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Effect of supplemental nutrient source on heifer growth and reproductive performance, and on utilization of corn silage-based diets by beef steers¹

Department of Animal Sciences, University of Kentucky, Lexington 40546-0215

	Abstract

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Two experiments were conducted to determine effects of oilseeds or soybean hulls on growth and reproductive performance of heifers and utilization of corn silage diets by growing beef cattle. In Exp. 1, 96 beef heifers (249 kg of BW) were used in a randomized complete block design. Treatments were as follows: 1) corn and soybean meal (CON) at 56% of the DMI; 2) whole linted cottonseed at 15% of the DMI (COT); 3) whole raw soybeans at 15% of the DMI (SB); or 4) pelleted soyhulls at 30% of the DMI (SH). Diets were formulated to be isonitrogenous (13.8% CP) and fed to achieve target weights equal to 65% of expected mature BW at the time of AI. Estrus was synchronized and heifers were inseminated by AI in response to detected estrus. Because the energy value for SH was underestimated, cumulative ADG for SH (1.03 kg/d) was greater (P 0.03) than for CON (0.89 kg/d), COT (0.87 kg/d), or SB (0.86 kg/d). Treatment did not affect (P > 0.10) the proportion of pubertal heifers at the beginning of the breeding season: CON (60%), COT (53%), SB (69%), SH (71%), or first-service conception rates: CON (37%); COT (38%); SB (57%); SH (42%). In Exp. 2, crossbred steers (387 kg) were used in a 6 x 6 Latin square design to evaluate the effects of supplemental nutrient source on utilization of corn silage diets. Treatments included diets used in Exp. 1, plus a negative control (soybean meal at 10% of the DMI; SIL) and whole raw soybeans at 25% of the DMI (SB25). Diets were formulated to be isonitrogenous (13.8% CP) except SB25 (17% CP), and were fed twice daily at 1.8 x NE_m. Oilseed inclusion decreased (P < 0.10) acetate:propionate ratios and (P < 0.10) apparent ruminal OM and ruminal and total tract NDF digestibilities. The CON and SH diets had the greatest (P < 0.10) total-tract OM digestibilities. Microbial efficiencies were greatest (P < 0.10), and long chain fatty acid flow to the duodenum increased (P < 0.10) with oilseeds. Biohydrogenation averaged 90.4% and increased slightly (P < 0.10) when oilseeds were added to the diet. Adding oilseeds or soybean hulls to corn silage-based diets did not affect reproductive performance of heifers. Although oilseed additions increased total fatty acid flow to the duodenum, a high degree of biohydrogenation occurred, greatly increasing C18:0, with only marginal increases in unsaturated fatty acid flow. Depending on diet and feeding conditions, inclusion of whole oilseeds may not be an effective means of increasing linoleic acid supply for ruminant animals.

Key Words: Digestion • Heifers • Oilseeds • Reproduction • Soybean Husks • Steers

	Introduction

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The lifetime productivity of a beef female is greatly affected by her age at first parturition. Management practices implemented in heifer development systems that enhance the ability to conceive early in the first breeding season will improve productivity over a lifetime in the herd (Lesmeister et al., 1973). Age and weight are two critical factors that work in concert to affect attainment of puberty by beef females. It has been shown that age at puberty can be reduced in females on a high plane of nutrition and delayed when on a low plane (Sorensen et al., 1959; Wiltbank et al., 1966). An increasing amount of evidence indicates that the ingestion and flow of supplemental fatty acids (FA) to the small intestine may benefit reproductive tissues and aid reproductive performance independent of energy supply (Thomas et al., 1997; Williams and Stanko, 2000; Bellows et al., 2001). Typically, heifers are developed on forage-based diets. Lipid-based supplementation has not been used to a great degree in forage-based beef cattle diets, presumably because of potential negative effects of fat on fiber utilization (Jenkins, 1993). Whole oilseeds have been shown to be a cost-effective, convenient approach to deliver dietary lipids and have been used to increase reproductive performance in beef females (Bellows et al., 2001). However, little is known about the influence of whole oilseeds on utilization of other dietary nutrients or their impact on flow of specific FA to the small intestine. Alternatively, energy sources high in digestible fiber, such as soybean hulls, may allow for increased energy intake without disruption of fiber digestion. The objectives of these studies were to determine the influence of oilseed or digestible fiber additions to corn silage diets on the following: 1) growth and reproductive performance of virgin heifers and 2) site and extent of OM and NDF digestion, ruminal fermentation, and flow of protein and FA to the duodenum in growing beef cattle.

