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ANIMAL NUTRITION |
* Department of Animal Science, North Carolina State University, Raleigh 27695; and
Ag ProVision LLC, Kenansville, NC 28349
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: meat goat • nitrogen balance • ruminally undegradable protein • secondary protein nutrient • steer
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Current daily throughput in many processing plants creates 3.8 to 7.6 million L of wastewater. Approximately 83% of surveyed plants used the treatment technology of dissolved air floatation (DAF) to aid in waste-water purification (Kiepper, 2001
). The DAF process, as described by Edzwald (1995), yields approximately 106,340 t of DAF sludge yearly (K. Custer, American Proteins Inc., Cummings, GA, personal communication).
The nature of DAF sludge (75% water, 15% fat, 10% nonfat solids) makes its utilization a challenge. It must be land-applied on sites near its point of origin or dried to facilitate handling, packaging, and transport. Nutrient content varies as a result of changes in the species being processed or in the specific procedures used by the processor (Fransen et al., 1995). Addition of flocculants (frequently trivalent salts of Fe and Al) is common and may result in high residual levels of Fe and Al in the sludge, raising toxicity concerns. Iron can cause autooxidation and rancidity in the fat (Black et al., 1992). Removal of a portion of the protein and fat and subsequent drying yields a proteinaceous meal called secondary protein nutrients (SPN). The nutritive value of SPN, as compared with traditional protein sources, must be determined. Our research objectives were 1) to evaluate the acceptability of SPN to cattle and meat goats, and 2) to determine the impact of substituting SPN for soybean meal (SBM) on an equal N basis on animal performance, N balance, and metabolic parameters.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Steer Growth Trial
Treatment of Animals. Seventy-two Angus and Angus-cross steers that graded USDA M-1 were purchased from North Carolina graded feeder cattle sales and transported to the NCSU Butner Beef Cattle Field Laboratory. Upon arrival, they were ear tagged and vaccinated to prevent bovine diseases of concern in central North Carolina (Bovi-Shield 4, Pfizer Inc., Exton, PA, and Vision 7 with SPUR, Intervet Inc., Millsboro, DE). They were also treated to eliminate internal and external parasites (Cydectin, Fort Dodge Animal Health, Fort Dodge, IA). After a 28-d quarantine period, 60 steers were selected, stratified by BW, and placed in groups of 12 in 2.74 x 9.14-m pens equipped with Calan gate electronic feeders (American Calan, Northwood, NH), 1 automatic water bowl per pen, and slatted floors. They were fed a corn silage-based diet while they acclimated to the feeding system.
The steers were implanted with Synovex S (Fort Dodge Animal Health) on d 0. Two steers per pen (mean initial BW ± SD = 255 ± 15 kg) were randomly assigned to 1 of 6 experimental diets. Initial (d 1) and final (d 85) shrunk weights were determined on all the steers by removing uneaten feed at 1600 and weighing them the following morning at 0700, before feeding. These weights were used to calculate ADG.
Feeding, Feed Sampling, and Analysis.
Diets based on corn silage and a supplement mix comprising varying amounts of ground corn, SBM, SPN, vitamins, and minerals were formulated according to NRC (1996) guidelines to allow for 1.0 kg of ADG. With the exception of a protein-deficient negative control diet, diets were formulated to be similar in energy (2.55 Mcal of ME/ kg of DM) and CP. The negative control diet (0 AN) contained no supplemental N source. For the purpose of formulating diets, an ME value of 3.4 Mcal/kg was assumed for SPN based on values listed in the NRC tables (NRC, 1996) for ingredients with similar protein and fat contents. Diet formulation is listed in Table 1. A positive control diet (0 SPN) contained all added N in the form of SBM. The remaining 4 diets (25 SPN, 50 SPN, 75 SPN, and 100 SPN) contained graded amounts of SPN, which replaced SBM on an equal-N basis to give 25, 50, 75, or 100%, respectively, of the added N from SPN.
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Days 63 through 70 of the feeding period were designated as a collection period for individual intake samples, with the intent of determining whether the steers were sorting their feed. The TMR were sampled daily during this collection period and composited for the week. Individual orts were saved quantitatively for the week and subsampled for laboratory analysis. The ratio of CP consumed to CP offered was used to detect sorting.
Feed and orts samples were dried in a forced-air oven at 55°C to a constant weight and then air-equilibrated for 48 h to determine air-equilibrated DM. Samples were then ground in a Model 4 Wiley mill (Arthur A. Thomas Co., Philadelphia, PA) to pass through a 1-mm screen. Dry matter (105°C), ash, and Kjeldahl N were determined according to AOAC (1999) procedures. Concentrations of NDF and ADF were determined with 
-amylase, as described by Van Soest et al. (1991), but modified for use with an Ankom apparatus (Ankom Technology, Macedon, NY). Protein fractions were determined for the feed ingredients by procedures recommended by Licitra et al. (1996). Nonprotein N was determined by the subtraction of N in the precipitate resulting from treatment of samples with trichloroacetic acid from total N. Soluble N was calculated by subtracting the N in the residue remaining after treatment with a borate-phosphate buffer from total N. Protein insoluble in neutral and acid detergents was determined by subjecting samples to neutral or acid detergent digestion prior to N analysis. Lipid levels were determined by ether extraction of SPN and TMR samples using the ether extraction technique of Goldfisch (AOAC, 1999) with a Labconco extraction apparatus (Model 35001, Labconco Corp., Kansas City, MO). Analysis of SPN and the 6 TMR samples for Ca, P, Na, Mg, S, K, Cu, Fe, Mn, and Zn was accomplished by optical emission spectrometry (Optima 5300, Perkin-Elmer, Shelton, CT) by the North Carolina Department of Agriculture and Consumer Services forage analysis laboratory (Raleigh, NC).
