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Effect of field pea replacement level on intake and digestion in beef steers fed by-product-based medium-concentrate diets¹

S. A. Soto-Navarro^*, G. J. Williams^*, M. L. Bauer^*, G. P. Lardy^*,2, D. G. Landblom and J. S. Caton^*

^* Department of Animal and Range Sciences, North Dakota State University, Fargo 58105, and and Dickinson Research Extension Center, Dickinson, ND 58601

Abstract

Four ruminally and duodenally cannulated steers (703.4 ± 41 kg initial BW) were used in a 4 x 4 Latin square to evaluate the effects of field pea inclusion level on intake and site of digestion in beef steers fed medium-concentrate diets. Steers were offered feed ad libitum at 0700 and 1900 daily and were allowed free access to water. Diets consisted of 45% grass hay and 55% by-products based concentrate mixture and were formulated to contain a minimum of 12% CP (DM basis). Treatments consisted of (DM basis) 1) control, no pea; 2) 15% pea; 3) 30% pea; and 4) 45% pea in the total diet, with pea replacing wheat middlings, soybean hulls, and barley malt sprouts in the concentrate mixture. Experimental periods consisted of a 9-d dietary adjustment period followed by a 5-d collection period. Grass hay was incubated in situ, beginning on d 10, for 0, 2, 5, 9, 14, 24, 36, 72, and 98 h; and field pea and soybean hulls for 0, 2, 5, 9, 14, 24, 36, 48, and 72 h. Total DMI (15.0, 13.5, 14.1, 13.5 ± 0.65 kg/d) and OM intake (13.4, 12.0, 12.6, 12.0 ± 0.58 kg/d) decreased linearly (P = 0.10) with field pea inclusion. Apparent ruminal (17.5, 12.0, 0.6, 6.5 ± 4.31%) and true ruminal CP digestibility (53.5, 48.7, 37.8, 46.2 ± 3.83) decreased linearly (P < 0.10) with increasing field pea. Neutral detergent fiber intake (8.9, 7.9, 7.8, 7.0 ± 0.3 kg/d) and fecal NDF output (3.1, 2.9, 2.6, 2.3 ± 0.2 k/d) decreased linearly (P < 0.03) with increasing field pea. No effects were observed for microbial efficiency or total-tract digestibility of OM, CP, NDF, and ADF (P 0.16). In situ DM and NDF disappearance rates of grass hay and soybean hulls decreased linearly (P < 0.05) with increasing field pea. Field pea in situ DM disappearance rate responded quadratically (P < 0.01; 5.9, 8.4, 5.5, and 4.9 ± 0.52 %/h, for 0, 15, 30, and 45% field pea level, respectively). Rate of in situ CP disappearance of grass hay decreased linearly (P < 0.01) with increasing field pea level. Field pea is a suitable ingredient for beef cattle consuming medium-concentrate diets, and the inclusion of up to 45% pea in by-products-based medium-concentrate growing diets decreased DMI, increased dietary UIP, and did not alter OM, NDF, or ADF digestibility.

Key Words: By-Product • Cattle • Digestibility • Field Pea • Intake

Introduction

Field pea acreage (Pisum sativum) has increased dramatically in North Dakota (NDASS, 2003). Reasons for increased production include benefits in crop rotations, adaptability to the northern Great Plains, use of conventional cultivation and handling equipment, and the fact that field peas are a legume and fix nitrogen in the soil (Anderson, 1998). In addition, field peas have potential to be used for livestock feeding (Reed, 2002).

In backgrounding systems for beef steers, gains of 0.7 to 1.2 kg/d are typically expected (Klopfenstein et al., 2000). Protein and energy requirements for such gains are typically achieved by either limit feeding high-concentrate diets or by feeding mixtures of grains and forages (NRC, 1996). Field peas, which contain approximately 24% CP and 48% starch, can be used as a protein and energy source (Petit et al., 1997). Dry matter intake was not affected when field peas replaced cereal grains in growing diets (Poland et al., 1996; Anderson, 1999). Improved feed efficiency was reported when field peas replaced barley and soybean meal in growing diets (Okine, 2001). Digestion characteristics of cattle consuming medium-concentrate diets that include field peas remain poorly quantified. This is especially true in medium-concentrate diets that contain by-products such as wheat middlings and soybean hulls. In addition, the optimal level of field pea inclusion in medium-concentrate diets is not well defined. We hypothesized that field peas can be successfully included into by-products-based, medium-concentrate diets with little or no adverse effects on digestive characteristics. Therefore, the objectives of this study were to evaluate the influence of field pea level on intake, site, rate, and extent of digestion, ruminal fermentation, and microbial efficiency in beef steers fed medium-concentrate diets.

