These results differed from Vander Pol et al. [
3] and Bremer et al. [
18], who observed a reduction in DMI when corn oil was added to feedlot diets. Intakes observed in Exp. 2 were numerically lower than what was observed in Exp. 1 (8.9 vs. 10.3 kg for CON, 9.3 vs. 10.8 kg for DODGS, 9.0 vs. 10.0 kg for DODGS + Oil, 9.4 vs. 10.2 kg for FFDGS). Dietary treatment had an impact on total tract DM digestibility (
p < 0.01), where the greatest DM digestibility was observed for CON treatment and differed from all other dietary treatments. Total tract DM digestibility results agree with Corrigan et al. [
19], Vander Pol et al. [
3], and Bremer et al. [
18], who reported that including DGS in the diet reduced DM digestibility when compared to feeding corn control diets with no DGS. They also noted that DM digestibility is reduced when corn oil is added to the diet. Results of OM intake (kg/d) and total tract digestibility followed the same trend as DM, where OM intake did not differ between dietary treatments (
p = 0.96) and total tract OM digestibility differed between treatments (
p < 0.01) with the corn CON treatment having the greatest OM digestibility. Jolly-Breithaupt et al. [
2] compared de-oiled MDGS and normal-fat MDGS and reported no differences in DM or OM intake and total tract digestibility.
A treatment effect was observed for NDF intake (
p < 0.01), with treatments containing MDGS (FFDGS; DODGS; DODGS + Oil) having greater NDF intake than CON (
Table 3). Greater NDF intake is due to greater NDF concentration in diets that contain DGS as DGS are approximately 33.7% NDF on a DM basis while dry-rolled corn is approximately 9.7% on a DM basis [
16]. There was a tendency (
p = 0.07) for total tract NDF digestibility to differ among treatments, with the greatest NDF digestibility observed for FFDGS and the lowest for CON and DODGS + Oil, with DODGS being intermediate and similar to all other treatments. Corrigan et al. [
19] and Bremer et al. [
18] also reported lower NDF digestibility for corn control treatments when compared to dietary treatments that included MDGS. Vander Pol et al. [
3] observed that total tract DM, OM, and NDF digestibility were less (
p < 0.10) for cattle fed a composite diet consisting of corn bran and corn gluten meal and a composite diet plus oil when compared to WDGS, a corn control diet, and a corn control diet plus oil. Vander Pol et al. [
3] concluded that equal amounts of fat provided from WDGS or corn oil do not result in similar impacts on NDF digestion, with added oil having a negative impact on NDF digestion compared to oil that is naturally present in distillers grains. Bremer et al. [
18] fed diets containing 8.5% lipid inclusion from varying fat sources and concluded that diets containing distillers grains that supplied lipid up to 8% of diet DM can be fed to cattle without having a negative impact on performance; however, 8% dietary lipid in the form of corn oil will have a negative impact on cattle performance. Digestion data from OM and DE are not consistent with the observed performance in Exp. 1 between full-fat MDGS (FFDGS) and adding corn oil back to de-oiled MDGS (DODGS + Oil). Results from the current experiment suggest that corn oil has a negative impact on NDF digestibility, which could be because corn oil is considered a free oil, which is available for biohydrogenation, or the feed particles could be coated with oil [
20]. Free oil can therefore impact fiber digestion in the rumen, while the fat in DGS is bound in the germ, so DGS feedstuffs will pass through the rumen without negatively impacting rumen fiber digestion.
Dietary fat intake (Mcal/d) was different among dietary treatments (
p < 0.01), with DODGS + Oil being greatest, FFDGS being an intermediate and similar to both DODGS + Oil and DODGS, and CON being the least (
Table 3). There was no treatment effect observed for total tract fat digestibility (
p = 0.83), with an observed range of 81.1 to 83.3%. Bremer et al. [
18] observed values greater than 90% for total tract fat digestibility for treatments that contained varying fat sources, including corn oil, beef tallow, condensed corn distillers solubles, and WDGS. Jolly-Breithaupt et al. [
2] also reported values around 90% for total tract fat digestibility, observing no difference between de-oiled and normal-fat MDGS which agrees with the results from the current study.