Blood, Ruminal Fluid, and Urine Sampling and Analysis. Blood was drawn on d 28, 56, and 84 by jugular venipuncture for determination of plasma Cu levels by using potassium EDTA-treated Vacutainer tubes (Becton Dickinson and Co., Franklin Lakes, NJ) and 3.81-cm x 20-ga needles and placed immediately on ice for transport to the laboratory. Plasma was obtained by centrifugation at 2,316 x g for 30 min and frozen for later analysis. Plasma Cu concentrations were determined for the supernatant resulting from the centrifugation of a mixture of 1 mL of thawed plasma and 3 mL of 5% nitric acid at 1,050 x g for 20 min, followed by aspiration into the flame of an atomic absorption spectrophotometer (Model AA-6701F atomic absorption flame emission spectrophotometer, Shimadzu, Kyoto, Japan).
Individual urine samples were collected from the steers on d 70 or 71 before the morning feeding by confining 1 group of 12 steers at a time in a narrow chute. To accomplish collections, plastic bags held open by rubber rings were secured with elastic straps over the steers’ abdomens to cover the prepuce. Bags were removed as soon as each animal urinated or after 1 h, at which time all the steers were returned to their pen. Urine pH was measured immediately after sample collection (Fisher Scientific Accumet AP63 portable pH meter, Cole-Parmer Instruments, Vernon Hills, IL). Samples were acidified to pH <3.0 with 6 M HCl and stored frozen for total N and urea N determination. Total N was determined as Kjeldahl N (AOAC, 1999) and urine urea was determined colorimetrically by an automated diacetyl-monoxime method (Marsh et al., 1965).
On d 84 of the trial at 2 h (±20 min) after feeding, ruminal fluid was collected via stomach tube from steers restrained in a squeeze chute. Ruminal fluid pH was determined immediately (Fisher Scientific Accumet AP63 portable pH meter, Cole-Parmer Instruments). Samples were strained through cheesecloth and frozen for later determination of concentrations of VFA and NH3. Additional blood samples were drawn by jugular venipuncture for determination of blood urea N (BUN) by using untreated Vacutainer tubes (Becton Dickinson and Co.). The blood was allowed to clot at room temperature for 30 min, centrifuged at 1,470 x g for 30 min, and serum was frozen and stored until BUN was determined, as previously described for the urine samples.
Statistical Analysis. All data were analyzed by using PROC GLM (SAS Inst. Inc., Cary, NC) with class variables of pen and dietary treatment. Orthogonal contrasts were made of the 0 AN diet against the added N diets and among the added N diets to determine whether linear and quadratic effects were present. With the exception of DMI, pen effects were not significant, so they were dropped from the model and only treatment effects are reported. Intake differences were expected among pens because the animals were penned by BW. The SE for steer initial BW within a pen was 1.72 kg.
Goat N Balance Trial
Treatment of Animals. Twenty-four weanling 7/8 Boer wether goats were selected from the NCSU herd. They were dewormed with an oral dose of 2 mL of Cydectin (5 mg/mL of moxidectin, Fort Dodge Animal Health) and moved to a lot at the NCSU Metabolism Educational Unit (Raleigh, NC) for adaptation to the experimental feed. The kids were subsequently dewormed with Valbazen (4 mL/45.4 kg of BW, 11.36% albendazole, Pfizer) 18 d after the initial deworming to ensure a minimal parasite load.
During adaptation, the goats were group fed 2.0% of BW daily as concentrate pellets and also offered free-choice hay and water. They were adapted gradually to the experimental diets by a regimen that made dietary changes every third day. The entire group was switched over the course of 21 d from a mixture of 50% commercial pellets and 50% 0 SPN pellets to experimental pellets containing 40% of added N as SPN (40 SPN) with a liquid molasses coating. When the goats accepted the molasses coating, it was continued throughout the experiment in all treatment groups. The pellets were well accepted by the goats, with the exception of 1 goat that refused to eat any of the experimental concentrates.