Materials and Methods

Animals
Four beef steers (703.4 ± 41 kg initial BW) with ruminal and proximal duodenum cannulas were used in a 4 x 4 Latin square to evaluate the effects of field pea inclusion on intake, digestion, and microbial efficiency in beef steers fed medium-concentrate diets. The North Dakota State University Institutional Animal Care and Use Committee approved all surgical animal care protocols for this experiment.

Diets
The composition of the experimental diets is shown in Table 1. Diets included 0.25% (DM basis) chromic oxide added as a digesta marker. Diets were mixed weekly and stored under roof in concrete feed bays. Diets consisted of 45% grass hay (6.8% CP, 11.2% ash, 45.9% ADF, and 67.2% NDF; chopped to pass through a 3.81 cm screen) and 55% by-product-based concentrate mixture. Treatments consisted of (DM basis) 1) control, no pea; 2) 15% pea; 3) 30% pea; and 4) 45% pea in the total diet (Table 1), with peas replacing wheat middlings, soybean hulls, and barley malt sprouts in the concentrate mixture. Field peas were processed (dry-rolled) using a roller mill (model K, Roskamp Mfg, Inc., Cedar Falls, IA) so that most were cracked in half. Diets were formulated to contain a minimum of 12% CP, 0.70% Ca, and 0.32% P (DM basis). Steers were offered feed ad libitum at 0700 and 1900 daily and were allowed free access to water.

Laboratory Analysis
Samples were stored frozen (–20°C) until analysis. Dietary, ort, ruminal, and fecal samples were dried at 50°C in a forced-air oven for 48 h. Dried samples were ground with a Wiley mill (2-mm screen; Arthur H. Thomas, Philadelphia, PA). Duodenal samples were lyophilized (Virtis Genesis 25LL; The Virtis Co., Inc., Gardiner, NY) and ground in a coffee grinder (KSM2 Braun, The Gillette Co., Boston, MA). Samples were subjected to all or part of the following analysis: DM (oven drying at 105°C until no further weight loss); ash, Kjeldahl N, ammonia N (AOAC, 1997); ADF and NDF (Robertson and Van Soest, 1991); starch (Herrera-Saldaña and Huber, 1989); purines (Zinn and Owens, 1986); VFA concentration of ruminal fluid (Goetsch and Galyean, 1983); and chromic oxide (Fenton and Fenton, 1979). In addition, cobalt concentrations were determined in ruminal fluid with an air+acetylene flame using atomic absorption spectroscopy (Uden et al., 1980).

Calculations
Microbial organic matter and N (MN) leaving the abomasum were calculated using purines as microbial markers (Zinn and Owens, 1986). Organic matter fermented in the rumen (OMF) was considered equal to OM intake minus the difference between the amount of total OM reaching the duodenum and microbial organic matter reaching the duodenum. Feed N escape to the small intestine was considered equal to total N leaving the abomasum minus MN, and thus includes any endogenous and ammonia N contribution. Dilution rate for Co was calculated by regressing the natural log of marker concentration on sampling time. Ruminal DM, NDF, and ADF disappearance (%/h) of grass hay, soybean hulls, and field peas were estimated using the model described by Mertens and Loften, (1980). The CP kinetic parameters of grass hay, soybean hulls, and field peas were estimated using the model proposed by Ørskov and McDonald (1979).

Statistical Analysis
Data were analyzed as a 4 x 4 Latin square using the MIXED procedure of SAS (SAS Inst., Inc., Cary, NC). The model included field pea level and period as fixed effects, and steer as random effects. Ruminal data over time were analyzed as a repeated measured design using the MIXED procedure of SAS. The model included field pea level, period, time, and the interaction of field pea level x time as fixed effects, and steer nested within period x field pea level as random effects. Orthogonal contrasts for linear, quadratic, and cubic effects of field pea level are discussed when a significant (P < 0.10) treatment F-test was detected.

