Granulated Cane Sugar as a Partial Replacement for Steam-Flaked Corn in Diets for Feedlot Cattle: Ruminal Fermentation and Microbial Protein Synthesis

Abstract

The aim of this study was to evaluate the influence of supplemental granulated cane sugar (GCS) levels (0, 13.3, 26.6, and 39.9% on a dry matter basis) in a steam-flaked corn-based finishing diet on measures of ruminal fermentation and the site and extent of nutrient digestion. Four Holstein steers (251 ± 3.6 kg live weight) with “T” type cannulas in the rumen and proximal duodenum were used in a 4 × 4 Latin square experiment to evaluate the treatments. The experiment lasted 84 d. Replacing steam-flaked corn (SFC) with GCS linearly decreased the flow of ammonia-N (NH3-N) to the small intestine, increasing the flow of microbial nitrogen (MN; quadratic effect, p = 0.02), ruminal N efficiency (linear effect, p = 0.03) and MN efficiency (quadratic effect, p = 0.04). The ruminal digestion of starch and neutral detergent fiber (NDF) decreased (linear effect, p ≤ 0.02) as the level of GCS increased. The postruminal digestion of organic matter (OM), neutral detergent fiber (NDF), and starch were not affected by the GCS inclusion. However, postruminal N digestion decreased (linear effect, p = 0.02) as the level of GCS increased. There were no treatment effects on total tract OM digestion. However, total tract NDF and N digestion decreased (linear effect, p ≤ 0.02) as the level of GCS increased. The ruminal pH decreased (linear effect, p < 0.01) as the GCS increased in the diet. The ruminal acetate molar proportion decreased (linear effect, p = 0.02) and the ruminal valerate molar proportion tended to increase (linear effect, p = 0.08) as the level of GCS increased. It is concluded that replacing as much as 13% of SFC with GCS in a finishing diet will enhance the efficiency of N utilization (g non-ammonia-N entering the small intestine/g N intake) without detrimental effects on total tract OM digestion. The inclusion of GCS decreased the ruminal proportion of acetate linearly without an effect on the acetate-to-propionate ratio or estimated methane production. Some of the effects on N utilization at a high level of GCS inclusion (27 and 40%) can be magnified by the differences in the CP content between diets. A higher level of GCS supplementation in the diet decreased the ruminal pH below 5.5, increasing the risk of ruminal acidosis.

