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Nonstructural carbohydrate supplementation of yearling heifers and range beef cows¹

Department of Animal and Range Sciences, Montana State University, Bozeman 59717

	Abstract

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A digestion study with 28 yearling heifers (428 ± 9.9 kg; Exp. 1) and a 2-yr winter grazing trial with 60 crossbred cows (552 ± 6.9 kg; Exp. 2) were used to determine the effects of level of nonstructural carbohydrate (NSC) supplementation on intake and digestibility of low-quality forage. Treatments were as follows: 1) control, no supplement; 2) 0.32 kg of NSC (1.8 kg/d of soybean hulls and soybean meal; DM basis); 3) 0.64 kg of NSC (1.7 kg/d of wheat middlings; DM basis); and 4) 0.96 kg of NSC (1.7 kg/d of barley and soybean meal; DM basis). Supplements provided 0.34 kg of CP/d and 5.1 Mcal of ME/d. In Exp. 1, heifers were individually fed hay (5.5% CP, DM basis) and their respective supplements in Calan gates for 28 d. Data were analyzed as a completely randomized design. In Exp. 2, cows were individually fed supplement on alternate days, and grazed a single rangeland pasture stocked at 1.8 ha/animal unit month. Two ruminally cannulated cows were used per treatment to obtain forage extrusa and to measure in situ DM disappearance (DMD) and carboxymethylcellulase (CMCase) activity of particle-associated ruminal microbes. Data were analyzed as a completely randomized design with the effects of treatment, year, and their interaction. In both experiments, Cr₂O₃ boluses were used to determine fecal output, individual animal was the experimental unit, and contrasts were used to test linear and quadratic effects of NSC level and control vs. supplemented treatments. In Exp. 1, hay and diet DM, NDF, and CP intakes and digestibilities were increased (P < 0.01) by NSC supplementation compared with the control. In Exp. 2, 72-h in situ DMD and CMCase were decreased linearly (P < 0.08) with increasing NSC supplementation. Intake of forage DM, NDF, and CP was decreased linearly (P < 0.01) with increasing NSC supplementation during both years. Supplementation with NSC decreased (P = 0.01) cow BW loss compared with the control in yr 1, whereas in yr 2, cow BW loss was linearly increased (P = 0.03) by increasing NSC supplementation. Supplements containing NSC improved forage digestion and intake when heifers consumed forage deficient in CP relative to energy (digestible OM:CP > 7), but decreased forage digestion and intake when cows grazed forage with adequate CP relative to energy (digestible OM:CP < 7). Forage and supplement digestible OM:CP seemed to be superior predictors of response to supplementation with NSC compared with forage CP levels alone.

Key Words: Forage Intake • Nonstructural Carbohydrates • Supplementation • Winter Range

	Introduction

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Low nutrient content and limited availability of forage for desired animal performance are two reasons why energy supplementation may be necessary for beef cows consuming forage diets (Horn and McCollum, 1987; Galyean and Goetsch, 1993). Supplements containing high levels of nonstructural carbohydrates (NSC), such as cereal grains, have decreased intake and digestibility of low-quality forages (Sanson et al., 1990; Olson et al., 1999; Bodine and Purvis, 2003). Mechanisms by which high levels of NSC could depress forage intake or digestibility include decreases in ruminal pH, decreased cellulolytic enzyme production and activity (Martin et al., 2001), impaired bacterial attachment to fibrous digesta (Hiltner and Dehority, 1983), and increased lag time for fiber digestion (Mertens and Loften, 1980). In addition, competition between cellulolytic and noncellulolytic ruminal microbes for essential nutrients may affect forage intake and digestibility (Firkins et al., 1991).

Limited amounts of NSC have stimulated fiber digestion, possibly by increasing microbial activity and attachment to fibrous digesta (Demeyer, 1981; Hiltner and Dehority, 1983; Piwonka and Firkins, 1996). High-fiber by-products containing low levels of NSC, such as soybean hulls and wheat middlings, have increased use of low-quality forage by cattle (Martin and Hibberd, 1990; Ovenell et al., 1991), possibly because shifts to starch-degrading microbes and fluctuations in pH are fewer (Highfill et al., 1987; Hsu et al., 1987).

