J. Anim. Sci. 2004. 82:2667-2678
© 2004 American Society of Animal Science
Effect of energy source and ruminally degradable protein addition on performance of lactating beef cows and digestion characteristics of steers1
T. A. Baumann*,2,
G. P. Lardy*,3,
J. S. Caton* and
V. L. Anderson
* Department of Animal and Range Sciences, and
and
Carrington Research Extension Center, North Dakota State University, Fargo 58105
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Abstract
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Two trials were conducted to determine the effect of energy source (ENG) and ruminally degradable protein (RDP) on lactating cow performance and intake and digestion in beef steers. In Trial 1, 78 cow-calf pairs were used in a 2 x 2 factorial design to determine the effect of ENG (corn or soyhulls; SH) and RDP (with our without sunflower meal) to a forage diet for lactating beef cows. The basal diet consisted of 75% grass hay (11.5% CP) and 25% wheat straw (7.4% CP). Supplement treatments and predicted RDP balances were corn (–415 g of RDP/d); SH (–260 g of RDP/d); corn plus RDP (0 g of RDP/d); or SH plus RDP (0 g of RDP/d). Data were analyzed as a split-plot in time, with pen as the experimental unit (two pens per treatment). No interaction between ENG and RDP was present (P 
0.08) for any response variable. No differences (P 0.39) due to ENG or RDP were noted for BW, BCS, or milk yield; however, final calf weight tended to increase with ENG (P = 0.06). In Trial 2, a 5 x 5 Latin square was used to determine effects of ENG and RDP on intake and digestion in steers (686 ± 51 kg BW). Treatments were arranged as a 2 x 2 plus one factorial and comprised a control (CON; grass hay, 7% CP), grass hay plus 0.4% BW SH, grass hay plus 0.4% BW SH and 0.15% BW sunflower meal, grass hay plus 0.4% BW corn, and grass hay plus 0.4% BW corn and 0.2% BW sunflower meal. Preplanned contrasts included main effects of ENG and RDP, ENG x RDP interaction, and CON vs. supplemented (SUP) treatments. Supplementation increased total DMI compared with CON (P = 0.001), but forage DMI was greater (P = 0.001) for CON than for SUP. An ENG x RDP interaction occurred for forage DMI (P = 0.02); addition of RDP to corn decreased forage intake, whereas addition of RDP to SH had no effect. There was an ENG x RDP interaction (P = 0.001) for ruminal pH; pH tended to increase with RDP addition to SH (P = 0.07), but decreased with RDP addition to corn (P = 0.001). Supplementation increased ruminal ammonia compared with CON (P = 0.001). Likewise, RDP increased ruminal ammonia (P = 0.001). An interaction occurred for OM disappearance (OMD; P = 0.01). The RDP addition to SH numerically decreased OMD (P = 0.23), whereas RDP addition to corn numerically increased OMD (P = 0.14). Intake and digestion seem to respond differently to RDP addition depending on supplemental energy source. Both corn or SH seem to be suitable supplements for the quality of forage used in this trial. Addition of supplemental protein did not improve cow or calf performance.
Key Words: Corn • Cows • Digestion • Protein • Soyhulls • Steers
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Introduction
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Low-quality forages such as dormant native pastures, crop residues, or mature grass hay often do not provide enough CP or energy to adequately maintain cow BW and BCS during early lactation. Fleck et al. (1988)
and Ovenell et al. (1991) reported feeding the proper amounts and type of supplements improved utilization of low-quality forage. Supplementation can positively or negatively affect forage digestion depending on the source of supplemental energy (starch or digestible fiber) and level of feeding (Chase and Hibberd, 1987; Grigsby et al., 1992; Pordomingo et al., 1991). Little research exists on the interaction of digestible fiber and ruminally degradable protein (RDP) addition. Soyhulls (SH), a soybean by-product high in digestible fiber, should increase energy intake without the adverse effects normally observed when high-starch feedstuffs are used to supplement low-quality forages (Marston et al., 1993). As an energy supplement in cattle diets, SH result in less negative effects on forage digestion than corn (Anderson et al., 1988; Grigsby et al., 1992; Martin and Hibberd, 1990); however, for energy supplementation to be effective, RDP requirements must be met. McCollum and Galyean (1985) reported protein supplementation for low-quality forages resulted in increased rate of forage digestion and particulate passage. They also stated that increased rate of particulate passage as a result of protein supplementation is a major factor associated with the increased intake of low-quality hay.
Therefore, our hypothesis was that SH supplementation would not have a negative effect on forage digestion compared with corn supplementation and that the addition of RDP would increase digestion of the forage. The objective of this study was to evaluate the use of SH vs. corn as a supplement with or without supplemental RDP in the diets of lactating beef cows and in the diets of steers fed a forage-based diet on intake and digestion.