1. Introduction

Granulated sugar (GS) is beet or cane sugar that has been processed, allowed to crystallize, and then dried so that the crystals do not clump together. The United States is the fifth largest producer of sugar in the world (approximately 80% from cane and 20% from beet). Raw cane sugar contains 94–98.5% sucrose, and 1.5–6% non-sucrose components such as reducing sugars, organic acids, amino acids, proteins, starch, gums, coloring matter, and other suspended matter [1]. GS has been used as a sweetener to enhance diet acceptability and feed intake in ruminants [2]. The metabolizable energy value of GS is around 93% of its gross energy value (3.85/4.10 Mcal/kg) [3], comparable to that of starch. The type of sugars (sucrose, maltose, galactose, and glucose) and the rate of hydrolysis affect the ruminal environment and VFA patterns [4]. Ruminally, sucrose is more readily soluble and hydrolysable than starch [5], resulting in a greater rate of fermentation [6] and potentially enhancing the transformation of rapidly degradable ruminal feed-N (i.e., Urea-N) into microbial protein (MP, synchrony with a rapid initial release of N during initial stages of postprandial fermentation). The response to moderate levels of sucrose supplementation (~8%) in dairy cows fed with a 40:60 forage-to-concentrate ratio diet decreased the ruminal ammonia concentration [7,8]. Reductions in ruminal NH3-N concentration are often associated with increased efficiency of dietary N conversion into MP [9]. With a forage-based substrate (bermudagrass hay), sucrose supplementation increased the MP synthesis in vitro [5]. Likewise, Huhtanen [10] observed an increased flow of microbial N to the small intestine with an increasing level of sucrose infusion (0, 450, and 900 g sucrose/d) into the rumen of cattle (live weight = 240 kg) fed a grass silage and barley grain diet (63:34 silage-to-barley ratio). Most studies evaluating the effects of supplemental sugars on ruminal digestion and fermentation involved isoprotein diets (~60:40 concentrate-to-forage ratio) offered to dairy cows [7,8], or the ruminal infusion of sugars to animals receiving the same forage-based diet [10,11]. On the other hand, replacing steam-flaked corn (SFC) with feed ingredients rich in sugars (i.e., sugar beets) increased the MP flow to the duodenum in feedlot cattle [12], and numerically increased (9.8%) the MP synthesis in lactating dairy cows [13]. Replacing corn with sugar beets did not greatly affect the diet composition (similar CP and NDF concentration between corn diets and sugar beet diets) [12]. This is not the case when sugar, at moderate to high levels directly replaces corn, as granulated sugar does not contain CP or NDF. Compositional and solubility differences between corn and sucrose are expected to affect the rate and extent of ruminal OM digestion, ruminal pH, and VFA patterns. Due to the rate of fermentation and intermediate products during sucrose degradation and utilization [4], we hypothesized that at some optimal level, sucrose supplementation as a partial replacement for starch (corn grain) might enhance ruminal microbial protein synthesis in the growing-finishing feedlot cattle. There is no information available regarding the effect of directly replacing corn (the principal cereal grain used in diet formulations for feedlot cattle in Mexico and North America) [14,15] with sugar on nutrient digestion, ruminal fermentation, and microbial protein synthesis. For this reason, the aim of this experiment was to evaluate the effects of different inclusion levels (0, 13.3, 26.6, and 39.9% in diet) of granulated cane sugar (GCS) as a partial replacement for SFC in finishing diets for feedlot cattle on the characteristics of site and extent of digestion, ruminal fermentation, and microbial protein synthesis.

2. Materials and Methods

The experiment was conducted at the University of California Desert Research & Extension Center (UC Davis), located in El Centro, California, USA (32°47′31″ N and 115°33′47″ W). El Centro is about −12 m below sea level and has a desert climate. All procedures involving animal care, surgery, and management were in accordance with and approved by the University of California, Davis, Animal Use and Care Committee (protocol #22362).

2.1. Animals, Experimental Design, and Dietary Treatments

Four Holstein steers (251 ± 3.6 kg) were used in a 4 × 4 Latin square experiment to study the treatment effects of the level of GCS addition to the finishing diet on characteristics of digestion, ruminal fermentation, and microbial protein synthesis. The steers were fitted with a 3.8 cm i.d. ruminal Tygon “T” cannula and a 1.9 cm i.d. Tygon “T” duodenal cannula, as have been described by Zinn and Plascencia [16]. Briefly, the steers fasted for 16 h before surgery. The steers were then intramuscularly given 0.25 mg/kg BW of xylazine (Rompun; Elanco, US Inc., Greenfield, IN, USA). When a sufficient depth of sedation was achieved, the steers were placed in left lateral recumbency and the surgical sites were shaved. The surgical sites were anesthetized by tissue infiltration using xylocaine. Using sterile techniques, a paracostal laparotomy (10 cm) was performed to gain entrance into the peritoneal cavity. A Tygon “T” cannula (1.9 cm i.d.) was inserted in the proximal duodenum (15 cm from the pyloric sphincter) and secured in the intestine with a purse-string suture about the barrel of the cannula. The cannula was then exteriorized through a stab incision approximately 4 cm above the original incision and perpendicular to the flexure of the 13th rib. The ruminal cannula (3.8 cm i.d. Tygon “T” cannula) was inserted through a stab incision in the region of the left paralumbar fossa, approximately 4 cm below the transverse processes of the lumbar vertebrae and 4 cm behind the costal arch. The steers were housed in individual pens (5.6 m²) in an indoor facility with a controlled climate. The pens had a concrete floor covered by a neoprene carpet, automatic waterers, and individual feed bunks. Dietary treatments consisted of an SFC-based finishing diet containing 0, 13.3, 26.6, or 39.9% GCS (DM basis) as a replacement for SFC (

Table 1. Composition of dietary treatments.