The objectives of this research were to determine the effects of increasing level of NSC supplementation on carboxymethylcellulase (CMCase) activity of particle-associated ruminal microbes, and forage intake and digestibility by yearling heifers fed low-quality forage, and by beef cows grazing native winter range.

	Materials and Methods

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Experiment 1
Heifer Digestion Trial.
Twenty-eight pregnant (second trimester) Angus x Hereford yearling heifers (average BW 428 ± 9.9 kg) were used in a 28-d completely randomized design to evaluate the effects of supplementation with increasing levels of NSC on forage intake and digestion. Heifers were randomly assigned to one of seven pens (3.5 x 12.2 m; four heifers per pen) with four Calan-Broadbent gates (American Calan, Inc., Northwood, NH) for individual feeding. Four supplement treatments were assigned randomly to gates (seven gates per treatment). The treatments were as follows: 1) control, no supplement; 2) 0.32 kg/d of NSC (1.8 kg/d of soybean hulls and soybean meal; DM basis); 3) 0.64 kg/d of NSC (1.7 kg/d of wheat middlings; DM basis); and 4) 0.96 kg/d of NSC (1.7 kg/d of barley and soybean meal; DM basis). Supplements were formulated to provide 0.34 kg/d of CP and 5.1 Mcal of ME/d (Table 1). Heifers were individually fed low-quality orchardgrass (Dactylis glomerata L.) hay (Table 1) and their respective supplement for 28 d. The experimental period consisted of 21 d for diet adaptation, followed by a 7-d collection period. Adaptation began on d 1, and heifers were weighed on d 9. Heifers were fed hay at 0500 and 1700, and supplement at 0500. Hay was chopped to 5.1 cm before feeding, and heifers were allowed ad libitum access to hay. Animals in each pen had ad libitum access to water and trace mineral salt blocks (NaCl 97.5%, Zn 0.35%, Fe 0.34%, Mn 0.20%, Cu 0.033%, I 0.007%, and Co 0.005%). Refused hay was weighed daily before feeding. Hay samples were taken daily throughout the experiment and composited on an equal weight basis. Samples of hay refused were collected and composited for each heifer. Hay and hay refusal samples were ground in a Wiley mill (Thomas Scientific, Swedesboro, NJ) to pass a 1-mm screen and analyzed for DM, OM, N (AOAC, 1997), NDF, ADF, ADL (Van Soest et al., 1991), and ADIN (Licitra et al., 1996).

Fecal Cr concentration and daily Cr release rate (0.985 g/d; supplied by the manufacturer) were used to estimate fecal output (FO) using the following equation:

[Eq. 1]

Fecal output, along with the measured forage and supplement intake, allowed for calculation of diet digestibility for each heifer.

Two ruminally cannulated crossbred steers with ad libitum access to low-quality hay were used to measure in situ DM and NDF disappearance of the three supplements. Supplement samples were ground to pass a 2-mm screen in a Wiley mill. Three polyester bags (10 cm x 20 cm; 50-µm pore size; Ankom Technology, Fairport, NY) containing approximately 3 g (as-fed basis) of each of the three supplements and one blank bag (a total of 10 bags for each time period) were placed into the rumen of each steer and removed after 0, 6, 24, and 48 h (each steer received a total of 40 bags). After removal from the rumen, bags were rinsed and manipulated in cold water until the water ran clear, and then squeezed by hand to remove excess water. Bags were dried at 55°C and residue remaining in the bags was analyzed for DM (AOAC, 1997) and NDF (Van Soest et al., 1991). In situ 48-h DM and NDF disappearance values were used to correct fecal output for DM and NDF originating from the supplement. This allowed for the calculation of hay DM and NDF digestibility.

Data were analyzed as a completely randomized design using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). The treatment sums of squares was partitioned into preplanned single-df orthogonal contrasts (Snedecor and Cochran, 1980) to determine: 1) the linear effect of supplemental NSC level; 2) the quadratic effect of supplemental NSC level, and 3) the comparison of unsupplemented control vs. supplemented treatments. An individual heifer was considered the experimental unit. Treatment least squares means and associated standard errors are reported.