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Materials and Methods
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Trial 1
All animal care, handling techniques, and surgical procedures were approved by the North Dakota State University Institutional Animal Care and Use Committee before the initiation of research.
Animals and Diets
The study utilized 78 mature, spring calving cow-calf pairs (Red Angus and Limousin cross) in a completely randomized design (610.5 ± 3.4 and 90.8 ± 1.5 kg initial BW, for cows and calves, respectively). Cow-calf pairs were confined in a drylot located at the Carrington Research Extension Center with 9 to 10 pairs per pen and two pens per treatment (eight pens total). Animals were stratified by calving date and BW and assigned randomly to treatment. Diets were formulated to provide 20 Mcal of NEm/d for 550-kg cows in early lactation with 9 kg of peak milk (NRC, 1996). A common basal diet comprising 75% bromegrass hay (11.5% CP, 65.9% NDF, 40.1% ADF; DM basis) and 25% wheat straw (7.4% CP, 75.9% NDF, 50.2% ADF; DM basis) was fed from May 16 (43 ± 10 d postpartum) to September 6.
Treatments
Composition of feedstuffs is shown in Table 1. Treatments were arranged as a 2 x 2 factorial. Factors were energy source (corn or SH) and RDP addition (with or without). Treatments and predicted RDP balances according to the NRC (2000) model were as follows: 1) hay/straw basal diet plus 4.78 kg of dry-rolled corn (–415 g of RDP/d); 2) basal diet plus 5.32 kg SH (–260 g of RDP/d); 3) basal diet plus 3.68 kg of dry-rolled corn and 1.55 kg of sunflower meal (0 g of RDP/d); and 4) basal diet plus 4.50 kg of SH and 1.05 kg of sunflower meal (0 g of RDP/d). Assumptions used for the NRC model requirement prediction were 550 kg BW, 1.5% BW DMI, 9 kg peak milk production, and 11% microbial efficiency. Diets were formulated for early lactation to provide 20 Mcal of NEm/d (Table 2). Diets were reformulated to meet lower energy requirement (15.6 Mcal of NEm/d) of late lactation (120 d) on July 25. Diets were mixed once daily as a total mixed ration and offered to cows once daily at 0800. Hay was fed ad libitum.
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Table 1. Analyzed composition (%, DM basis) of bromegrass hay, wheat straw, corn, soyhulls, and sunflower meal used in Trial 1
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Table 2. Formulated and analyzed nutrient composition of early and late lactation diets used in Trial 1 (%, DM basis)
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Data Collection
Cow BW, BCS, milk yield, and calf BW were recorded at d 1, 28, 56, 84, and 112 for all response variables except milk yield which was not recorded at d 1. Cow BW was measured before morning feeding and cows were visually evaluated for BCS (1 = emaciated, 9 = obese; Wagner et al., 1988) in late afternoon. Cow BCS was recorded individually by the same three persons each period, with the median value reported. Before the start of the trial, five cows from each treatment were chosen randomly for milk sample analysis. Each teat was stripped before obtaining an approximately 100-mL milk sample. Milk was analyzed for fat (Babcock method), protein (Method 990.02, AOAC, 1997), and solids (DM). Milk yield from cows was determined by the weigh-suckle-weigh method (Boggs et al., 1980). Calves were separated from 0600 until 0900 and allowed to suckle for one half hour. Calves were then separated from their dams for 6 h from 0930 until 1530. At 1530, calves were weighed, allowed to suckle to completion, and reweighed to determine milk intake by difference in weight and that value was multiplied by four to determine 24-h milk production. All calves had access to creep feed from June 18 until trial completion. Composition (% DM basis) of creep feed offered to nursing calves was 30% wheat middlings, 30% SH, 20% corn, 15% grass hay, and 5% mineral (70% 12-12 mineral and 30% limestone).
Jugular blood was collected on d 1, 28, 56, 84, and 112. Blood was collected as cows were weighed in the morning before feeding. Blood was collected via jugular venipuncture into K3 EDTA Vacutainers (Becton Dickinson, Franklin Lakes, NJ) and placed on ice immediately until transport to the laboratory. Tubes were centrifuged (3,000 x g; 30 min at 4°C), and serum was separated into polyprolylene vials and frozen at –20°C until analysis. Serum was analyzed for glucose using a hexokinase/glucose-6-phosphate dehydrogenase couple enzyme assay (procedure described by Sigma Chemical Co., St. Louis, MO). Nonesterified fatty acids were analyzed with a NEFA C kit (Wako Chemicals U.S.A., Richmond, VA). Both glucose and NEFA procedures were adapted for 250-µL, 96-well microtiter plates. Plasma urea nitrogen (PUN) was measured by using a urease/alkaline hypochlorite/phenol determination (urea nitrogen, Procedure No. 640, Sigma Diagnostics, St. Louis, MO). Samples were included with all procedures for inter- and intraassay CV comparisons.