	Materials and Methods

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All procedures involving animals were approved by the University of Kentucky Animal Care and Use Committee (protocol number 31A2000). In Exp. 1, 96 crossbred, virgin beef heifers (249 kg), averaging 9 mo of age at the beginning of the experiment, were used in a randomized complete block design to determine the effects of source of supplemental nutrients on ADG, attainment of puberty, and first service conception to AI. Heifers were grouped by weight and assigned randomly to 12 pens of eight heifers each, such that there were four pens of each of three weight blocks. Pen groups were assigned randomly within blocks to receive one of four corn silage-based diets, in which corn silage comprised 42% of the DM, and supplemental protein and energy sources comprised 56.5% of the DM (Table 1). Treatments were as follows: 1) corn-based control (CON) and three experimental diets based on CON in which 2) whole linted cottonseed (COT) was incorporated at 15% of diet DM, 3) whole raw soybeans (SB) were incorporated at 15% of diet DM, or 4) pelleted soybean hulls (SH) were incorporated at 30% of diet DM. Soybean meal was incorporated in the supplements at appropriate levels to ensure that all diets were isonitrogenous (Table 1). Diets were fed once daily at 0800, and were fed at approximately 2.0 x NE_m to achieve target weights equal to 65% of expected mature body weight at the time of AI.

Heifers were weighed every 28 d for 112 d, with weights obtained on two consecutive days at the beginning and end of the treatment period. After each weighing, feeding levels were adjusted based on the average weight in each pen to allow for increased maintenance requirements due to growth. Thus, all groups were fed to achieve the same rate of gain, with no adjustment from initial calculated dietary energy concentrations. Beginning on d 113, treatments were discontinued and all groups were fed the CON diet at an appropriate level to maintain target gains. On d 112 and 119, blood samples for serum progesterone analysis were obtained via jugular venipuncture and drawn into vacutainers which were placed on ice, transported to the laboratory, and centrifuged (1,500 x g for 15 min at 4°C). Harvested serum was stored at -30°C until analysis.

Serum progesterone was quantified using a commercial RIA kit (Coat-A-Count; Diagnostic Products Corp., Los Angeles, CA) as outlined by Imwalle et al. (1998). The procedure is a solid-phase, antibody-coated tube assay using ¹²⁵I-P₄ as the tracer and requires no extraction of serum. Inter- and intraassay CV were 3.7 and 6.9%, respectively. Heifers were considered to have initiated estrous cycles if progesterone concentrations exceeded 0.5 ng/mL in either sample.

Estrus was synchronized by feeding 0.5 mg•animal^-1•d^-1 melengestrol acetate (MGA) for 14 d (d 119 to 132), followed by i.m. administration of 25 mg of PGF₂ (Lutalyse, Pharmacia Upjohn, Kalamazoo, MI) 19 d after cessation of MGA feeding. Heifers were inseminated by AI in response to detected estrus on d 154 to 156 (beginning 48 h after PGF₂ administration). Conception rates to AI were determined on d 189 using transrectal ultrasonography.

In Exp. 2, six crossbred steers (387 kg) were used in a 6 x 6 Latin square design to evaluate supplemental energy source effects on utilization of corn silage diets. Ruminal and double-L duodenal cannulas (Streeter et al., 1991) were surgically placed in each steer. Treatments (Table 2) comprised six corn silage-based (CP = 9.1%, TDN = 67%) diets. Treatments differed primarily in the amount and type of supplemental energy sources. Amounts of soybean meal also varied in order to maintain similar N concentrations among diets. Treatments were as follows: 1) a negative control, which contained silage and no added supplemental energy source (SIL); 2) a corn-based positive control (CON); 3) pelleted soybean hulls incorporated at 30% of diet DM (SH); 4) whole linted cottonseed, incorporated at 15% of diet DM (COT); 5) whole raw soybeans, incorporated at 15% of diet DM (SB); and, 6) whole raw soybeans, incorporated at 25% of diet DM (SB25). Corn silage comprised 88% of the DMI in diet 1 and 42% of the DMI in diets 2 through 6. Cottonseed, whole soybeans, and soyhulls in diets 3 through 6 replaced a portion of the corn and soybean meal from diet 2. Diets were formulated to be isonitrogenous (13.8% CP), except for SB25, which had 17% CP. Diets were mixed daily and fed at 1.8 x NE_m. The energy value of the silage was calculated from chemical composition using a summative approach outlined by Weiss et al. (1992). Steers were housed in 1.2-m x 1.8-m individual pens in an enclosed facility with free access to water. Lighting was controlled to provide 16 h of light and 8 h of darkness daily.