Once the goats had been adapted to the 40 SPN pellets, 20 goats were selected for a N balance trial (BW = 17.0 ± 1.8 kg). The wethers were blocked by BW (4 goats/block) and placed into metabolism crates. Fecal egg counts were performed by using a Paracount-EPG fecal analysis kit (Chalex Corp., Issaquah, WA) on a composite of fresh feces collected from 1 wether randomly selected from each BW block on the second day the goats were housed in metabolism crates. High counts (> 1,000 eggs/g of feces) resulted in the retreatment orally of all the wethers with 2 mL of Cydectin (Fort Dodge Animal Health). Egg counts were reduced to <750 eggs/g by this treatment. All the goats were dusted with CoRal (1% coumaphos, AgriLabs, St. Joseph, MO) to eliminate lice. Two goats (1 receiving 20 SPN and 1 receiving 40 SPN) developed coccidial diarrhea on d 10 of the feeding period. Both were treated for 5 consecutive days with Corid (9.6% amprolium, Merial Ltd., Duluth, GA) given at a rate of 20 mg/kg of BW. The diarrhea was resolved and feces were normal by the third day of treatment.
Feeding, N Balance Sampling, and Analysis.
Diets were formulated according to NRC (1981) to achieve 100 g/d of ADG when concentrates and hay were each fed as 50% of the total diet. As in the steer trial, a negative control was formulated with no supplemental N (0 AN). Concentrates containing supplemental N in the form of SBM (0 SPN) and in which SPN replaced SBM to provide 20 or 40% of supplemental N (20 SPN and 40 SPN, respectively) were also formulated. These levels of SPN inclusion were selected based on the steer performance data combined with previous experiences with feeding goats novel feed ingredients. Poultry oil was added to the formulations in an attempt to balance fat across the rations, and a pellet binder (Super-Bind, Uniscope Inc., Johnstown, CO) was included to improve pellet quality. All concentrates were pelleted through a 5-mm die (Model D1173 Pellet Mill, Sprout-Waldron, Muncy, PA). Concentrate formulations are listed in Table 2.
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The day the goats were placed into metabolism crates, they were fed an amount equal to 3.5% of their individual BW. Half of the feed (by weight) was fescue hay xand the other half was 40 SPN concentrate. They were then switched to their randomly assigned treatments over 2 d. One wether in each BW block received each treatment. Once the animals were receiving 100% of their assigned treatments, total feed was offered at a rate to obtain 10% total feed refusals. Concentrate was offered 30 min before hay to allow its consumption before hay addition to reduce the amount of hay the goats would spill. By the end of a 14-d ad libitum feed intake period, intake levels had reached a plateau. From this point on, the goats were fed a constant amount of feed equal to 110% of the average intake for the last 3 d of the 14-d ad libitum period.
Fecal collection bags were placed on the goats at the initiation of the constant feeding period. After a 3-d adaptation to the collection bags, total feces and urine were collected for 5 d. Fecal collection bags were emptied twice daily and the contents were weighed once daily. The total feces were subsampled at a percentage set for individual goats to obtain approximately 100 g of DM from each animal over the 5-d collection. The fecal samples were composited daily into containers in a forced-air oven at 55°C. Following d 5 additions for the collection, the feces were allowed to dry for an additional 48 h, after which they were air-equilibrated for 48 h and then weighed for air-equilibrated DM determination. Dry samples were placed in sealed plastic bags and stored for later laboratory analysis.
Urine was acidified by the addition of 6 M HCl to the collection vessels to maintain a pH of 3 to 5, as determined with pH paper (pHydrion 1 to 12, Micro Essential Laboratory Inc., Brooklyn, NY). Acid additions varied by animal but were in the range of 10 to 30 mL per day. Additionally, 100 mL of deionized water was added to the collection vessels to minimize the crystallization of urinary compounds that are sensitive to low pH. Total urine was removed from the collection vessels daily. Total weight and volume of the acidified urine were determined, and a subsample was obtained by weight. The percentage of the total urine saved also varied by animal and was established on d 1 of the collection period. It was set to yield 50 g of urine and remained constant for the 5-d collection. Urine samples were composited, frozen, and stored at –10°C for later laboratory analysis.
In addition to fecal and urine samples, feed samples (100 g of each feed/d) were collected, and feed refusals were saved and composited quantitatively. Subsamples of the feed refusals were taken at the end of the 5-d period. All feed and feed refusal samples were placed into a forced-air oven at 55°C and dried for 48 h. After a 48-h air-equilibration period, the samples were weighed for barn DM determination. Procedures for laboratory analysis of these samples were those described for similar samples generated during the steer trial, with the exception of fat determination. Feed samples from the goat trial were analyzed for fat by Dairy One Laboratory (Ithaca, NY) by ether extraction with anhydrous diethyl ether (Soxtec HT6, Foss Tecator AB, Höganäs, Sweden).
Ruminal Fluid and Blood Sampling and Analysis.
After total excreta collection, the goats were put onto a staggered feeding regimen in which the initial groups of 4 goats blocked by BW were fed at 15-min intervals. On the second day of the staggered feeding regimen, blood samples were collected into untreated 10-mL Vacutainer tubes (Becton Dickinson and Co.) with 20-ga x 2.5-cm needles via jugular venipuncture before feeding (time = 0 h) and at 2, 4, and 8 h after feeding. All blood samples were immediately placed on ice and later processed to obtain serum, as previously described.