Statistical Analysis
The trial was designed as a completely randomized design with treatments arranged as a 2 x 2 factorial. There were two pens per treatment with 9 to 10 animals per pen. Data were analyzed as a split plot in time with pen as the experimental unit. The model included effects of energy source, RDP addition, and the energy source x RDP. Pen within treatment was used to test for treatment effects. For measures such as blood metabolites, a repeated-measures analysis was used (Gill and Hafs, 1971). The model included effects of energy source, RDP addition, energy source x RDP addition interaction, period, and the three-way interaction. Pen within treatment was used to test for treatment effects.
Trial 2
Animals and Diets.
All animal care, handling techniques, and surgical procedures were approved by the North Dakota State University Institutional Animal Care and Use Committee before the initiation of research.
Five mature steers (686.2 ± 51.4 kg BW) with ruminal and duodenal cannulas were weighed at the initiation of the trial and housed in a climate-controlled room in individual pens (3.0 x 3.7 m) during each 14-d adaptation period and restrained with neck collars in individual stalls (1.2 x 2.0 m) during each 7-d collection period. Stalls were constructed to allow for separation of urine and feces. Total fecal collections were performed using stainless steel pans placed directly behind the stalls.
Treatments consisted of the following: 1) control (CON; bromegrass hay, 7.0% CP, DM basis); 2) grass hay plus 0.4% BW SH (13.5% CP; DM basis); 3) grass hay plus 0.4% BW SH and 0.15% BW sunflower meal (35.0% CP; DM basis); 4) grass hay plus 0.4% BW corn (9.5% CP; DM basis); and 5) grass hay plus 0.4% BW corn and 0.2% BW sunflower meal. Diets supplemented with RDP were formulated to have a 0-g RDP balance using the 1996 NRC model (NRC, 1996). Corn was processed similarly to Trial 1. Table 3 shows nutrient composition of feedstuffs.
Animals were fed forage twice daily (0700 and 1900) and supplements once daily (0700). Grass hay was chopped through a 10.16-cm screen. Steers were acclimated to ad libitum intake for 14 d, with DMI measured during the following 7 d. Supplements were fed at 0700 before the steers received hay. Steers had continuous access to a salt block and clean water.
Sampling Protocol.
Samples of diet components were collected weekly, composited by period, dried (55°C, 48 h, forced-air Grieve oven SB-350, Round Lake, IL), and ground to pass a 2-mm screen in a Wiley mill (model No. 3, Arthur H. Thomas, Philadelphia, PA). Orts were collected daily, subsampled (10% of total weight), composited within period, dried, and ground in a Wiley mill. During each collection period, daily total fecal output was weighed, subsampled (10% of total net weight), and stored (–20°C). Total subsample was mixed in a commercial rotary mixer (Hobart H-600, Troy, OH), subsampled, dried (55°C, 48 h, forced-air Grieve oven SB-350) and ground to pass a 2-mm screen. Cobalt-EDTA was dosed (10 g of cobalt-EDTA/200 mL of solution) at 0700 to determine fluid dilution rate. Ruminal fluid was collected with a suction-strainer apparatus starting at 0700 on d 6 of collection and continuing every 2 h for a 12-h period. Approximately 100 mL of ruminal fluid was placed into a plastic bag (Whirlpak, Nasco, Ft. Atkinson, WI), pH recorded (Beckman pH meter, model 2000; West Chester, PA), and stored (–20°C) until analyzed for ammonia and cobalt. Ruminal fluid (3 mL) at each time point was added to 0.75 mL metaphosphoric acid (25% wt/vol) and frozen (–20°C) for VFA analysis. Ruminal contents were evacuated on d 7 of each collection period, weighed, and two samples were taken. One sample was dried (55°C, 48 h, forced-air Grieve oven SB-350) and ground to pass 1-mm screen in a Wiley mill for analysis of DM and OM and the second sample was for future bacterial separation.
Laboratory Analysis.
Composited diet components, orts, and fecal samples were analyzed for DM, OM, and CP using standard procedures (AOAC, 1997). Acid detergent fiber and NDF were analyzed using procedures (Van Soest et al., 1991) modified for use in an Ankom 200 fiber analyzer (Ankom, Fairport, NY). Ruminal fluid samples were centrifuged (20,000 x g, 20 min) and analyzed for ammonia-N (Broderick and Kang, 1980).
Duodenal CP was corrected for N of bacterial origin by purine analysis (Zinn and Owens, 1986) of a bacteria sample. Rumen contents (4 kg) were combined with 1 L of a formalin-saline solution and incubated 2 to 4 h in the refrigerator before freezing. Samples were blended and strained through four layers of cheesecloth. Liquid was centrifuged at 500 x g for 20 min to remove protozoa and feed particles. Supernatant was removed and spun again at 500 x g for 20 min. Particle-free supernatant was then spun at 30,000 x g for 20 min to collect bacteria.