Each 21-d period in the experiment consisted of a 10-d adaptation, 7 d of total fecal collection, 3 d of duodenal collection, and 1 d of ruminal fluid sampling. Steers were tied with neck chains and sequestered with a gate during fecal collections. Grab samples of feedstuffs were obtained daily during the fecal collection phase of each period, dried, and pooled within period for subsequent analysis. Fecal collections were performed daily by gathering the total fecal output from the floor. Total fecal output was weighed and sampled (1% of total fecal output) daily. Duodenal samples (approximately 350 g/sample) were collected four times daily at 6-h intervals, with the collection times advanced 2 h each day. Steers were weighed at the start of the experiment and at the end of each 21-d period following an overnight stand without access to feed or water.

Beginning on d 4 of each period and continuing until the end of the period, 8 g of chromic oxide was administered ruminally at each feeding (16 g of Cr₂O₃•steer^-1•d^-1) to serve as a duodenal flow marker. A pulse dose of Co-EDTA was administered at 0630 (1 h before feeding) on d 20 as a liquid flow marker. Each 500-mL dose was injected by syringe and dispersed throughout the rumen to facilitate uniform distribution. Ruminal fluid samples were collected before (0 h) and at 3, 6, 9, 12, and 24 h after dosing for determination of pH and of VFA, NH₃-N, and Co concentrations. All samples were analyzed for pH at the time of sampling using a portable pH meter fitted with a combination electrode (IQ Scientific Instruments, Inc., San Diego, CA). Samples were divided into two aliquots for subsequent analysis; 16 mL of each sample was frozen for Co analysis (all samples) and 8 mL was combined with 2 mL of 25% metaphosphoric acid and frozen for subsequent VFA and NH₃-N analyses (0- to 12-h samples).

On d 21, immediately before (0 h) and 1 h after the morning feeding, 1-kg samples of ruminal contents were collected from each steer, combined with 500 mL of a solution of cold, 10% formalin in physiological saline and frozen for subsequent isolation of ruminal bacteria. Prior to bacterial isolation, samples were thawed, processed through a blender, and strained through two layers of cheesecloth. Bacteria were isolated and prepared for analysis by differential centrifugation as described by Hannah et al. (1991).

Feed, fecal, and ruminal evacuation samples were dried at 50°C in a forced-air oven for 48 h. Individual duodenal samples were frozen and lyophilized. All dried samples were ground through a 1-mm screen using a Wiley Mill. Feed, fecal, duodenal, ruminal digesta, and ruminal bacterial samples were analyzed in duplicate for DM and OM as previously described. Crude protein concentration of feed, duodenal digesta, and bacterial samples were determined as described for Exp. 1. Chromium concentrations in feces and duodenal contents were determined using a Unicam 929 (ATI Unicam, Cambridge, U.K.) atomic absorption spectrophotometer after samples were prepared according to Williams et al. (1962). Fecal Cr recoveries for individual observations were used to correct duodenal flow values, which had been estimated from known dose rates and duodenal Cr concentrations. This correction assumes that loss of chromic oxide during transit through the gastrointestinal tract occurs before the duodenum, or that marker is not released in a steady-state fashion from the rumen. This assumption is substantiated by at least one study in sheep in which recovery of dosed chromic oxide from re-entrant duodenal cannulas was 79.8% (Mercer et al., 1980) and from human studies in which orally dosed chromic oxide continued to be excreted sporadically for up to 42 d after dosing had ceased (Allen et al., 1979). Portions of each lyophilized duodenal sample were reconstituted to 3% DM in 0.1 N HCl, mixed, and centrifuged (20,000 x g for 20 min). Then, the supernatant was analyzed for NH₃-N as described below for ruminal fluid. Ruminal bacteria and duodenal samples were analyzed for purine concentration as a marker for the calculation of microbial N flow to the small intestine and efficiency of microbial protein synthesis, using a modification of the procedure described by Zinn and Owens (1986). Modifications to the Zinn and Owens procedure included use of 2 N, rather than 12 N HClO₄, for sample hydrolysis (Makkar and Becker, 1999) and use of the precipitating solution to wash the sedimented pellet after centrifugation (Obispo and Dehority, 1999). Both modifications were incorporated because results in our laboratory (unpublished) supported the authors’ conclusions that recoveries of purines were increased by use of these procedures. True ruminal OM digestibility (OMD) was calculated as the apparent amount of OM digested in the rumen corrected for bacterial OM flow to the duodenum.