Ruminal fluid was also obtained by rumenocentesis 2 h after feeding, as described by Nordlund and Garrett (1994) and modified for goats using 14-ga x 5-cm needles and with no local anesthesia. A minimum of 5 mL of fluid was withdrawn from each goat. Ruminal fluid pH was determined immediately upon sample removal by using a Fisher Scientific Accumet AP63 portable pH meter (Cole-Parmer Instruments), after which the samples were placed on ice. Ruminal fluid and serum samples were then frozen and stored at –10°C. These were analyzed as described for the steer trial.
Statistical Analysis. Statistical analysis of goat data was by PROC GLM (SAS Inst. Inc., Cary, NC). The model included class variables of treatment, BW block, and crate type (wood or metal). No significant differences (P > 0.10) were detected for crate type or BW block, so they were removed from the model and only treatment differences are presented. Orthogonal contrasts between 0 AN N-supplemented diets and among N-supplemented diets were conducted. For the BUN data, the independent variables were goat within treatment, treatment, time, and the time x treatment interaction. The latter played no significant role in the model and was dropped. The goat within treatment term was used as the error term to test for treatment differences.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Steer Growth Trial.
The SPN used in this trial contained 46.5% CP (Table 3), with 90% of CP being slowly degradable or undegradable in the rumen. It also contained 27.8% ether extract, 0.95% Ca, and 1.78% P. Because ferric chloride (FeCl3) had been used as a floc-culating agent during water treatment, the SPN was exceptionally high in Fe (28,200 mg/kg). The concentrations of Zn and Cu were also substantial (329 mg/kg and 178 mg/kg, respectively).
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and 3); therefore, the diets contained less than the 13% CP for which they were formulated (Table 4). Nonetheless, the 5 N-supplemented diets were similar in CP content (mean CP ± SD = 10.7 ± 0.33%), with the 0 AN (negative control) diet lower in CP (7.6%). Iron and Cu contents of the diets increased with the increase in SPN, as did the ether extract content of the diets (Table 4).
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). The protein fractions of the feeds were determined by laboratory analysis rather than by calculation to account for changes that may have occurred in the fractions as a result of pelleting the feed.
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Steer Growth Trial.
Dry matter intake showed both linear and quadratic decreases in response to dietary additions of SPN (Table 6). Intake peaked when SPN contributed 25% of added N and declined dramatically when SPN was increased to 75 and 100% of added N. Steers that received their entire supply of supplemental N as SPN consumed quantities of DM similar to those of the negative control (0 AN) steers.
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), with similar gains for the 0 SPN and 25 SPN steers and with additional SPN in the diet causing ADG to decline. As was observed for DMI, steers receiving the 0 AN diet performed similarly to those receiving the 100 SPN diet.
Daily CP intake was determined for the steers for d 63 to 70 based on the feed and orts samples collected during the intake measurement week (Table 6). Intake of CP followed the same pattern as DMI, with both linear and quadratic declines in CP intake as SPN increased. Crude protein ratio was calculated as the percentage of CP in feed consumed divided by the percentage of CP in feed offered to evaluate the extent to which the steers sorted the SPN out of their feed (Table 6). Linear and quadratic declines in this ratio suggest that the steers began sorting their feed as the SPN level increased to more than 75% of added N.
Diets containing added N had higher G:F than the 0 AN diet (P < 0.001). The substitution of SPN for SBM caused greater reductions in gain than in DMI because G:F decreased linearly as SPN increased (Table 6).
Goat N Balance Trial.
The addition of a supplemental N source to diets for the goats increased DMI (P = 0.01) and daily N intake (P < 0.001); however, it did not change the digestibility of DM, NDF, or ADF (P > 0.10; Table 7). Fecal N increased linearly (P = 0.06) as the proportion of SPN increased in the diet (Table 7); however, there was no change (P > 0.10) in N absorption as a result of increasing SPN in the feed. Goats that received the unsupplemented feed absorbed less N than supplemented goats (P < 0.001; Table 7).
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; P < 0.001), whereas urinary N was similar among supplemented goats (P > 0.10). Nitrogen retention was also similar among supplemented goats, but there was a trend (P = 0.11) for a quadratic relationship between SPN level and N retention, with peak retention being obtained with the 20 SPN diet. Goats that did not receive supplemental N retained less N than their supplemented peers (P = 0.01; Table 7).
Despite the differences in the quantities of N retained between the supplemented and unsupplemented goats, there were no differences between treatment groups in the proportion of feed N retained or in the proportion of absorbed N retained (P > 0.10; Table 7).
Blood, Urine, and Ruminal Parameters
Steer Growth Trial.
All the steers consuming diets with added N had higher BUN than did the negative control steers (P < 0.001), and BUN exhibited a linear decline as the proportion of SPN in the diet increased (P < 0.001; Table 8). Plasma Cu was quadratically related to the proportion of SPN at d 28, but this relationship was no longer present by d 84 (Table 8). Because d-0 blood samples were not obtained, we could not determine whether this shift represented an overall change in Cu status for the steers that was the result of the diet. Changes in plasma Cu level between d 28 and 84 (Table 8) were not affected by treatment.