In situ DM disappearance (ISDMD) of grass hay, SH, and corn was determined with Dacron bags (10 x 20 cm, Ankom) with heat sealed edges. Bags containing 5 g of sample (grass hay, SH, or corn) were incubated in the rumen for 2, 5, 9, 14, 24, 36, 48, 72, and 98 h. Steers receiving supplemental SH received duplicate in situ bags containing grass hay and SH. In situ bags containing corn or forage were incubated in steers receiving supplemental corn. Dacron bags were placed in large mesh bags placed below the ruminal fiber mat and added in a manner that allowed all bags to be removed at the same time. Bags were machine washed on gentle cycle for seven (2-min) cycles (GE heavy-duty washer, Appliance Park, KS). Bags were then dried (55°C, 48 h, forced-air Grieve oven SB-350) to determine ISDMD. Forage was also analyzed for NDF and ADF disappearance.
Experimental Design and Statistical Analysis.
The experiment was a 5 x 5 Latin square with treatments arranged as a 2 x 2 plus one factorial. Data were analyzed with the GLM procedure of SAS (SAS Inst., Inc., Cary NC; Version 8.2). The model included steer, period, and treatment. For data collected over time within each sampling period (ruminal pH, VFA, etc.), a repeated-measures analysis was conducted (Gill and Hafs, 1971). Effects in the model included steer, period, treatment, time, treatment x time interaction, and the three-way interaction. Period x steer x treatment was used as the error term for the repeated-measures analysis. Preplanned contrasts included main effects of ENG and RDP, ENG x RDP interaction, and CON vs. supplemented treatments (SUP).
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Results and Discussion
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Trial 1
Initial and final measurements for cow BW, BCS, milk yield, and calf BW are reported in Table 4. An interaction of ENG and RDP was not present for cow BW, BCS, milk yield, or calf BW (P 0.08). Energy source and RDP had no effect on final cow BW (P = 0.55 and 0.51, respectively), BCS (P = 0.53 and 0.89, respectively), or milk yield (P = 0.87 and 0.48, respectively). Final calf BW tended (P = 0.06) to be greater from cows supplemented with SH than from cows supplemented with corn. Addition of RDP did not increase performance over the nonprotein-supplemented treatments. The basal diet (forage plus corn or SH) supplied adequate protein to meet the lactation requirements of these animals; therefore, milk yields also were not affected by SUP, ENG, or RPD (P 0.48). Although milk yield was not affected by treatment, milk composition was affected. Milk solid content increased (P = 0.02) with RDP supplementation and milk fat content tended to increase (P = 0.07) with RDP supplementation. Clark and Armentano (1997) found increased milk fat with increased dietary NDF; however, NDF levels in their study were lower than the levels of NDF used in our study. Sanson et al. (1990) reported greater BW gain after calving for cows fed low-quality hay supplemented with ear corn compared with cows supplemented with protein (32% all-natural commercial protein pellet containing various oilseed and vegetable proteins) or the combination of ear corn and protein, but calf weaning weights were not different among treatments. Sanson et al. (1990) also suggested that the higher BW gain of cows was at the expense of milk production. Our data indicate that milk production is not affected by supplementation.
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Table 4. Effect of energy source and protein addition on BW, BCS, milk yield, milk composition, and calf BW in cows consuming medium-quality forage diets
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During the 112-d study, cow BW decreased (P = 0.001) from 610.5 to 584.2 ± 3.4 kg and BCS decreased (P = 0.001) from 5.58 on d 1 to 5.01 ± 0.05 on d 112 (data not shown). Decreases in BW and BCS may be expected as requirements for peak lactation may not be met with dietary sources. All treatments were formulated to provide similar amounts of NEm. Because no difference in BW or BCS between treatments was noted, we concluded that corn and SH supplements had similar effects on forage digestion. Equal amounts of corn or SH fed to nonpregnant cows resulted in no difference in digestible OM intake or total-tract OM disappearance, suggesting that the higher TDN from corn was not realized in the study by Chan et al. (1991). Wheeler et al. (2002) reported less BW and body condition loss in spring-calving cows fed protein supplements compared with control cows. Furthermore, increased amounts of supplemental protein did not result in an increase in BW and body condition retained during early lactation.
Daily calf milk intake decreased (P = 0.001) from 13.1 kg on d 28 to 7.7 ± 1.1 kg on d 112 (data not shown). Loy et al. (2002) reported no change in calf milk intake with advancing season although milk intake as a percentage of BW decreased. Milk intakes in the study by Loy et al. (2002) were lower (average 4.23 kg/d) than our study. As expected, calf BW increased (P = 0.001) from 90.7 to 219.1 ± 1.5 kg during the 112 d trial for 1.15 kg ADG (data not shown).