	Granulated Cane Sugar Level (%)
Item	0	13.3	26.6	39.9
Ingredient composition (% DM basis)
Steam-Flaked corn	59.68	46.38	33.08	19.78
Granulated cane sugar	0.00	13.30	26.60	39.90
Distillers grains plus solubles	17.58	17.58	17.58	17.58
Yellow grease	3.60	3.60	3.60	3.60
Cane molasses	5.78	5.78	5.78	5.78
Sudangrass hay	3.60	3.60	3.60	3.60
Alfalfa hay	7.20	7.20	7.20	7.20
Urea	0.72	0.72	0.72	0.72
Trace mineralized salt 1	0.33	0.33	0.33	0.33
Limestone	1.02	1.02	1.02	1.02
Magnesium oxide	0.14	0.14	0.14	0.14
Chromic oxide	0.35	0.35	0.35	0.35
Nutrient composition (DM basis) 2
Net energy (Mcal/kg)
Maintenance	2.26	2.27	2.27	2.28
Gain	1.58	1.58	1.58	1.58
Crude protein (%)	14.93	13.67	12.40	11.13
Ether extract (%)	8.22	7.65	7.08	6.50
Neutral detergent fiber (%)	18.86	17.66	16.46	15.26
Calcium (%)	0.64	0.64	0.63	0.63
Phosphorous (%)	0.37	0.32	0.28	0.24
Potassium (%)	0.89	0.84	0.80	0.75
Magnesium (%)	0.28	0.26	0.25	0.24

¹ Trace mineral salt contained: CoSO₄, 0.068%; CuSO₄, 1.04%; FeSO₄, 3.57%; ZnO, 0.75%; MnSO₄, 1.07%; KI, 0.052%; and NaCl, 93.4%. ² Based on tabular values for individual feed ingredients [17].

).

Chromic oxide (0.35% of the diet DM) was used as an indigestible marker for the estimation of nutrient flow and digestion. Chromic oxide was premixed with minor ingredients (urea and mineral supplements) before incorporation into the complete mixed diets. In order to adapt the steers to the high-grain diet and determine the ad libitum intake of each steer, all steers were given ad libitum access to the diet without GSC inclusion for 21 days before the initiation of the trial. The steers were fed at 08:00 and 20:00 h daily. In order to avoid feed refusals, DMI was restricted to 90% of ad libitum intake during the last 7 d of the adaptation period [average of 5.6 kg/d, (as feed basis) equivalent to 2.23% of LW daily]. Experimental periods were 21 d, with 17 d for dietary treatment adjustment, and 4 d for collection.

2.2. Sampling

During collection, duodenal and fecal samples were taken twice daily as follows: d 1, 07:50 and 13:50 h; d 2, 09:00 and 15:00 h; d 3, 10:50 and 16:50 h, and d 4, 12:00 and 18:00 h. Individual samples consisted of approximately 700 mL of duodenal chyme and 200 g (wet basis) of fecal material. Samples from each steer within each collection period were composited for analysis. During the final day of each collection period, ruminal samples were obtained from each steer via the ruminal cannula 4 h after feeding. The ruminal fluid pH was determined on fresh samples. Samples were strained through four layers of cheesecloth. Two milliliters of freshly prepared 25% (wt/vol) metaphosphoric acid were added to 8 mL of strained ruminal fluid. The samples were then centrifuged (17,000× g for 10 min), and the supernatant fluid was stored at −20 °C for VFA analysis. Upon completion of the experiment, ruminal fluid was obtained via the ruminal cannula from all steers and composited for the isolation of ruminal bacteria by differential centrifugation [18], as follows: (1) the ruminal fluid was diluted 50:50 with 0.16 N saline (37 °C), agitated gently for about 30 sec and then strained through four layers of cheesecloth; (2) the strained fluid was promptly transferred into centrifuge bottles and spun at 2000× g for 10 min at 10 °C; (3) the supernate was decanted and centrifuged at 43,000× g for 20 min at 10 °C, and (4) the supernate was then decanted and the pellet isolated, oven-dried (70 °C) and then ground with a mortar and pestle. The microbial isolate served as the purine: N reference for the estimation of microbial N contribution to chyme entering the small intestine [19].