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) did not differ when negative control animals were compared with N-supplemented animals (P > 0.10). Increasing the proportion of supplemental N supplied by SPN, however, decreased urine pH linearly (P < 0.001). The percentage of urinary N as urea N (%UUN; Table 8), which can be an indicator of the overall N status of ruminants, declined linearly (P < 0.001) and quadratically (P = 0.01) as SPN increased in the diet. Additionally, steers receiving supplemental N in their feed excreted urine with higher %UUN than the negative control steers (P < 0.001).
Ruminal NH3 (Table 9) declined linearly with each addition of SPN to the diets (P < 0.001) and was higher in fluid from N-supplemented steers than from 0 AN (P < 0.001). Ruminal pH was unaffected by increasing the proportion of SPN in the diets (Table 9). Similarly, total VFA concentration (Table 9) did not vary as a result of the SPN substitution level in the diets, although there was a trend (P = 0.11) toward a linear decline with increasing levels of SPN. Unsupplemented steers had lower total VFA than steers supplemented with N (P = 0.002; Table 9).
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). Butyrate and isobutyrate declined linearly as SPN increased, and propionate exhibited both linear and quadratic increases with SPN concentration (Table 9). Changes in propionate proportions resulted in a linear decrease in the acetate to propionate (A:P) ratio as the proportion of SPN increased.
Goat N Balance Trial.
Goats that received supplemental N in their feed excreted larger quantities of urea than goats that received no supplemental N (Table 7). The %UUN was similar for goats on all treatments (Table 7).
The addition of SPN to the concentrates had several effects on ruminal parameters (Table 10). Ruminal pH declined linearly (P = 0.10) as SPN increased in the diets, but no difference was observed between supplemented and unsupplemented animals.
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). It was similar among the goats receiving varied levels of SPN in their feed, with only a slight trend (P = 0.14) for a linear decline in ruminal NH3 concentration. Total VFA were lower in ruminal fluid from the goats receiving the 0 AN diet than in the goats receiving supplemented diets (P = 0.07). There were no differences among the supplemented goats in total VFA concentrations.
The molar proportion of acetate increased linearly as SPN increased in the diet, and the proportion of propionate declined linearly (Table 10). This resulted in a linear increase in the A:P ratio, with a trend toward a quadratic relationship (P = 0.11). There were few differences in the proportions of the other ruminal acids when treatments were compared (Table 10), although goats receiving no supplemental N had reduced proportions of butyrate (P = 0.04) and valerate (P = 0.03) as compared with supplemented animals.
There were no treatment differences in BUN as a result of increasing SPN in the feed (Table 10). Negative control goats had lower BUN than supplemented goats (P = 0.03).
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The protein content of the SPN used in our studies (Table 3) was similar to the range of protein concentrations reported by Fransen et al. (1995), who surveyed similar material from 5 poultry-processing plants and 9 swine-processing plants. The products evaluated by Fransen et al. (1995), which were produced with a similar chemical treatment, also contained high levels of Fe, Cu, and Zn (21,832 to 49,444 mg/kg of Fe; 38 to 183 mg/kg of Cu; 165 to 550 mg/kg of Zn). The product used in our studies fell into the reported ranges for these minerals. It appeared likely that for use in ruminant feeds, SPN would be characterized in a manner similar to other nonruminant, nonspecified risk material protein by-products, such as blood meal or meat and bone meal.
The high Fe content raised concerns that animals eating this product might develop Cu deficiency. We did not measure mineral digestibility or balance in the goat trial, nor are any data on these parameters available in the literature. Because we did not see any treatment impact on plasma Cu changes over the course of the steer trial, however, we believe that the available Cu concentration in the SPN was high enough to balance available Fe concentrations.
The protein fraction profile of SPN (Table 3) indicated that it may be a good source of ruminally undegradable protein (RUP). The B2 and B3 protein fractions of SPN constituted 41 and 44% of the CP, respectively, whereas SBM, which is a more common source of protein in ruminant diets, contained 70.2% as B2 and 5.4% as B3.
The negative values obtained for the B1 fraction in the corn silage and SBM (Table 3) are not biologically possible. This anomaly in the data is the result of the analyzed amount of NPN present in these feeds being slightly higher than the calculated amount of soluble true protein present. Soluble true protein values were obtained by subtracting the amount of insoluble CP (which was assumed to be all true protein) and NPN from total protein. It may therefore contain error caused by the presence of insoluble NPN compounds. An overestimation of insoluble true protein would result in an underestimation of soluble true protein. When the NPN and soluble true CP fractions are similar in size, the calculation of soluble true protein (CP – insoluble CP– NPN) may result in a negative number if insoluble protein is overestimated.
The effects of SPN additions on proportions of the protein fractions were apparent in the goat concentrates (Table 5). Although the total percentage of N as fractions B2 plus B3 remained fairly constant, the B2 fraction declined as the B3 fraction increased. Additionally, the unavailable C fraction increased, again reflecting the increasing SPN concentration in the goat concentrates.