Effects of treatment on plasma concentrations of NEFA, glucose, and PUN are shown in Table 5. No ENG x RDP interactions were present (P 0.40) for these variables. The level of NEFA was not affected by ENG (P = 0.67), but RDP increased (P = 0.02) NEFA compared with no RDP. Plasma NEFA are negatively correlated with energy balance (Erfle et al., 1974). An increase in plasma NEFA concentration is an indicator of body fat mobilization (Blauwiekel and Kincaid, 1986). In general, NEFA decrease during lactation. In lactating cows, serum NEFA is typically highest immediately postpartum (Blauwiekel and Kincaid, 1986). In contrast to our results, Caton et al. (1988) reported that supplementation of low-quality forage with cottonseed meal did not affect serum NEFA concentrations. Rusche et al. (1993) observed a similar effect of RDP or RUP supplements on plasma NEFA concentrations of beef cows. Corn and SH supplementation seemed to have the same effect on energy balance.
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Table 5. Effect of energy source and RDP addition on plasma concentrations of nonesterified fatty acids, glucose, and plasma urea nitrogen (PUN)
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Glucose concentrations were decreased (P = 0.04) with RDP addition compared with no RDP. There was no effect of ENG (P = 0.22). Glucose is one of the main substrates extracted from the blood and used by the lactating mammary gland (Larson, 1985). Anthony et al. (1986) found prepartum glucose concentration was lower in beef heifers fed 81% of NRC protein requirements compared with heifers fed 141% of NRC (1984) protein requirements.
Concentration of PUN was not affected by ENG (P = 0.64) or RDP (P = 0.24). Other research indicates that protein supplementation increases PUN (Caton et al., 1988; Cheema et al., 1991; Barton et al., 1992). Variation in response may be due to stage of gestation or lactation. During lactation, PUN decreased (Sletmoen-Olson et al., 2000). Recycling of PUN to the rumen may have increased, thereby resulting in decreased PUN.
The NRC (1996) computer model was also used to predict degradable intake protein (DIP) and metabolizable protein (MP) balances based on actual data from Trial 1 (data not shown). On d 28, predicted DIP balances were –243, +178, –35, and +202 g/d for the corn, corn/RDP, SH, and SH/RDP diets, respectively. Predicted MP balances on d 28 were 43, 83, 52, and 74 g/d for the corn, corn/RDP, SH, and SH/RDP diets, respectively. On d 112, predicted DIP balances were –171, +175, –36, and +203 g/d for the corn, corn/RDP, SH, and SH/RDP diets, respectively. Predicted MP balances on d 28 were 239, 252, 249, and 250 g/d for the corn, corn/RDP, SH, and SH/RDP diets, respectively. Even though the NRC (1996) computer model predicted highly negative DIP balances for the diets without supplemental RDP, production responses were not observed. This indicates that the NRC (1996) model either overpredicted DIP requirements or underestimated DIP supply, which may be a function of improperly estimating dietary TDN, microbial efficiency, or ruminal CP degradabilities of various feedstuffs (Lardy et al., 2004).
Trial 2
Forage Intake.
Supplementation increased (P = 0.001) total DMI compared with CON (1.67 vs. 1.45 ± 0.04% BW, respectively); however, forage DMI was decreased (P = 0.001) for SUP compared with CON (1.25 vs. 1.45 ± 0.03% BW, respectively; Table 6). Other research with corn supplementation (Chase and Hibberd, 1987) and SH supplementation (Martin and Hibberd, 1990) suggests that the substitution rate of supplement for hay is lower for SH than for corn. Conversely, Sanson (1993) found no difference in forage intake with addition of corn to low-quality meadow hay. Substitution rates may be influenced by animal factors. Chase and Hibberd (1987) and Martin and Hibberd (1990) used mature cannulated beef cows, whereas Sanson (1993) used intact lambs. An ENG x RDP interaction (P = 0.02) occurred for total and forage DMI when expressed as a percentage of BW. Addition of RDP to SH had no effect (P = 0.31) on forage DMI (1.24 vs. 1.29 ± 0.03% BW), whereas addition of RDP to corn decreased (P = 0.01) forage DMI (1.28 vs. 1.16 ± 0.03% BW). The addition of RDP to the corn diet may have relieved a RDP deficiency. Sanson et al. (1990) reported decreased forage DMI with a greater level of corn and protein supplement compared with a lower level of corn and protein supplement. Decreased forage DMI with increasing amounts of an energy source was also reported by Chase and Hibberd (1987), Pordomingo et al. (1991), and Sanson (1993). The reason for the substitution (Chase and Hibberd, 1987) was because of the combination of the deficiency in RDP and the increased amount of supplemental corn. Bodine et al. (2000) reported a quadratic increase in hay and total intake when increasing levels of RDP from soybean meal were added to forage-based diets supplemented with corn. The reason for the contradictory data is not immediately evident. It may be related to the differences in forage quality between the study of Bodine et al. (2000) and our study. Bodine et al. (2000) used 6% CP prairie hay, whereas the bromegrass hay and straw we used had greater levels of CP. Another possible reason for the interaction we observed may be due to the differences in DIP balance between the corn and SH treatment. Based on the calculations used for Trial 1, the DIP balance for the corn diet was approximately 150 g/d lower than the SH diet. This may have contributed in part to the interaction we observed.