Energy intake (Mcal/d) and digestible energy (DE; Mcal/d) were not impacted by treatment (
p ≥ 0.46;
Table 3). Energy intake ranged from 38.6 to 45.0 Mcal/d, while DE ranged from 30.97 to 34.31 Mcal/d. Digestible energy measured as Mcal/kg did not differ between treatments (
p = 0.13) and averaged 3.57 Mcal/kg. Wilson et al. [
21] reported a DE (Mcal/kg) value of 3.68 for a 40% WDGS diet, which was similar to the current experiment. Wilson et al. [
21] concluded that there was an additional supply of DE when diets contain DGS compared to a corn-based diet, which is likely due to the higher protein and fat content of DGS. The increased supply of DE in diets containing DGS is a new concept and has not been researched in detail. This concept could help explain the improvement in cattle performance from including DGS in the diet. The DE results from Exp. 2 do not match the performance results from Exp. 1, where cattle fed DODGS + Oil were numerically the most efficient and had the greatest ADG.
Dietary NE values that were calculated from NASEM [
16] equations using DE from Exp. 2 were similar to values calculated from performance characteristics in Exp. 1 which used dietary NE equations from the NRC [
7], except for net energy values for the DODGS + Oil treatment (
Table 4). The difference observed between the NE values calculated from Exp. 1 and Exp. 2 for the DODGS + Oil treatment are proposed to be due to a difference in the previously assumed 82% conversion of DE to ME. Through back calculations, a DE to ME conversion of 87% was calculated for the DODGS + Oil treatment. Although NE values from Exp. 1 are assumed to be more correct than values from Exp. 2, there is confidence that DE values from Exp. 2 are correct due to the similarity between NE values for CON, DODGS, and FFDGS dietary treatments.
Dietary treatment did not impact ruminal pH average, maximum, minimum, or magnitude of change (
p ≥ 0.14;
Table 5). Jolly-Breithaupt et al. [
2] observed an increase in average ruminal pH for cattle fed normal-fat MDGS compared to de-oiled MDGS. Bremer et al. [
18] reported that ruminal pH was greatest for cattle fed a diet containing corn oil, which was similar to the numerical results observed in the current study.
The total VFA production rate (mM/hr) was impacted by dietary treatment (
p < 0.01), with DODGS having the greatest total VFA production differing from the other three treatments (
Table 6). The production rate of acetate and butyrate did not differ among treatments (
p ≥ 0.40). Propionate production was greatest for DODGS (
p < 0.01), intermediate for CON and DODGS + Oil, and lowest for FFDGS. The total VFA production agrees with observed pH data, where DODGS + Oil and FFDGS had numerically greater pH and a lower rate of production, while CON and DODGS had numerically lower pH with a greater rate of VFA production. There were no hour × treatment interactions for molar proportion of VFA (
p ≥ 0.96). The molar proportion of acetate was impacted by dietary treatment (
p < 0.01) with FFDGS having the greatest acetate molar production, DODGS and DODGS + Oil having the least, with CON as an intermediate and similar to all other treatments. There was a tendency for the molar proportion of propionate (
p = 0.06) to differ between dietary treatments, with DODGS and DODGS + Oil having the greatest molar propionate production, and FFDGS having the least. There was no dietary treatment effect observed for molar proportion of butyrate (
p = 0.57). The acetate/propionate (A:P) molar proportion tended to differ between dietary treatments (
p = 0.07) and was greatest for the cattle fed FFDGS and least for the cattle fed DODGS and DODGS + Oil. Similar to the results of the current experiment, Carlson [
22] tested a corn-based control compared to a diet with 40% inclusion of MDGS on a DM basis and observed no differences in VFA molar proportion. Ham et al. [
23] tested a DRC-based control diet compared to a diet with 40% WDGS on a DM basis and observed no difference in the amounts (mM) of acetate, propionate, butyrate, or A:P.
Total gas production was impacted by dietary treatment (p = 0.02), with FFDGS and DODGS + Oil having the greatest total gas production, DODGS having the least total gas production, and CON as an intermediate and similar to all treatments. The rate of gas production (%/hr) was impacted by dietary treatment (p = 0.03), with DODGS + Oil having the greatest rate of gas production, DODGS and FFDGS having the least total gas production, and CON as an intermediate and similar to all other treatments.