2.3. Samples Analyses

Dried (55 °C) and grounded (1 mm sieve) feed, duodenal and fecal samples were subject to the following analyses: dry matter (oven drying at 105 °C until no further weight loss; method 930.15); ash (method 942.05), and Kjeldahl N (method 984.13) according to AOAC [20]. Neutral detergent fiber [NDF, corrected for NDF-ash, incorporating heat stable α-amylase (Ankom Technology, Macedon, NY, USA) at 1 mL per 100 mL of NDF solution (Midland Scientific, Omaha, NE, USA)] was determined following the procedures described by Van Soest et al. [21]. Chromic oxide and starch were determined according to Hill and Anderson [22] and Zinn [23], respectively. Ammonia-N (NH3-N; method 941.04) [20] and purines [14] were determined on duodenal samples. VFA concentrations of the ruminal fluid were assessed by gas chromatography following procedures and equipment described by Zinn [24].

2.4. Calculations

Total DM flow to the duodenum and fecal DM excretion was estimated as follows: DM flow to the small intestine or fecal excretion, g/day = g Cr₂O₃ intake/Cr₂O₃ concentration of duodenal or fecal DM. The organic matter (OM) content of the duodenal feed and fecal samples was estimated as the DM concentration minus the ash content. Microbial organic matter (MOM) and microbial nitrogen (MN) leaving the abomasum were calculated using purines as microbial markers [19]. Organic matter fermented in the rumen (OMF) is considered equal to the OM intake minus the difference between the amount of total OM reaching the duodenum and the MOM reaching the duodenum. The feed-N escaping to the small intestine is considered equal to the total N leaving the abomasum minus ammonia-N and MN and, thus, includes any endogenous contributions. The ruminal microbial efficiency was estimated as the duodenal MN, g/kg of OM fermented in the rumen, and N efficiency represents the duodenal non-ammonia-N, g/g of N intake. Methane production was estimated based on the theoretical fermentation balance for the observed molar distribution of the VFA and the OM fermented in the rumen [25].

2.5. Statistical Analysis

Experimental data were analyzed as a replicated 4 × 4 Latin square according to the following statistical model:

Y_ijk = µ + S_i + P_j + T_k + E_ijk

where Y_ijk is the response variable, µ is the common experimental effect, S_i is the steer effect, P_j is the period effect, T_k is the treatment effect, and E_ijk is the residual error. Treatment effects were tested by means of orthogonal polynomials equally spaced. Least mean squares and standard error of the means are reported, and contrasts were considered significant when the p-value was ≤0.05, and tendencies are considered when the p-value was >0.05 and ≤0.10. Analysis was performed using Statistix^®10 (Analytical Software, Tallahassee, FL, USA).

3. Results

Cubic effects were not significant (p > 0.10). Thus, the p-values for those components are not presented in the

Table 2. Influence of granulated sugar inclusion on ruminal, postruminal, and total tract digestion in cannulated Holstein steers (251 kg LW).