Duodenal and feed samples were analyzed for long-chain FA according to Sukhija and Palmquist (1988). Briefly, 2 mL of heptane with an internal standard (C 17:0) and 3 mL of freshly prepared methanolic-HCl were added to 100 to 300 mg of sample in a screw-cap culture tube. Samples were gassed with N₂, vortexed gently, and incubated for 2 h at 70°C for feed and 90°C for duodenal samples. Tubes were allowed to cool, 5 mL of 6% K₂CO₃ and 2 mL of heptane were added, and samples were vortexed and centrifuged for 5 min at 2,500 x g. Fatty acids were quantified using an HP 6890 gas chromatograph (Hewlett Packard, Avondale, PA) fitted with a 2-m x 2-mm column packed with SP2330 (Supelco, Bellefonte, PA). Nitrogen was used as the carrier gas at a rate of 20 mL/min. Inlet and flame ionization detector temperatures were set at 225 and 250°C, respectively, and the oven temperature was ramped from 130 to 210°C at a rate of 3.5°C/min. Ruminal biohydrogenation was calculated using an equation that accounted for the total number of C₁₈ double bonds that were saturated, rather than simply accounting for the percentage of FA that were saturated during passage through the forestomach (Eq. 2 of Tice et al., 1994).

Ruminal fluid samples collected for NH₃-N, VFA, and Co analysis were thawed and centrifuged at 39,000 x g for 20 min. Ruminal NH₃-N concentration was determined using a glutamate dehydrogenase procedure (171-B; Sigma Chemical Co., St. Louis, MO) adapted for use on a COBAS FARA II centrifugal analyzer (Roche Diagnostic Systems, Montclaire, NJ). Ruminal VFA concentrations were determined by gas chromatography as described by Vanzant and Cochran (1994). Cobalt concentration in the ruminal fluid samples was measured on a Unicam 929 atomic absorption spectrophotometer using an air-acetylene flame; ruminal fluid dilution rate was calculated as the slope of the regression of the natural log of sample concentration against sampling time, as described by Warner and Stacy (1968).

For Exp. 1, the GLM procedure of SAS (SAS Inst., Inc., Cary, NC) was used with a model appropriate for a randomized complete block design (terms for treatment and block were included in the model statement) to identify significant treatment effects for all variables using pen as the experimental unit. Means were separated using preplanned contrasts, in which treatments COT, SB, and SH were each tested against the control (CON) diet.

For Exp. 2, the MIXED procedure of SAS was utilized with the model including terms for steer, period, and supplement. Steer was included as a random effect, and period as a repeated effect. Fermentation characteristics were measured using a model including steer, period, supplement, and sampling time, where sampling time was specified as a repeated effect. Errors of repeated measures were modeled with an autoregressive correlation structure. When significant time x treatment interactions occurred, means within sampling times were evaluated to determine the consistency of treatment responses across sampling times using a protected (P < 0.10) Fisher’s LSD to separate treatment means. Differences between means were considered significant at P < 0.10.

	Results

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When formulating diets for Exp. 1, we used a low net energy value for soybean hulls (1.44 Mcal of NE_m/kg from NRC [1984] compared with 1.86 Mcal of NE_m/kg from NRC [1996]). Since the energy value for soybean hulls was underestimated, cumulative ADG (Table 3) for SH was greater (P = 0.01) than for CON, whereas COT and SB did not differ (P > 0.10) from CON. The percentage of pubertal heifers prior to synchronization and AI conception rates were unaffected (P > 0.10) by treatment.