Experiment 2
Grazing Trial.
Sixty pregnant crossbred (Angus x Hereford) cows were assigned to one of four treatments in the fall after weaning in each of 2 yr (average initial BW; 565 ± 12.9 kg in yr 1; 538 ± 14.3 kg in yr 2). The treatments were as follows: 1) control, no supplement; 2) 0.32 kg/d of NSC (1.8 kg/d of soybean hulls and soybean meal; DM basis); 3) 0.64 kg/d of NSC (1.7 kg/d of wheat middlings; DM basis); and 4) 0.96 kg/d of NSC (1.7 kg/d of barley and soybean meal; DM basis). Supplements were formulated to provide 0.34 kg/d of CP and 5.1 Mcal of ME/d (Table 1). All cows, along with eight ruminally cannulated cows (two ruminally cannulated cows per treatment), grazed a single native rangeland pasture (257 ha) located on the Red Bluff Research Ranch in Norris, MT. The pasture contained sandy and silty range sites typical of the foothills of southwest Montana. Elevation at the study site ranged from 1,400 to 1,900 m, and long-term annual precipitation was from 350 mm to 406 mm. Pasture vegetation was composed of 65% grasses and 35% forbs and woody species. Dominant grasses included bluebunch wheatgrass (Agropyron spicatum), needle and thread (Stipa comata), and Idaho fescue (Festuca idahoensis). Cows grazed the pasture from December 15 to February 24 during yr 1, and from December 23 to February 24 during yr 2. Calving began approximately March 1 both years. The suggested stocking rate for this area was 1.3 ha/animal unit month (AUM; Lacey and Taylor, 1985). The stocking rate for this pasture was 1.7 ha/AUM in yr 1 and 1.9 ha/AUM in yr 2, which ensured that intake was not limited by forage availability. The total snowfall during the study period was 3.8 cm in yr 1 and 71.1 cm in yr 2. Average daily temperature during the study period was –0.4°C in both yr 1 and 2. Average minimum temperature during the study period was –5.1°C in yr 1 and –4.9°C in yr 2 (Western Regional Climate Center, 2003).

Cows were individually fed two times the daily ration of supplement on alternate days. All cows were gathered (including those on the control treatment and the ruminally cannulated cows) beginning at 0700 on supplementation days and penned in a corral facility. Groups of eight cows on the supplement treatments were rotated through eight separate stalls, where they were individually fed their appropriate supplement allotment. Consumption of the supplements was complete and rapid. After all cows on supplement treatments had received their supplement, the herd was turned back out to graze.

Cows were weighed initially on December 14 and 15 (d 1) in yr 1, and December 22 and 23 (d 1) in yr 2. The average of weights taken on the two consecutive days was used as the initial cow BW for each year. Cows were turned on to the pasture on d 1 in both years. Supplement feeding was initiated on d 3 in yr 1, and d 5 in yr 2. Supplement grab samples were taken and analyzed (Table 1) for DM, N, OM (AOAC, 1997), NDF, ADF (Van Soest et al., 1991), lignin, and ADIN (Licitra et al., 1996). Sustained-release boluses (Captec Chrome, Nufarm) were administered to all cows (including the cannulated cows) on d 43 in yr 1, and d 40 in yr 2 to provide Cr₂O₃ as an external marker to estimate FO. Fecal grab samples were taken on d 51, 53, and 57 in yr 1, and on d 47, 49, and 51 in yr 2. Fecal samples were dried in a forced-air oven at 60°C for 72 h, ground to pass a 1-mm screen Wiley mill, and composited for each cow on an equal dry-weight basis. Fecal composites were then analyzed for DM, OM (AOAC, 1997), NDF (Van Soest, et al., 1991), and Cr by atomic absorption spectrophotometry (Ellis et al., 1982).

One sustained-release bolus was weighed, placed in the rumen of each cannulated cow for 15 d beginning on d 43 in yr 1, and on d 40 in yr 2, removed, and weighed again to estimate daily Cr release rate from the boluses. Chromium release rate averaged 0.965 g/d during yr 1 and 0.963 g/d during yr 2. Fecal Cr concentration and daily Cr release rate were used to estimate FO using Eq. [1] given above.