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Table 6. Effect of energy source and RDP addition in Trial 2 on DMI, total intake, forage intake, ruminal DM fill, fluid dilution rate, and ruminal pH
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Ruminal DM fill (% BW) was not affected by ENG (P = 0.51) or RDP (P = 0.74), but unsupplemented cattle had increased (P = 0.01) ruminal fill, indicating longer retention time corresponding to lower total intake for control steers (Table 6
). In steers grazing rangeland and consuming increasing amounts of whole corn, Pordomingo et al. (1991) reported a quadratic response for ruminal retention time with unsupplemented steers having the longest retention time.
Fluid dilution rate (%/h) was not affected by ENG (P = 0.66) or RDP (P = 0.18), but SUP tended to increase (P = 0.08) fluid dilution rate, which corresponds with the increased total intake in supplemented steers. Heldt et al. (1999) also reported increased fluid dilution rate for supplemented vs. control steers and an interaction for starch vs. fiber x level of RDP x level of carbohydrate.
Ruminal Fermentation Characteristics.
No time x treatment interaction was present for ruminal pH (P = 0.79); therefore, main effects of treatment are reported in Table 6. The relatively low level of supplementation (<0.4% BW) may explain the lack of pH effects across time. Ruminal pH was higher (P = 0.001) for CON than for SUP (6.69 vs. 6.56 ± 0.02). Grigsby et al. (1992) observed a linear decrease in pH when SH replaced up to 60% of a bromegrass hay diet fed to steers. A decrease in ruminal pH with SH and corn supplementation (corn and SH fed at 0.63% BW) was also observed by Grigsby et al. (1993). There was an ENG x RDP interaction (P = 0.001) for ruminal pH as pH tended to increase (P = 0.07) with RDP addition to SH (6.58 vs. 6.63 ± 0.02), but decreased (P = 0.001) with RDP addition to corn (6.60 vs. 6.46 ± 0.02). The difference in pH is relatively minor and may not be biologically significant. Although pH was depressed with addition of RDP to corn, it is unlikely bacterial populations were altered as Ørskov (1982) and Mould et al. (1983) indicated that ruminal fiber digestion would not be impacted with pH above 6.2. Similar to our results, Bodine et al. (2001) reported decreased ruminal pH in steers fed low-quality prairie hay and supplemented with a protein supplement, a digestible fiber-based supplement, or a grain-based supplement. Bodine et al. (2000) also reported a quadratic decrease in ruminal pH with increasing level of DIP addition to a low quality forage-based diet in either the presence or absence of supplemental corn.
Ammonia concentrations in this study (Table 7) seemed to be relatively low compared with studies with similar diets (Pordomingo et al., 1991; Sanson et al., 1990). A time x treatment interaction occurred in ruminal ammonia data (P = 0.001). The interaction was a result of the RDP-supplemented treatment ammonia concentration increasing from 0 to 2 h after feeding, whereas the other treatments decreased during that time period. Ammonia concentration for the CON diet peaked 4 h after feeding. Supplementation in this study increased ammonia concentration compared with CON (P = 0.001), which agrees with the results of Sanson et al. (1990), but not with those of Chase and Hibberd (1987), who found lower ruminal ammonia with the addition of corn to a hay diet. Forage used in the study by Chase and Hibberd (1987) was of lower quality than the forage used in this study and the forage used by Sanson et al. (1990). Addition of RDP increased ruminal ammonia (P = 0.001). Ruminal bacteria require ammonia for microbial growth. Low ruminal ammonia has the potential to inhibit microbial activity and decrease rate of fiber digestion. Diets without RDP supplementation were generally deficient in ammonia. Satter and Slyter (1974) reported microbial CP production increased and then leveled off when ruminal ammonia reached 50 mg/L of ruminal fluid.