	Discussion

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To estimate adequacy of dietary protein supply with these diets, degradable intake protein (DIP):TDN ratios were calculated in Exp. 2 by assuming that non-ammonia, non-bacterial nitrogen represented the undegradable intake protein fraction of dietary CP, that all remaining dietary CP was DIP, and that TDN was estimated by total-tract OMD. These values ranged from 13.0 to 18.0%, meeting requirements for DIP suggested by NRC (1996). Furthermore, duodenal N flows that were substantially less than N intakes support the concept that DIP was adequate for these diets. Thus, it is unlikely that DIP or ruminal NH₃-N concentrations (which ranged from 6.2 to 11.3 mM) limited microbial digestion or microbial protein production in the rumen.

The primary differences in molar proportions of acetate were between the two high-fiber diets (SIL and SH) and the diets receiving starch or oilseed supplements (CON, COT, SB, and SB25). It is well recognized that fiber-based diets result in higher proportions of acetate than starch-based diets (Jenkins, 1993). Although some researchers have observed small depressions in ruminal acetate proportions with unprotected lipid supplementation (Whitney et al., 2000), others have found no differences (Krysl et al., 1991; Scollan et al., 2001). The increase in molar proportions of propionate with dietary lipid is in agreement with Chalupa et al. (1984), Krysl et al. (1991), and Whitney et al. (2000). Changes in acetate:propionate ratio are mediated both through depressions in fiber digestion and through direct metabolism of glycerol backbones of triglycerides to propionic acid (Church, 1976; Noble, 1978). Decreased butyrate levels in oilseed-supplemented groups vs. CON is consistent with data from Whitney et al. (2000). Hess et al. (1996) suggested that increases in molar proportions of butyrate were elicited by increases in corn intake. Accordingly, our values may be related to replacing corn with oilseed sources in the diet. The greatest proportions of branched-chain VFA in the present study were found in the SB25 treatment. This can most likely be explained by their derivitization from fermentation of branched-chain AA, which is consistent with observed changes in ruminal NH₃-N concentrations. Requirements for isobutyrate and isovalerate by ruminal cellulolytic bacteria have been estimated to be between 0.05 and 0.5 mM (Dehority et al., 1967). Thus, it is unlikely that availability of branched-chain VFA was limiting fiber digestion for any of our dietary treatments.

Apparent ruminal digestibilities tended to be lower when oilseeds were included in the diet. These effects can be attributed to toxic effects of polyunsaturated FA on ruminal protozoa and bacteria (Sklan et al, 1985; Tamminga and Doreau, 1991; Jenkins, 1993). True ruminal OMD was not affected by treatments in this study, although numerical trends were generally similar to the responses seen with apparent ruminal OMD. The magnitude of the difference between apparent and true ruminal OMD was greater for the SIL and oilseed-supplemented groups, as indicated by the relatively large bacterial OM flows for these treatments. Furthermore, these differences in microbial OM flow were reflected in microbial growth efficiency. Because there were no differences in apparent postruminal OMD, apparent total tract OMD reflected responses seen with ruminal digestion. The observed total-tract apparent digestibilities compared favorably with our predicted dietary TDN concentrations shown in Table 2, with some small, but notable, differences. These differences could be viewed as an indicator of associative effects, in that the observed digestibilities reflect deviations in digestion from an additive model. To evaluate associative effects, we converted the observed OMD value for the SIL diet to a TDN value by: 1) assuming that each gram of digestible OM (without added fat) provides the same amount of DE as each gram of TDN (Heaney and Pigden, 1963); 2) converting all values to a DM basis; and 3) using 87% TDN for soybean meal (NRC, 1996) to calculate the observed basal silage TDN (68.5 % of DM). Using this value for the silage TDN and reported values for the TDN concentration of other feeds (NRC, 1996), we calculated expected TDN values using an additive model and compared these with values derived as above for our other diets while allowing for fat to contribute 2.25 times as much TDN as carbohydrate and protein (assuming digestibilities were similar for these three components). Differences from this additive model (observed–expected) were 0.6, 1.7, -4.1, -1.5, and -3.2, percentage units for CON, SH, COT, SB, and SB25, respectively. Thus, there was little evidence of associative effects for the CON, SH, or SB diets, whereas negative associative effects were evident for COT and SB25 groups. These observations correspond closely with differences seen in fiber digestion, discussed below.