		Granulated Cane Sugar Level (% DM Diet)				p-Value
Item	0	13.3	26.6	39.9	SEM	Linear	Quadratic
Intake (g/d)
Dry matter	4975	4975	4975	4975	7.28	0.24	0.78
Organic matter	4705	4705	4714	4718	6.97	0.18	0.78
Neutral detergent fiber	738	689	645	593	2.40	<0.01	0.92
Starch	2369	1858	1354	840	11.08	<0.01	0.91
Nitrogen	111	103	95	86	0.05	<0.01	0.34
Flow to duodenum (g/d)
Organic matter	2665	2744	2680	2713	98.6	0.87	0.84
Neutral detergent fiber	473	553	645	594	68.1	0.12	0.31
Starch	536	459	500	465	57.1	0.53	0.75
Nitrogen	127	133	120	120	4.45	0.10	0.55
NH3-N	4.04	3.71	2.96	2.58	0.57	<0.01	0.94
Non-ammonia-N	124	129	116	117	4.02	0.13	0.57
Microbial N	74.6	90.8	80.2	75.5	4.04	0.55	0.02
Feed-N	48.9	38.6	36.9	41.7	5.86	0.36	0.23
Ruminal digestion (% of intake)
Organic matter	59.24	60.96	60.07	58.52	2.66	0.81	0.58
Neutral detergent fiber	36.51	18.95	1.24	1.18	10.32	0.02	0.40
Starch	77.37	75.31	62.72	44.28	5.82	<0.01	0.24
Feed-N	56.01	62.59	61.20	51.59	5.84	0.55	0.20
MN efficiency 1	27.26	31.43	28.88	27.57	1.06	0.72	0.04
N efficiency 2	1.11	1.26	1.22	1.36	0.049	0.03	0.93
Postruminal digestion (% entering the duodenum)
Organic matter	59.94	60.64	60.13	61.20	2.48	0.79	0.85
Neutral detergent fiber	10.69	10.24	13.43	23.05	1.95	0.47	0.70
Starch	91.16	92.35	92.19	92.71	1.87	0.62	0.87
Nitrogen	72.74	73.30	69.14	69.45	0.83	0.02	0.89
Fecal excretion (g/d)
Dry matter	1166	1190	1162	1161	50.43	0.86	0.83
Organic matter	1054	1082	1061	1050	49.07	0.89	0.73
Neutral detergent fiber	368	431	443	454	38.87	0.18	0.55
Starch	47.65	33.57	39.92	31.39	9.88	0.39	0.80
Nitrogen	34.60	35.53	36.18	36.32	1.28	0.36	0.78
Total tract digestion (% of intake)
Dry matter	76.59	76.09	76.64	76.63	1.03	0.89	0.84
Organic matter	77.59	77.04	77.50	77.75	1.06	0.86	0.74
Neutral detergent fiber	49.44	37.04	30.01	23.09	5.39	0.02	0.65
Starch	97.98	98.19	96.93	96.23	0.59	0.06	0.50
Nitrogen	68.92	65.40	61.74	57.80	1.45	<0.01	0.90

¹ Microbial efficiency estimated as duodenal MN, g/kg OM truly fermented in the rumen; ² N efficiency estimated as duodenal nonammonia-N, g/g N intake.

and

Table 3. Influence of granulated sugar inclusion on ruminal pH, VFA concentration, and estimated methane production.

	Granulated Cane Sugar Level (% DM Diet)				p-Value
Item	0	13.3	26.6	39.9	SEM	Linear	Quadratic
Ruminal pH 1	5.89	6.10	5.55	5.20	0.14	<0.01	0.09
Total VFA, moles	70.18	60.92	68.28	67.90	5.67	0.98	0.46
Ruminal VFA (mol/100 mol)
Acetate	51.81	51.98	49.02	48.85	0.88	0.03	0.85
Propionate	35.25	33.87	35.91	36.46	1.80	0.51	0.61
Butyrate	8.67	8.76	9.37	8.91	0.59	0.65	0.69
Isobutyrate	0.66	0.64	0.42	0.95	0.03	0.66	0.42
Isovalerate	1.17	1.02	0.58	0.97	0.003	0.46	0.40
Valerate	2.42	3.72	4.72	3.84	0.005	0.08	0.10
Acetate:propionate ratio	1.47	1.53	1.36	1.33	0.11	0.33	0.74
Methane production 2	0.410	0.423	0.389	0.384	0.020	0.26	0.65

¹ Average of the ruminal samples taken at 4 h post-feeding; ² Methane, mol/mol of glucose equivalent fermented [25].