Effects of treatment on ruminal VFA concentrations are shown in Table 8. An ENG x RDP interaction was present for valerate concentration (P = 0.04) and total VFA (P = 0.001). Total VFA concentrations were not changed with the addition of RDP to SH (P = 0.23), but increased for RDP addition to corn (P = 0.001). Concentration of propionate was not affected by SUP (P = 0.47). However, acetate concentration decreased (P = 0.001) with SUP compared with CON. Concentrations of isobutyrate, valerate, and isovalerate were increased (P = 0.001) for SUP compared with CON. Concentrations of butyrate were also increased for except for SUP compared with CON (P = 0.02). Acetate concentrations were higher (P = 0.001) for SH compared with corn and RDP treatments had lower (P = 0.001) acetate concentrations compared without RDP. Diets supplemented with SH had higher (P = 0.001) propionate concentrations compared with diets supplemented with corn. The addition of RDP increased propionate concentration (P = 0.001) and decreased acetate concentration (P = 0.001). Decreased acetate and increased propionate that we observed are in agreement with previous research (McCollum and Galyean, 1985; Köster et al., 1996). Butyrate concentration decreased (P = 0.001) with SH compared with corn and also decreased (P = 0.002) without RDP compared with RDP. Isobutyrate was not affected by ENG (P = 0.66), but increased (P = 0.001) with RDP. Isovalerate was lower for SH (P = 0.004) and no RDP (P = 0.001) compared with corn and with RDP, respectively. Supplementation increased (P = 0.001) total VFA compared with CON, illustrating the ability of supplementation to increase fermentative activity. Total VFA increased in response to RDP fed to beef cows (Köster et al., 1996) and did not change when corn was supplemented (Chase and Hibberd, 1987). Olson et al. (1999) also reported these same outcomes with supplemental protein and starch. The interaction observed in the current study may be due to ENG because it is likely SH do not change the ruminal bacterial population to the extent that corn does. In addition, the increased total diet intake would contribute to a greater total VFA concentration.
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Table 8. Effect of energy source and ruminally degradable protein addition on ruminal volatile fatty acid concentration in steers fed medium-quality grass hay
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Digestibility.
Table 9 shows the effects of treatment on OM digestibility. These effects were generally similar to the effects on DM digestibility (data not shown). Supplementation (P = 0.001) and RDP (P = 0.001) addition increased OM intake compared with CON and diets without RDP, respectively. Bacterial OM flow at the duodenum was increased (P = 0.007) with SUP compared with CON, and the ENG x RDP interaction was significant (P = 0.05). The addition of RDP to SH increased flow to a greater degree than corn. Increased bacterial OM flow at the duodenum is a result of RDP being incorporated into ruminal bacteria, thereby allowing an increase in bacteria turnover. This indicates that the SH diet was deficient in RDP. The interaction was significant (P = 0.04) for fecal OM output, with increased flow with RDP addition to SH, but not with RDP addition to corn. The apparent rate of OM digestion, disappearance (g/d), and disappearance as a percentage of intake from the rumen was increased (P = 0.03) with SUP compared with CON. Organic matter disappearance corrected for OM of bacterial origin (g/d and % intake) was increased (P = 0.001 and 0.02, respectively) for SUP compared with CON, and true OM disappearance also increased with RDP (P = 0.005). The increase in OM intake resulted in increased OM digestion with little affect of passage rate. When apparent OM disappearance was corrected for OM of bacterial origin, true OM disappearance remained higher for SUP and RDP steers indicating that a greater amount of OM in the feedstuffs was digested along with an increase in bacteria turnover. An ENG x RDP interaction occurred for apparent total-tract OM disappearance (P = 0.01). Total-tract OM disappearance for SH and SH+RDP treatments did not differ (P = 0.23); however, for corn, the addition of RDP tended to increase OM disappearance (P = 0.14), which agrees with the data found for fecal OM output. Bodine et al. (2000) reported a quadratic increase in hay OM digestibility when increasing levels of DIP were supplemented to steers fed low-quality prairie hay (6% CP) and corn. Bodine and Purvis (2003) also reported increased OM intake and digestibility in steers supplemented with corn and SBM vs. either corn or SBM alone. Our data with corn supplementation seem consistent with their conclusion that when grain-based supplements are balanced for DIP in relation to dietary TDN, negative effects on forage intake and digestibility are minimized.
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Table 9. Effect of energy source (corn vs. soyhulls) and RDP addition on OM intake, digestibility, and duodenal flow
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Effect of treatment on NDF digestibility is reported in Table 10. Total NDF intake was greater (P = 0.001) for SH than for corn. In the rumen, NDF disappearance (g/d) was greater for SUP compared with CON (P = 0.03), and greater for SH compared with corn (P = 0.001), due to the nature of the feedstuff composition, but no differences for SUP (P = 0.35), ENG (P = 0.12), or RDP (P = 0.91) were observed for NDF disappearance as a percentage of intake. An ENG x RDP interaction occurred (P = 0.04) for total-tract NDF digestibility that was similar to the interaction for OM digestibility. The addition of RDP to SH resulted in a decrease in NDF disappearance (P = 0.06); however, the addition of RDP to corn resulted in no difference in NDF disappearance (P = 0.25). Increased intake likely resulted in increased passage rate, which would lead to overall decreased total-tract NDF digestibility. Sanson et al. (1990) reported protein supplementation did not affect NDF digestibility, which we also found with corn supplementation, but not for SH. Corn-supplemented treatments had lower NDF disappearance compared with SH-supplemented treatments, which corresponds with the findings of Grigsby et al. (1993), who reported a linear decrease (P = 0.06) in NDF digestibility as corn replaced SH. Increasing the level of corn (0 to 3 kg/d) had a cubic response to NDF digestibility in a study conducted by Chase and Hibberd (1987).