The decrease in ruminal NDF digestibility with oilseeds is in agreement with other studies evaluating lipid inclusion to forage-based diets in ruminants (Vernon, 1976; Tamminga and Doreau, 1991; Jenkins, 1993). Ruminant diets containing in excess of 2 to 4% added lipid from plant oils are likely to depress fiber digestion within the reticulo-rumen (Jenkins, 1994). Alternatively, Krysl et al. (1991), Kouakou et al. (1994), and Brokaw et al. (2001) found no changes in ruminal NDF digestibility when cattle were fed hay and given supplemental soybean oil. The reason for this is unclear, although added lipids were below 5% of DMI in each case, which may have been insufficient to elicit such reductions.

With soybean hulls, a greater total-tract NDF digestibility (compared with SIL and CON) was accounted for by an increase in postruminal NDF digestion. There was essentially no postruminal digestion of NDF for the SIL or CON diets, in agreement with work by Tice et al. (1993) and Brokaw et al. (2001). Additionally, Tice et al. (1993) found a similar increase in postruminal NDF digestion when corn silage diets were supplemented with whole soybeans, whereas Brokaw et al. (2001) found no effects when free soybean oil was supplemented. This suggests that some fiber in whole soybeans and soybean hulls escapes the rumen and is fermented in the hindgut. Also, in the soybean and COT treatments, the shifting of the site of digestion to the hindgut may have been a result of the oil inhibiting fiber digestion in the rumen. With the inhibitory FA being absorbed in the small intestine, the deleterious effect may not have been evident in the large intestine and cecum. Demeyer (1991) noted that fat supplementation, which decreases ruminal fermentation, might tend to shift carbohydrate fermentation to the large intestine. Total-tract NDF digestibilities were the lowest for the oilseed treatments, suggesting that the negative effect of oils on the ruminal environment was not overcome postruminally.

On all treatments, N intake exceeded requirements (NRC, 1996) for growing beef steers consuming diets at 1.8 x NE_m. Differences among treatments were a consequence of planned differences in diet formulation. Although no differences were detected among treatments for bacterial N flow to the duodenum, it essentially paralleled bacterial OM flow. Accordingly, no differences were detected in bacterial N concentration, which averaged 7.6% of bacterial OM. Although statistical differences were identified for NH₃-N flow to the duodenum, the magnitude of the differences was unlikely to have substantial biological importance. As with bacterial N flow, nonammonia, nonbacterial N flow was not different among treatments. However, total duodenal N flows were greatest with SIL, least with CON, and intermediate with the groups receiving either fiber or oilseed energy supplements. Numerically, most of these differences were related to differences in bacterial N flow. Differences in bacterial nutrient flow to the duodenum are a consequence of differences in quantity of substrate fermented in the rumen and in microbial efficiency. In this study, with no differences detected in true ruminal OMD, results are largely explained by differences in microbial efficiency.

Our values for microbial efficiency are consistent with other literature reports (Krysl et al., 1991; Pantoja et al., 1995; Brokaw et al., 2001). Microbial efficiency was highest for the oilseed-supplemented diets. A decrease in protozoa, which are predatory on ruminal bacteria, may occur when oilseeds are supplemented, thus increasing efficiency of bacterial growth (Ushida et al., 1984; Kayouli et al., 1986). The lower microbial efficiency for CON compared with SIL could be related to a numerically lower dilution rate with CON (Isaacson et al., 1975; Meng et al., 1999). Additionally, fiber-digesting bacteria have been shown to have lower maintenance carbohydrate requirements than nonstructural carbohydrate fermenting bacteria (Russell et al., 1992), which might also help explain the relatively low efficiencies with high levels of corn in the CON diet.