Animal Performance and Metabolic Parameters
The changes observed in both steer and goat performance and metabolic status are those expected when a protein-deficient diet is supplemented with protein and when a ruminally degradable protein source is replaced by one of lower degradability. The DMI results presented from our steer trial (Table 6) were similar to those of Swartz et al. (1991), who replaced SBM with blood meal in diets for growing Holstein calves. Blood meal also contains high levels of B2 and B3 protein and is considered a good source of RUP.
Increased DMI when a source of supplemental N was added to the diets was a common response in both steers and wethers. This change may have been the result of improved fiber digestion caused by more NH3 being available for use by cellulolytic bacteria, which prefer NH3 as their N source. The ruminal NH3 concentrations we observed (Tables 9 and 10) are consistent with this explanation.
Reported values of optimal NH3 for ruminal fermentation vary from 5 to 23.5 mg/dL (Satter and Slyter, 1974; Mehrez et al., 1977; Van Soest, 1994) and likely vary with species and diet. Ammonia in ruminal fluid from our steers fell below or toward the lower end of this range (Table 9). Goat ruminal fluid, with the exception of that from the negative control group, exceeded this range (Table 10). The reduced rate of fiber degradation as a result of lower ruminal NH3 could therefore have limited the ability of the cattle and 0 AN goats to digest fiber as quickly as animals with higher NH3. This could have resulted in reduced passage from the rumen, which in turn would have caused reduced intake. Total tract fiber digestibility, however, was not influenced by treatment (Table 7).
Declines in ruminal NH3 as SPN increased in the steer diets (Table 9) reflected both the lower ruminal degradability of SPN as compared with SBM and the reduced DMI of the steers. Similar reductions in ruminal NH3 were seen by Cunningham et al. (1994) and Bohnert et al. (1998) when they substituted high-RUP protein sources for lower RUP protein sources in diets for dairy cattle and steers, respectively.
The lack of change in DMI and ruminal NH3 in the goats receiving SPN (Tables 7 and 10) may have resulted from the lower inclusion rates used for the N balance study. The SPN inclusion rates for the goat concentrates were intentionally chosen to be below the levels at which negative responses were seen in cattle.
Another explanation for the difference in responses between steers and goats could be the difference in fat content. The steer diets were not balanced for fat in the SPN, so the fat content increased from 2.4 to 5.1% as SPN increased (Table 4). The goat diets were balanced for fat content and averaged 5.2% fat (Table 5). Onetti et al. (2001) found that adding fat to the diets of lactating dairy cows reduced DMI and ruminal NH3. Lower ruminal NH3 was attributed to a reduced protozoa population, which was also observed with fat supplementation. Although we did not measure microbial populations, these findings agreed with what we observed with regard to other ruminal parameters and could explain why the species seemed to respond differently to SPN.
Data presented by Ikuta et al. (2005) demonstrated that changes in BUN levels occur approximately 2 h after changes in ruminal NH3. The changes in BUN observed in our trials (Tables 8 and 10) therefore likely reflected the changes in ruminal NH3 and may have been in response to declining protein degradability and increased fat as SPN increased in the feed. They were also in agreement with the findings of other trials (Knaus et al., 1998; Huntington et al., 2001; Knaus et al., 2002).
Palatability may have been an issue at higher SPN substitution levels. The decline in CP ratio as the SPN level increased in the steer diets (Table 6) did not become obvious until SPN supplied 100% of the supplemental protein, suggesting that the animals accepted the SPN until it exceeded 8.6% of DM. Sungwaporn (2004) noted declines in intake when SPN from the same batch of product was added to broiler diets at the rate of 7.5% of DM (equivalent to a 64.7 SPN diet in our steer trial). They offered palatability as an explanation for the decreased DMI with SPN diets.
Reduced ADG among steers receiving SPN in their feed may have been caused by a combination of reduced feed intake and lower G:F when SPN increased in the diets (Table 6). Sungwaporn (2004) attributed decreased gain in broilers to reduced feed intake when SPN was added to the feed in place of SBM. They also reported linear declines in feed efficiency (the reciprocal of the efficiency of gain, which is represented by G:F) as SPN increased from 7.5 to 20% of DM. El Boushy et al. (1984) described reduced gain and efficiency among broilers as dehydrated poultry slaughterhouse effluent (37.5% CP, 28.6% fat, 41,000 mg/kg of Fe, air-dried basis) increased from 2 to 7% in their diets. Nelson et al. (1985) observed a depression in ADG in steers fed wheat straw and corn silage diets supplemented with blood meal instead of SBM, although they did not observe reduced DMI. This depression was partially alleviated when the wheat straw was ammoniated, suggesting that limiting ruminal NH3 may have played a role. The negative control steers in our study had lower G:F than the supplemented animals (Table 6), indicating that even the 100 SPN diet had benefits over offering no supplemental protein source. The low N level of the 0 AN diet and the reduced protein degradability of the higher SPN diets were apparently limiting animal performance.