Results and Discussion

Dry matter intake (kg/d and g/kg BW) decreased linearly (P < 0.07) with increasing field pea level (Table 2). In other research conducted at our laboratory, Encinias et al. (2000) reported forage intake linearly declined and total intake linearly increased with field pea supplementation level in gestating cows fed forage-based diets (8.7% CP hay, DM basis). Also at our laboratory, Reed (2002) conducted a study in which field pea replaced corn in a diet that consisted of 50% corn, 23% corn silage, 23% alfalfa hay, and 4% supplement (DM); field pea level did not alter DMI in that study. Similar to the results of Reed (2002), no differences in DMI have been observed when field pea replaced mixtures of barley and SBM (Poland et al., 1996) or barley and canola meal (Okine, 2001). Greater forage intakes have been observed with limited quantities of supplemental grain, when N was not limiting for microbial growth (Guthrie and Wagner, 1988). However, it has been observed that starch has detrimental effects on fiber utilization of roughage-based diets (Sanson et al., 1990). Since the starch content of field peas is lower than that of other cereal grains (48% vs. 57 to 77%; Huntington, 1997; Petit et al., 1997), no differences in DMI were expected in this trial. Pordomingo et al. (1991) reported that forage OM intake tended to increase with whole corn supplementation at 0.2% BW, but forage OM intake linearly decreased with 0.4 and 0.6% BW. Assuming that starch concentration of corn was 72% (Huntington, 1997), then the proportions of starch consumed by the 0.2, 0.4, and 0.6% treatments were 4.6, 10.6, and 16.5%, respectively. In the present study, starch concentration increased from 9.5 to 16.3% for the 0 and 45% field pea concentration. Besides starch intake, other factors, such as rate of nonstructural digestion, may impact forage intake and characteristics of digestion. Perhaps the modest increase in starch level (Table 1) associated with increasing level of field pea resulted in the decline in intake observed in this study. It seems that starch has more of a negative effect when fed with low-quality forage (Sanson et al., 1990; Pordomingo et al., 1991) than when fed with moderate- to high-quality hay (Encinias et al., 2000; Reed, 2002). Alternatively, if the increase in field pea inclusion were associated with chemostatic satiety regulation, then a decrease in DMI would be expected (Grovum, 1988).

Increasing field pea level decreased OM intake (kg/d and g/kg BW basis; linear; P < 0.07; Table 3). However, there were no treatment effects on ruminal, postruminal, or total-tract OM digestibility (P 0.11). Likewise, total, microbial, and nonmicrobial OM flowing to the duodenum were not affected by treatment. The reduction in OM intake reflected the decrease in DMI with increasing field pea level reported in Table 2. Because the TDN content of field peas is higher than that of barley malt sprouts, soybean hulls, and wheat middlings (NRC, 1989; NRC, 1996), an increase in OM digestibility was expected. However, field pea level reduced rate of in situ forage degradability (Table 4), which likely explains why total-tract OM digestibilities were not affected by field pea inclusion.

The influence of field pea level on characteristics of starch digestion is shown in Table 5. Increasing field pea level linearly increased starch intake (P < 0.01) and starch reaching the small intestine (P < 0.03), and quadratically reduced (P < 0.04) total-tract starch digestibility. The increase in starch intake was by design; two of the fiber sources (soybean hulls and barley malt sprouts) were low in starch. Therefore, as expected, the replacement with field pea increased starch content of the diet. The decrease in starch digestion was unexpected because it is well documented that ruminants have a high capacity to digest starch. Owens et al. (1986) summarized data from 40 trials where high concentrate diets were fed and reported that the total-tract starch digestibility was 92.1 ± 5.5%. Waldo (1973) summarized results of 51 experiments dealing with starch digestion and utilization by sheep and cattle and reported that total-tract digestion of starch, across various grain sources, was 99 ± 1.2%. In our laboratory, Reed (2002) observed that the replacement of corn with field pea in a medium-concentrate growing diet did not affect starch digestibility, and mean values for digestibility ranged from 93 to 96%. In the present study, field pea replaced a fibrous by-product-based concentrate mix and resulted in a decline in starch digestibility from 96.2 to 81.8% when field pea level increased from 0 to 30%. Starch digestion was 87.3% at the 45% field pea level. These data may indicate that ruminal bacterial population shifted slightly from cellulolytic to amylolytic populations when field pea inclusion was between 15 and 45%. This seems likely since starch intake increased from 9.52 to 16.27% of the DMI for 0 to 45% field pea, respectively (Table 1). No treatment effects were observed on ruminal or postruminal starch digestion. However, the quadratic contrast tended (P = 0.13) to be significant for ruminal starch digestion, with the lowest ruminal digestibility observed in the 30% field pea treatment.