Two ruminally cannulated cows per treatment were used to obtain forage extrusa samples via rumen evacuation on d 4, 39, 53 and 80 in yr 1, and on d 19, 47 and 61 in yr 2. Forage extrusa samples were air-dried, ground to pass a 2-mm screen in a Wiley mill, and analyzed for DM, N, OM (AOAC, 1997), NDF, ADF, ADL (Van Soest et al., 1991), and ADIN (Licitra et al., 1996). Extrusa composition values used in the statistical analyses (see Table 4) were the mean of extrusa samples collected on all dates within a year (n = 4 in yr 1, and n = 3 in yr 2) because extrusa composition did not vary by date (P > 0.10; data not shown). Extrusa collected on d 53 in yr 1 and on d 47 in yr 2 was used to determine in situ DM disappearance (DMD) and CMCase specific activity. Three polyester bags (10 cm x 20 cm; 50-µm pore size; Ankom Technology) containing the respective extrusa of each ruminally cannulated cow, one blank bag, and two bags containing supplement from the cows’ respective supplement treatment, were placed in the rumen at time 0 relative to supplementation on d 67 in yr 1, and d 51 in yr 2. Bags were incubated for 0, 2, 6, 24, 48, and 72 h. Each ruminally cannulated cow received a total of 36 bags, except for cows on the control treatment, which received 24 bags (no bags containing supplement). After removal from the rumen, two bags containing extrusa and two bags containing supplement were washed with cold water until the rinse water ran clear, and then dried in a forced-air oven at 60°C for 48 h. At each time point, the third bag containing extrusa was frozen and reserved for CMCase activity analysis (Silva et al., 1987). Dried bag residues were analyzed for DM (AOAC, 1997), corrected with blank bag DM values, and in situ DMD at each time point calculated. In situ 48-h DM indigestibility and FO were used to estimate forage intake using the following equation:

Cows were weighed at calving each year. At calving, calf birth date and calf birth weight were recorded. At weaning, calf weaning weight (WW) was recorded and WW adjusted for age of calf was calculated using the following equation:

Cows were rectally palpated for pregnancy diagnosis at weaning. In addition, the calf birth date the following year was recorded (calf birth date in yr 2 for the yr 1 study, and calf birth date in yr 3 for the yr 2 study) and used to calculate the calving interval for individual cows during both years.

Particulate-associated CMCase was determined on the residues in the bags that had been frozen after removal from the rumen at 2, 6, 24, 48, and 72 h of incubation (Silva et al., 1987). After incubation, residue in the bags contained 17.8% DM on average. Approximately 1 g (wet weight) of the residue was placed in 50-mL centrifuge tubes. Another 1 g (wet weight) was dried at 105°C for 24 h to determine DM. Twenty milliliters of 10 mM sodium phosphate buffer containing 20 µg/mL of lysozyme (Sigma Chemical, St. Louis, MO) and adjusted to pH 6.8 with 50% (wt/vol) NaOH was added to the centrifuge tubes, followed by 2.5 mL of CCl₄. Tubes were vortexed and incubated in a water bath at 37°C for 3 h. After incubation, tubes were centrifuged (29,000 x g; 4°C) for 15 min. The supernatant fluid was frozen and saved for later analysis of CMCase activity as described by Groleau and Forsberg (1981). The supernatant fluid was thawed, and 1 mL was placed into 15-mL culture tubes along with 1.5 mL of prewarmed (39°C) 2% (wt/vol) sodium carboxymethylcellulose containing 0.1 mg/mL of thimerosal. Tubes were vortexed and incubated in a water bath at 39°C for exactly 30 min. After incubation, hydrolysis of carboxymethylcellulose was measured by the formation of reducing sugar using 3,5-dinitrosalicylic acid reagent (DNS; Miller et al., 1960). On removal from the water bath, 3.0 mL of DNS was added to each tube. Color was developed by placing the tubes in a boiling water bath for 5 min. After cooling in tap water for 5 min, absorbance at 560 nm was measured. D-Glucose was used as the standard, and CMCase activity was expressed as micromoles of glucose released per gram of DM per minute.

Data were analyzed as a completely randomized design with the effects of treatment, year, and their interaction (SAS Inst., Inc.). The treatment sums of squares were partitioned into preplanned single-df orthogonal contrasts (Snedecor and Cochran, 1980) to determine 1) the linear effect of supplemental NSC level; 2) the quadratic effect of supplemental NSC level; and 3) the comparison of unsupplemented control vs. supplemented treatments. Because cows were fed supplement individually, an individual cow was considered to be the experimental unit. Performance and intake data from the ruminally cannulated cows were excluded from statistical analysis. Treatment least squares means and associated standard errors are reported.