Another possible explanation for the reduction in performance was the high Fe content (1,004 to 3,135 mg/ kg DM) of the SPN-supplemented feed caused by the FeCl3 flocculant. Harrison et al. (1992) found that additions of 100 to 1,000 mg of Fe/mL of ruminal fluid resulted in depressed degradation rates for forage substrate DM in vitro. Total VFA concentrations were not altered by the Fe, but rates of production were reduced. They suggested that feed Fe concentrations in excess of 500 mg/kg (DM basis) could supply adequate Fe to have toxic effects on ruminal microorganisms and result in reduced intakes and daily gains. The TMR containing SPN all contained more than 500 mg/kg DM of Fe, meaning toxic effects on ruminal microorganisms could have occurred, reducing intake and gain by depressing the rate of fiber degradation.
In addition to reduced intake and feed efficiency, Sungwaporn (2004) reported an increased incidence of rickets among broiler chicks receiving the higher levels of SPN in their diets. The rickets were attributed to a vitamin D deficiency, which was the result of dietary vitamin D destruction by Fe and other pro-oxidants contributed to the feed by the SPN.
As mentioned earlier, preventing fat oxidation and rancidity and the destruction of fat-soluble nutrients is one of the challenges presented by using DAF sludge as a nutrient source. Rancidity does not have detrimental impacts on palatability or fat utilization in cattle (Zinn, 1995), but can reduce milk fat and protein production in lactating animals (Heinrichs et al., 2005). We did not monitor the feeds for the presence of rancidity or nutrient destruction, and we did not observe any indications in the animals that these issues were presenting themselves.
According to the NRC (1981), goats similar to those used in our trial require 10.6 g of N/d to gain 100 g/d. With the exception of the 0 AN treatment, N intake by the goats was similar across treatments (Table 7), exceeded their requirement, and therefore should have supported 100 g/d of gain or more if no other nutrients were lacking. Nitrogen intake for the 0 AN diet exceeded the maintenance recommendation of 6.1 g of N/ d, suggesting that the animals on the 0 AN treatment should have maintained their BW if the diet was not lacking in other nutrients. This expectation was also supported by a N retention value greater than zero (Table 7).
The lower fecal excretion of N by the unsupplemented animals compared with the N-supplemented animals was expected. Linear increases in fecal N as SPN increased in the diet were anticipated because of increases in both the slowly degraded B3 fractions and the largely indigestible C protein fraction contributed by the SPN. Despite the increase in fecal N caused by the SPN, all supplemented goats absorbed similar amounts of N, but N digestibility declined linearly with SPN additions to the diet. This result was expected because the C protein fraction increased in the diets. Our results were similar to those reported in other trials when RUP was increased in feed (Cunningham et al., 1994, with dairy cows; Bohnert et al., 1998, with steers).
Similar urinary excretion of N among supplemented goats suggests that a large portion of the B3 protein described earlier was indeed being digested and absorbed in the lower gastrointestinal tract. In agreement with our findings, Brun-Bellut et al. (1991) found that similar quantities of urinary N were excreted by nonlactating does receiving diets containing both high and low levels of ruminally degradable protein.
The lack of a decline in N retention in goats as SPN increased seemed to contradict the decline in ADG observed in steers; however, as stated earlier, the goat diets were designed to contain SPN levels lower than those that caused negative responses in the steers (50% SPN inclusion for ADG). There was a trend (P = 0.11) for a quadratic relationship between SPN level and the amount of N retained. If the SPN content of the goat diets had been increased beyond the 40% level, declines in N retention may have occurred. The trend suggested that the optimal inclusion rate for SPN in the diet would be approximately 20% of supplemental N. With only 3 levels of SPN, however, precise estimation of the optimal SPN level was difficult. The 20 SPN level in the goat diets and the 25 SPN level in the steer rations (which produced the highest ADG among the levels tested in cattle) were similar in dietary proportion (% of dietary DM), and this supports the theory that the optimal level of SPN inclusion would be to supply 20 to 25% of supplemental N as SPN. Additional support for the 20 to 25% optimal inclusion rate range came from data on retained N as a percentage of N intake (Table 7), which also peaked with the 20 SPN goat diet and which was similar to rates reported by others (Lindberg, 1989; Woodard and Reed, 1997; Merkel et al., 2001). Retaining the highest proportion of dietary N is of paramount importance to maximizing profit and minimizing the proportion that is excreted into the environment.
The %UUN can provide a glimpse into the N status of a ruminant animal. Increasing the N intake above the required level resulted in higher %UUN (Archibeque et al., 2001; Huntington et al., 2001). Based on the reductions in CP intake with increasing SPN, and assuming that the differences between supplemented and unsupplemented steers reported from d 63 to 70 (Table 6) are representative of protein intakes for the entire trial, we expected our observations that the 0 AN steers had lower %UUN than steers receiving the supplemented diets and that %UUN declined among the supplemented steers (Table 8) with increasing SPN.
Declining %UUN among supplemented treatment groups with increasing SPN and the lower %UUN of the negative control steers (Table 8) might also have indicated increased urea recycling to the rumen associated with reduced ruminal NH3. Rémond et al. (1993) demonstrated that increasing NH3 concentrations within the rumen limited urea uptake by the rumen. Applied in reverse, the reduced NH3 concentrations observed with the higher levels of SPN in the diets and when the 0 AN diet was fed would have led to enhanced urea uptake by the rumen, leaving less urea in the blood to be excreted in the urine. The reduced ruminal degradability of the dietary protein created by adding SPN to the feed caused lower ruminal NH3, which compounded with reductions in intake to depress %UUN.