Fatty acid intakes, particularly intakes of unsaturated FA, were increased with oilseed inclusion in the diets and were greatest with the high level of soybeans. Primarily as a result of greater intakes of C18:1 and C18:2, total FA intake was slightly greater for CON than for the SIL or SH diets. Net flow of total FA to the duodenum was positive for all treatments, indicating that more total FA passed to the duodenum than were consumed. Positive net flows of FA are commonly reported and can be explained by the presence of endogenous FA and de novo microbial synthesis of FA (Palmquist and Jenkins, 1980; Klusmeyer and Clark, 1991; Christensen et al., 1998). Total FA flow at the duodenum ranged from 430 to 669 g/d with oilseed diets. Increased duodenal flows of individual and total FA tended to correspond to the intake of each diet. The most notable changes were in the flows of C18:0 in relation to intake indicating a large degree of biohydrogenation in the rumen. Unless protected, the major FA in duodenal digesta is C18:0 because of hydrogenation of unsaturated FA by ruminal microorganisms. Within the oilseed-supplemented groups, biohydrogenation ranged from 91.6 to 92.0%, which was slightly greater than in the other three treatment groups, which ranged from 88.5 to 89.9%. This is in agreement with Bauchart et al. (1990), who demonstrated that biohydrogenation of linoleic acid increases with increasing dietary concentration of linoleic acid. Doreau and Ferlay (1994) compiled data from several studies to conclude that hydrogenation of linolenic and linoleic acid averaged 92% and 80% respectively. Scollan et al. (2001) reported that 90.2% of C₁₈ FA were biohydrogenated when linseed oil was supplemented at about 3% of the DMI to perennial ryegrass haylage. Doreau and Ferlay (1994) suggested that feeding whole oilseeds may partially protect FA from ruminal metabolism. However, our biohydrogenation and ruminal NDF digestion data do not support this contention. Others evaluating whole soybean supplementation have reported lower levels of biohydrogenation. Tice et al. (1994) reported only 56.9% biohydrogenation when whole raw soybeans were supplemented on corn silage diets. Similarly, Christensen et al. (1998) found that 57.2% of C₁₈ were hydrogenated when soybeans were added at 10% of DMI on mixed diets. The latter two studies used lactating dairy cows fed at a much higher level of intake than in the present study. With higher intakes and associated rapid passage rates from the rumen, ruminal alterations of FA would be expected to be decreased.

Results from other studies (Whitney et al., 2000; Bellows et al., 2001) suggest that oilseeds may enhance reproductive performance of beef females under certain conditions. Additionally, evidence from a variety of studies implicates the involvement of linoleic acid in stimulatory effects on ovarian function (De Fries et al., 1998; Williams and Stanko, 2000; Bellows et al., 2001). Although we increased linoleic acid flow to the duodenum with oilseeds, the magnitude of this increase was minimal, amounting to only 7 to 14 g of linoleic acid per day. Likewise, our additions of oilseeds did not impact the reproductive performance of the heifers in the first experiment. A variety of dietary factors can influence the ability of linoleic acid to flow from the rumen. Such factors may help to explain discrepancies in existing studies evaluating effects of oilseed supplementation on reproductive performance of heifers and cows.

	Implications

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Although some decrease was detected for fiber digestibility, oilseeds (up to 15% of dry matter intake from cottonseeds or up to 25% of dry matter intake from soybeans) were added to corn silage-based diets for growing beef cattle with little evidence of adverse effects on growth or efficiency of feed utilization. Soybean hulls added at 30% of corn silage-based diets increased postruminal and total tract fiber digestibility. Although inclusion of oilseeds increased flow of total fatty acids to the duodenum, a high proportion of the dietary unsaturated fatty acids were saturated in the rumen. Thus, depending on diet and feeding conditions, inclusion of whole oilseeds may not be an effective means of stimulating linoleic acid supply for ruminant animals.

	Footnotes

 
¹ This research was supported by the University of Kentucky Agr. Exp. Stn. and is publication No. 02-07-145 of the Kentucky Agric. Exp. Stn. The authors express their appreciation to J. Randolph, K. B. Combs, S. Hamilton, J. Greenwell, S. Rudd, J. Piel, and R. B. Hightshoe for their expert assistance in data collection and cattle management.

² Current address: 154 Kinkaid Dr., Lafayette, IN 47909.

⁴ Current address 203 Animal Sciences Bldg., Stillwater, OK 74078.

⁵ Current address: P.O. Box 342, Buckeystown, MD 21717.

³ Correspondence: 805 WP Garrigus Bldg. (phone: 859-257-9438; fax: 859-257-3412; E-mail: evanzant{at}uky.edu).

Received for publication September 13, 2002. Accepted for publication May 21, 2003.

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