	Results and Discussion

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Experiment 1
Heifer Digestion Trial.
Nutrient intake and digestibility of hay and total diet by heifers supplemented with increasing levels of NSC are presented in Table 2. Intakes of hay DM, OM, NDF, CP, and total diet DM, OM, NDF, and CP were increased (P < 0.01) by all levels of NSC supplementation. Hay and total diet digestible intakes of DM, OM, and NDF were also increased (P < 0.006) by all levels of NSC supplementation compared with the unsupplemented control. However, there was a quadratic response (P = 0.09) to increasing NSC supplementation in hay digestible NDF intake, which was increased (P = 0.09) to a greater degree when heifers were fed 0.64 kg of NSC compared with those fed 0.32 or 0.96 kg of NSC. Total diet NDF intake and diet digestible NDF intake decreased linearly (P = 0.03) with increasing NSC supplementation. All levels of NSC supplementation increased (P < 0.01) apparent total-tract digestibilities of hay and total diet DM, OM, and NDF compared with the unsupplemented control. Hay DM, OM, and NDF digestibilities responded quadratically (P < 0.04) to increasing level of NSC supplementation, with the highest digestibility seen when 0.64 kg of NSC was fed. These data agree with results reported by Martin and Hibberd (1990) and Ovenell et al. (1991) that supplementation with fibrous by-products containing low to moderate levels of starch can increase use of low-quality forage diets by cattle. In our study, digestibility of hay DM, OM, and NDF was highest when an intermediate level of NSC was fed. Heldt et al. (1999a) supplemented steers consuming low-quality tall grass prairie hay with starch and found a decrease in forage OM intake and NDF digestibility when 0.30% BW starch was fed compared with 0.15% BW starch. Our 0.64-kg NSC supplement was equivalent to 0.15% BW starch, and our 0.96-kg NSC supplement was equivalent to 0.23% BW starch. Farmer et al. (2001) supplemented steers consuming low-quality hay with supplements containing 30% CP and either 0.026, 0.057, or 0.107% BW starch, and found all supplement treatments to increase forage OM intake and digestibility.

Experiment 2
Grazing Trial.
No year x treatment interaction (P > 0.30) was observed for in situ DMD of supplements containing increasing levels of NSC after 2, 6, 24, 48, or 72 h of incubation (Table 3). In situ DMD of supplements after 6 h incubation was higher (P = 0.01) in yr 2 compared with yr 1. In situ DMD of the supplements after 2, 6, and 24 h increased linearly (P = 0.001) with increasing level of NSC in the supplement, with the lowest disappearance for the soybean hulls supplement, followed by wheat middlings and the barley-soybean meal. No effect (P > 0.44) of NSC level was observed on DMD of supplements at 48 h of incubation. However, DMD at 72 h responded quadratically (P = 0.006) to NSC level, being higher for the soybean hull supplement and the barley-SBM supplement compared with the wheat middlings supplement. Our results agree with findings reported by Bhatti and Firkins (1995), who found a longer lag but a greater extent of NDF digestion for soybean hulls than for wheat middlings. Soybean hulls have a relatively low lignin content, are low in ferulic and paracoumaric acids and noncore lignin phenolics that decrease digestibility of the structural carbohydrate fraction (Garleb et al., 1988), and contain highly degradable carbohydrates (Miron et al., 2001). In vitro digestibility of total carbohydrates and NDF in soybean hulls has been reported to be 90 and 83%, respectively, whereas in wheat bran, which makes up between 40 and 59% of wheat middlings (Blasi et al., 1998), in vitro digestibility of total carbohydrate and NDF was found to be 76 and 51%, respectively (Miron et al., 2001).