. The influence of GCS inclusion in diets on the characteristics of ruminal, postruminal, and total tract digestion are shown in Table 2. As was planned, there were no fed refusals during the experiment averaging 4.975 kg/d across all treatments. Because of the difference in chemical composition between GCS and SFC, increasing GCS in the diet decreased linearly (p < 0.01) the intake of N, NDF, and starch. Replacing SFC with GCS linearly decreased the flow of NH3-N to the small intestine, increasing quadratically the flow of microbial N (p = 0.02). Even though the ruminal digestion of starch and NDF decreased (linear, p ≤ 0.02) as GCS increased in the diet, the ruminal digestion of OM was not affected (p ≥ 0.58). The GCS inclusion improved the ruminal N efficiency (linear component, p = 0.03) and the microbial N efficiency (quadratic component, p = 0.04), being maximal at the 13.3% GCS inclusion level. The postruminal digestion of OM, NDF, and starch was not affected by the GCS inclusion. However, the postruminal N digestion decreased (linear effect, p = 0.02) with increased GCS levels. The lower ruminal digestion was observed for NDF and starch, and the lower postruminal digestion of N was reflected in a lower total tract digestion of N and NDF (p < 0.02). The treatment effects on ruminal pH, VFA molar proportions, and estimated methane production is shown in Table 3. The ruminal pH decreased (linear effect, p < 0.01) with increasing levels of GCS. The ruminal acetate molar proportion decreased (linear effect, p = 0.03) and valerate molar proportion tended to increase (linear effect, p = 0.08) with increasing levels of GCS. There were no treatment effects (p ≥ 0.26) on acetate levels, either on the propionate molar ratio or estimated methane production.

4. Discussion

Although low levels (<10%) of sucrose supplementation have been evaluated in dairy and beef cattle that are fed high-forage diets, very little research has been conducted evaluating sucrose addition which replaces steam-flaked corn for feedlot cattle that are fed high-grain finishing diets. Consistent with the present study, Chamberlain et al. [26] observed an increased microbial N flow to the small intestine and an increased ruminal microbial efficiency in lambs fed with grass silage supplemented with 4.8% sucrose. This effect is not a simple matter of increased ruminal substrate, as sucrose has been shown to be superior to starch as an energy source for microbial N fixation [26,27]. Ruminally, sucrose is more readily soluble and hydrolysable than starch [5], resulting in a greater rate of fermentation [6] and potentially enhancing the capture of rapidly degradable feed-N (i.e., Urea-N) into microbial protein (synchrony with the rapid initial release of N during the initial stages of postprandial fermentation). This effect is consistent with the linear decrease in ruminal NH3-N concentration with increasing sucrose addition. It is important to note that this decrease of NH3-N was not due to dietary differences in the CP among treatments, since the relative proportion of NH3-N flowing to the duodenum as a percentage of intake-N decreased from 3.65% for 0% GCS inclusion to 3.0% for 39.9% GCS inclusion. Likewise, Sannes et al. [7] and Broderick et al. [8] observed a decreased ruminal NH3-N concentration with the addition of up to 7.5% sucrose. The ruminal infusion of 1 kg of sucrose (equivalent to 18.8% of total DMI) in steers that are fed a silage-based diet increased the microbial N entering the small intestine, but it did not affect the efficiency of net ruminal microbial synthesis (g N kg⁻¹ OM apparently digested in the rumen) [28]. The increase in MN efficiency in the present experiment was noted only at 13.3% supplemental GCS. Increasing the GCS level beyond this led to a microbial efficiency similar to the control. The positive effect on the microbial protein flow to the duodenum with GCS supplemented at moderate levels (13%) could be of greater benefit to lightweight calves during the early growing phase when metabolizable protein is most likely to be deficient. Future research is needed to determine the role of GCS supplementation on growth performance and the feed efficiency of feedlot cattle during these early phases of growth.

Whereas the effect of sucrose addition had a quadratic effect on the microbial N efficiency, ruminal N efficiency (nonammonia-N, g/g N intake) increased linearly with increasing levels of GCS substitution for SFC. This additional effect of increasing sucrose level on ruminal N efficiency may be attributable to changes in the ruminal dilution rate (not measured in the present study). Huhtanen and Khalili [29] and Pfau et al. [30] observed an increased ruminal dilution rate with sucrose supplementation. In as much as the ruminal fluid dilution rate is negatively associated with ruminal starch digestion [31,32], the increased dilution rate with an increasing level of GCS substitution for SFC might explain the decreasing ruminal starch digestion observed in the present study.