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Table 10. Effect of energy source (corn vs. soyhulls) and RDP addition on NDF and ADF intake, digestibility, and duodenal flow
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Table 10 shows the effects of treatment on ADF digestibility. Total ADFI and ADF disappearance (g/d) from the rumen was increased (P = 0.04 and 0.01, respectively) for SUP compared with CON, and increased for SH compared with corn. When expressed as a percentage of intake, ADF disappearance from the rumen did not differ for SUP (P = 0.25), ENG (P = 0.09), or RDP (P = 0.85). Total-tract ADF digestibility was not different (P = 0.79) for CON compared with SUP or with or without RDP (P = 0.52). Corn supplementation resulted in lower (P = 0.002) ADF digestibility (45 vs. 54%) compared with SH. Decreased ADF digestibility for supplemental corn compared with SH indicates that corn exhibited a negative associative effect for forage digestibility, decreasing ADF digestibility. Chase and Hibberd (1987) suggested that corn supplements formulated with inadequate RDP will decrease hay digestibility and intake, whereas low ruminal ammonia concentrations coupled with decreased ruminal pH may inhibit microbial growth, resulting in decreased fiber digestibility. Our study indicates that RDP was inadequate in the corn diet because the addition of RDP to corn resulted in increased total-tract disappearance of not only ADF, but also of NDF and OM.
The effect of treatment on CP disappearance is reported in Table 11. By design, CP intake was increased for SUP compared with CON (P = 0.001), SH compared with corn (P = 0.02), and with RDP compared with no RDP (P = 0.001). Duodenal flow of total CP and bacterial CP were increased for SUP vs. CON (P = 0.005 and 0.001, respectively) and with RDP vs. without RDP (P = 0.002 and 0.001, respectively). Bacterial CP (g/d) was increased (P = 0.001) for SUP compared with CON and for addition of RDP (P = 0.001). Increased bacterial CP, which corresponded with the increased fluid dilution rate and increased intake reported, helps explain the increase observed in DM, OM, NDF, and ADF digestibilities. Fecal CP output was also increased for SUP vs. CON, and the interaction was also significant (P = 0.04), with the addition of RDP to SH resulting in an increase in fecal CP output and no change with the addition of RDP to corn. The additional fecal CP output suggests that SH were adequate in RDP and the additional RDP resulted in CP not utilized by the digestive tract or N recycling to the hindgut. Apparent CP disappearance (g/d and % intake) was increased (P = 0.01) with RDP compared with no RDP. Correction for CP of bacterial origin resulted in an increase (P = 0.001) in true CP disappearance (g/d) for RDP-supplemented steers compared with no RDP supplementation. This finding suggests that RDP was incorporated into microbial protein. Intestinal CP disappearance (g/d and % entering) was increased for with RDP compared with no RDP (P = 0.004 and 0.04 respectively). Supplemented steers had greater (P = 0.02) total tract CP disappearance than CON steers. There was an ENG x RDP interaction for CP disappearance (P = 0.01). The addition of RDP to SH and corn resulted in increased CP disappearance; however, the magnitude of increase was greater for corn compared with SH. This increase for corn with RDP suggests that RDP was indeed limiting digestion of the diet.
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Table 11. Effect of energy source (corn vs. soyhulls) and RDP addition on CP intake, digestibility, and duodenal flow
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Implications
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Corn or soyhulls are suitable as an energy supplement for medium-quality forage. For the medium-quality forage fed in this study, intake and digestion seemed to respond differently to ruminally degradable protein addition depending on supplemental energy source. Effects on intake seem to have the greatest effect on forage utilization. Addition of ruminally degradable protein helped alleviate the known protein deficiency of lower-quality forages. The observed decreases in digestion of soyhulls with ruminally degradable protein pose new questions about potential interactions. Thus, additional research is needed to determine the optimal supplemental ruminally degradable protein level and responses in diets using soybean hulls as the supplemental energy source.
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Footnotes
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1 Partial funding for this project was obtained from the North Dakota State Univ. BeefLine Initiative 2001. 
2 Current address: North Dakota State University Ext. Service, Box KK, Amidon, ND 58620-0446.
3 Correspondence: 177 Hultz Hall (phone: 701-231-7660; fax: 701-231-7590; e-mail: glardy{at}ndsuext.nodak.edu).
Received for publication November 20, 2003.
Accepted for publication May 17, 2004.
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Abstract
Introduction