The influence of field pea level on ruminal fermentation characteristics is shown in Table 6. Overall, increasing field pea level tended (P < 0.14) to reduced ruminal pH. Although dietary starch concentration increased with increasing field pea level, pH changed minimally (6.48 to 6.28). These relatively small changes in pH likely helped minimize changes in ruminal bacterial population and in VFA production (Yokoyama and Johnson, 1993). However, our in situ data indicates rate field pea DM disappearance and CP effective degradability both responded quadratically with increasing level of field pea (Tables 4 and 7).

The influence of field pea level on the rate of ruminal DM, NDF, and ADF disappearance (%/h) of grass hay, soybean hulls, and field peas in beef steers is shown in Table 7. Forage DM, NDF, and ADF rate of ruminal disappearance linearly decreased (P = 0.02) with increasing field pea level. In situ rate of soybean hulls NDF disappearance linearly decreased (P < 0.04) with increasing field pea level. In contrast, the rate of in situ DM disappearance for field peas responded quadratically with increasing field pea level (P < 0.02). In general, the response to field pea level was more pronounced between the 15 and 30% field pea levels, which was not expected. Negative effects of adding starch-based concentrates to roughage-based diets may be limited when small quantities of supplemental grain (<0.4% BW) are used if CP is not limiting, whereas higher proportions of concentrate can compromise fiber digestion (Vanzant et al., 1990). Soluble carbohydrates, such as starch or sugar, may impede cellulose digestion due to factors such as lowered pH, competition between cellulolytic and noncellulolytic bacteria for essential nutrients other than energy, or use of alternative energy sources by certain of cellulolytic bacteria per se (Bryant, 1973). Bayourthe et al. (2000) reported a slower rate of degradation for cracked field peas vs. hammer-milled field peas. In this study, the field peas were dry-rolled. Bayourthe et al. (2000) did not include field peas in the basal diet when they measured rates of degradation, which may account for the relatively slow rates of degradation that they reported for cracked field peas.

When no field pea was included in the basal diet, in situ rates of field pea and soybean hull DM disappearance were similar (Table 7). However, when field peas were included in the diet at the 15% level, the in situ rate of soybean hull DM disappearance did not change, but the rate of field pea DM disappearance increased substantially. This may indicate some microbial adaptation to field peas is necessary. It is also possible that the in situ rate of field pea DM disappearance at the 15% level is abnormally high, since the rates for both the 30 and 45% levels were similar to the 0% level.

Forage in situ CP degradation rate (Table 4) decreased with field pea level and, as with DM, NDF, and ADF digestion rates, the response was more pronounced between the 15 and 30% field pea levels. The CP degradability of soybean hulls and field pea responded quadratically (P < 0.10) to increasing field pea level with the lowest degradability occurring at 30% field pea inclusion. The lower CP degradability of soybean hulls and field peas at the 30% field pea level partially explains the higher nonmicrobial CP flow at that level observed in this trial. In situ degradation data for CP are in agreement with our hypothesis that the ruminal bacterial population likely changed slightly from a cellulolytic to an amylolytic population when field pea inclusion level increased from 15 to 45%. After the transition of the ruminal population from a cellulolytic to an amylolytic bacterial population, decreases in forage degradability and an increase in field pea degradability would be expected (Yokoyama and Johnson, 1993). Therefore, the decrease on in situ CP degradation rate of forage and field pea CP degradability was not unexpected

In summary, increasing the level of field pea inclusion in medium-concentrate by-product-based growing diets resulted in decreased DM, OM, ADF, and NDF intakes and starch digestibility. In addition, increasing the level of field pea inclusion resulted in increased starch intake and nonmicrobial protein reaching the small intestine. However, no effects were observed regarding OM, ADF, and NDF digestibilities or ruminal fermentation characteristics as a result of increasing field pea inclusion.

Implications

Field peas may be included in diets for growing beef cattle. Our research indicates that the field pea is a suitable substitute for a combination of barley malt sprouts, soybean hulls, and wheat middlings in medium-concentrate diets. Inclusion of field pea in by-product-based medium-concentrate diets resulted in minimal effects on digestion characteristics; however, dry matter intake was decreased. When formulating diets, field pea cost and availability should also be considered.

Footnotes

¹ This material is based on work supported by the Cooperative State Research, Education, and Extension Service, USDA, under a subcontract with the University of Idaho, Cool Season Food Legumes Project. Gratitude is expressed to the employees of the Animal Nutrition and Physiology Center and NDSU Nutrition Laboratory for their valuable assistance with this project.

² Correspondence—phone: 701-231-7660; fax: 701-231-7590;e-mail: glardy{at}ndsuext.nodak.edu.

Received for publication August 26, 2003. Accepted for publication March 1, 2004.

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