No year x treatment interactions (P > 0.15) were seen for in situ DMD or CMCase specific activity of extrusa after 2, 6, 24, 48, or 72 h of incubation, so the main effects of year and supplemental NSC level are presented in Table 5. In situ DMD of extrusa was greater (P < 0.03) at all time points of incubation in yr 2 compared with yr 1. Particle-associated CMCase activity was greater (P = 0.001) at 2 and 72 h in yr 2 compared with yr 1. An increase in particle-associated CMCase activity and resulting DMD may have resulted from the increased CP content seen in the extrusa during yr 2. Griswold et al. (2003) found increases in microbial protein synthesis and DM and NDF digestion in continuous culture when urea or ruminally degradable protein were added to diets low in ruminally degradable protein. No effect (P > 0.19) of NSC supplementation was seen on in situ DMD of extrusa after 2 or 6 h. However, in situ DMD of extrusa at 24, 48, and 72 h was decreased (P < 0.07) by all levels of NSC supplement. In situ CMCase activity of particle-associated bacteria was decreased (P = 0.001) with all levels of NSC compared with the unsupplemented control at all incubation times. In situ CMCase activity decreased linearly (P < 0.08) with increasing level of NSC supplementation at 2, 24, 48, and 72 h of incubation. Supplying NSC may have selected for noncellulolytic bacteria at the expense of cellulolytic, and thereby decreased cellulase activity and forage digestion (Firkins et al., 1991). These results agree with Martin et al. (2001), who reported decreased cellulolytic activity of particle-associated bacteria, and decreased rate of in situ forage digestion when ruminally cannulated cows were supplemented with higher levels of NSC.

In contrast to reports of negative effects, Ovenell et al. (1991), Matejovsky and Sanson (1995), and Heldt et al. (1999b; Exp. 2) reported improvements in forage use when NSC was supplemented. Our results from the grazing study (Exp. 2), where forage CP averaged 5.7%, are also in contrast to what was found in the heifer digestion study (Exp. 1), where forage CP averaged 5.5%. Forage CP content alone was not a good predictor of responses to supplementation with increasing levels of NSC. Moore et al. (1999) reported the change in forage intake when a supplement was added was generally negative when the unsupplemented forage had a DOM:CP of <7. In the grazing study, forage DOM:CP was estimated as <7 for both years, indicating adequate N relative to energy. Since N was not limiting in the base forage, forage digestion was already maximized. Any additional DOM provided by the supplements, unless accompanied by adequate N for its use, would actually create a ruminal deficiency of N relative to energy that was not previously present. All of the supplements had similar estimated DOM:CP (9.7, 9.5, and 9.7 for 0.32, 0.64, and 0.96 kg, respectively), which indicated deficient N relative to energy, and thus resulted in the observed decreases in DMD and CMCase activity of ruminal bacteria. This is consistent with the predicted depression in forage intake with supplementation of forages with DOM:CP <7 according to the theory of Moore et al. (1999). Matejovsky and Sanson (1995) reported a linear increase in low-quality grass hay (calculated DOM:CP = 8.2) DMI by lambs with increasing levels of corn supplementation, and linear decreases in medium- (calculated DOM:CP = 4.3) and high-quality (calculated DOM:CP = 3.4) hay DMI with increasing corn supplementation.

Klevesahl et al. (2003) provided steers consuming low-quality grass hay with supplements that varied in the ratio of rumen degradable protein (RDP) to starch. When supplement RDP was low, starch decreased forage intake and digestion, but when supplement RDP was high, starch had little effect on fiber digestion. These results indicate that in addition to the DOM:CP of the base forage, the energy and protein content of the supplement may greatly influence responses to supplementation of forage diets.

	Implications

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Providing supplements containing low (0.32 kg), intermediate (0.64 kg), and high (0.96 kg) levels of nonstructural carbohydrate increased hay digestible dry matter intake by heifers consuming low-quality hay with a deficiency of protein relative to energy, by supplying additional nitrogen to ruminal microbes. The same supplements resulted in decreased forage intake and digestibility by cows grazing native range that had adequate protein relative to energy. In this case, the supplements, which themselves were unbalanced in protein relative to energy, added energy without adequate nitrogen for its use, thereby creating an unbalanced energy:protein ruminal environment. Forage and supplement energy:protein content seemed to be better predictors of the response in forage intake and digestion to supplementation with increasing levels of nonstructural carbohydrate than forage protein levels alone.

	Footnotes

 
¹ This study was funded in part by Cenex Harvest States Cooperatives, Inver Grove Heights, MN.

³ Current address: Campbell County Extension, 1000 S. Douglas Hwy., Ste. A, Gillette, WY 82716.

² Correspondence—phone: 406-994-5563; e-mail: jbowman{at}montana.edu.

Received for publication August 28, 2003. Accepted for publication May 27, 2004.

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