The decrease in ruminal NDF digestion is consistent with the sucrose level effects on the ruminal pH (Table 3). Ruminal pH has been shown to be a primary limiting factor affecting fiber digestion [33]. In the present study, sucrose inclusion linearly decreased the ruminal pH from 5.89 to 5.29. However, with the inclusion of 13.3% GSC, the ruminal pH was not different (p > 0.10) from 0% inclusion levels. Therefore, soluble sugars such as sucrose may also inhibit ruminal NDF digestion via enzymatic interference. In vitro, the presence of soluble sugar in the media decreased the NDF rate of digestion even when the pH is maintained above 6.2 [34,35]. The effects of sucrose supplementation on NDF digestion have not been consistent. As with the present study sucrose supplementation has decreased NDF digestion both in vitro [5] and in vivo [36,37]. In other studies [8,12,38], the partial replacement of starch with sucrose did not affect ruminal and total tract NDF digestion, whereas even at moderate levels of inclusion (8%), sucrose increased ruminal NDF digestion [39,40]. The depression in fiber digestion with sucrose supplementation is more consistent when the ruminal pH is sufficiently depressed to affect the activity of the ruminal fibrolytic microbes. For example, Campos et al. [41] observed that the NDF digestion of a corn silage-based diet was only reduced when the addition of sugar was greater than 20% (DM basis). It might be expected that when the ruminal digestion of NDF and starch are reduced, a concomitant reduction of ruminal OM digestion would also occur. As previously mentioned, in the present study the ruminal digestion of starch and NDF decreased with increasing levels of GCS substitution for SFC. But the ruminal OM digestion was not affected. However, this was expected due to the ready fermentability of the sucrose, itself. It was estimated that the ruminal degradation rate of the sucrose was 8.5-fold greater than that of the incorporated grain starch [22].

There is no information about the effects of starch replacing sucrose on nutrient postruminal digestion. In the present study, only the postruminal N was affected by GSC supplementation. The negative effect of GCS on the postruminal digestion of N is surprising. A possible explanation is the higher ratio of dietary to microbial protein entering the intestine for the GSC treatments (the ratio of dietary to microbial protein entering the intestine for GSC treatments averaged 47.5, whereas, for controls, the ratio was 65.3). Feed protein is usually more digestible than microbial protein [42]. However, the rate of passage might also have played a role, as mentioned previously. In the present study, the lower total tract digestion of NDF with GSC supplementation was due to depressed ruminal digestion. The postruminal NDF digestion did not compensate for the lower ruminal digestion. Indeed, there is a close relationship between the total tract and the ruminal NDF digestion. In a meta-analysis, Huhtanen et al. [43] observed that greater than 90% of total NDF digestion occurs in the rumen.

The effects of the sucrose supplementation of high-energy finishing diets on ruminal pH and VFA molar proportions have not been previously evaluated. However, by nature, finishing diets, having limited amounts of roughage, are expected to result in lower ruminal pHs [44]. As mentioned above, GCS inclusion at the higher levels (27 and 40%) markedly decreased the ruminal pH, increasing the risk of ruminal acidosis. Due to its ready fermentability, the substitution of SFC with higher levels of sucrose is expected to exacerbate this effect, as we observed in this study. Whereas increasing the levels of sucrose did not affect ruminal acetate, either propionate molar ratio or estimated methane production, we observed an increasing molar proportion of acetate and valerate. Likewise, Arrizon et al. [12] observed that replacing SFC with increasing levels of dried shredded sugar beets (≈74% sucrose) in finishing diets increased both acetate and valerate molar ratios.

5. Conclusions

It is concluded that replacing as much as 13% of SFC with GCS in a finishing diet will enhance the efficiency of ruminal N (greater g non-ammonia-N entering the small intestine/g N intake) without detrimental effects on total tract OM digestion. The inclusion of GCS linearly decreased the ruminal proportion of acetate without an effect on the acetate-to-propionate ratio or estimated methane production. Some of the effects on N utilization due to GCS at high inclusion levels may be magnified by differences in the dietary CP content. Higher levels of GCS supplementation (27 and 40%) decreased the ruminal pH below 5.5, increasing the risk of ruminal acidosis. Future research is needed to determine the role of GCS supplementation on growth performance and the feed efficiency of feedlot cattle during these early phases of growth.