The absence of a difference in %UUN between the negative control goats and the N-supplemented goats (Table 7) was unexpected and is difficult to explain. The concentrate fed to this group of animals was 90% corn (Table 2) and so provided a substantial supply of rapidly fermented carbohydrate along with protein, which was 87% B2 and B3 fractions (Table 5). The hay, which was fed 30 min later than the pellets, contained 74.6% B2 and B3 fractions. The diets and feeding regimen may have resulted in an asynchronous delivery of protein and energy, preventing the protein from being efficiently utilized by the animal and resulting in it being excreted as urea in the urine. Cronjé (1992) found that inadequate energy reduced the percentage of N retention in goats fed adequate levels of protein and that N recycling increased as the supply of energy increased. Other possible explanations were an imbalance in the AA supplied to the goats, which prevented them from being utilized efficiently, or that the reduced DMI among the negative control goats (Table 7) resulted in a greater proportion of the available AA being used as an energy source.
An explanation for the decline in steer urine pH as the SPN level increased was not readily apparent. Huntington et al. (2001) also reported declines in urine pH when the proportion of ruminally degradable protein in the diet decreased. An explanation for the observation was not offered. It is possible that the FeCl3 used in the production of the SPN was involved in the pH decline we observed. The diets were not analyzed for Cl content; however, if it was present in the same magnitude as Fe, it could have created an anionic physiological state within the steers, which would have resulted in reductions in urine pH. Sodium remained fairly constant in the diets at 0.15% of DM (Table 4), but potassium declined as SPN increased. This could have compounded with high Fe concentrations to lower urine pH.
Lower total VFA concentrations in the negative control steers and wethers (Tables 9 and 10, respectively) likely reflected the lower DMI of these animals. The trend for a linear decline in total VFA concentration among SPN-supplemented cattle can be explained by falling DMI as the SPN proportion in the diet increased. Changes in ruminal VFA proportions as a result of SPN additions to the diets, however, varied with species and are more difficult to explain.
The increase in molar proportion of propionate in ruminal fluid from cattle receiving increasing levels of SPN (Table 9) could have been the result of decreased acetate production, which in turn could have been caused by low ruminal NH3 and the inhibition of fiber fermentation (Van Soest, 1994). It could also have been explained by the declines in the C4 acids (Table 9), which are produced by the fermentation of AA (isobutyrate) or fiber (butyrate). The increased proportion of propionate in ruminal fluid accompanied by no change in acetate across treatments explained the decrease in the A:P ratio (Table 9). The alterations in VFA proportions (Table 9) also could have resulted from high ruminal Fe concentrations. Harrison et al. (1992) noted that the A:P ratio increased with 100 to 500 mg/L of added Fe but then dropped with 1,000 mg/L additions to in vitro fermentation flasks. Concentration of Fe was not determined in the ruminal fluid collected during our study.
The increase in fat in the steer diets (Table 4) also added a plausible explanation for the changes seen, particularly because poultry fat is less saturated than tallow, which is a common source of fat in cattle diets. Pantoja et al. (1994) observed no change in total VFA or rumen pH when fat was added to dairy cow rations. Onetti et al. (2001) found that adding fat caused a trend for declines in total VFA and that the A:P ratio declined because of decreased acetate and increased propionate proportions as fat increased from 2 to 4% in dairy cow diets.
The changes seen in the VFA proportions in goats (Table 10) seemed to contradict those reported for the cattle. Increased acetate and decreased propionate, and the concomitant increase in A:P ratio suggested improved fiber digestion with increasing SPN rather than impaired fermentation; however, this was not supported by the NDF and ADF digestibility data (Table 7). The largest decline in the cattle A:P ratios was observed at SPN levels beyond those fed to the goats. When the cattle results from 0, 25, and 50 SPN were compared with the goat results, the conflict diminished. Additionally, when the trend toward a quadratic relationship between A:P ratio and SPN in the goats was considered (P = 0.11; Table 10), the possibility existed that further increases in SPN in goat diets would have led to reduced A:P, as was reported in the cattle. Balancing fat in the goat diets, but allowing fat levels to increase in the steer diets, may have also contributed to the difference observed between goat and steer A:P responses.
Analyses of SPN suggested that it could be a useful source of protein in livestock diets. It would fit especially well into ruminant feeds, providing RUP based on its high B2 and B3 protein fractions. Steers and goats receiving SPN performed as well as control animals when the SPN supplied up to 40% of supplemental N; however, including SPN to supply more than 50% of added N caused reductions in DMI and ADG. Inclusion of SPN in ruminant diets in place of traditional protein sources should therefore be limited to 40% of added N. This use of secondary protein nutrients would provide an environmentally and economically sound means of recycling the nutrients in DAF sludge. Additional research is needed to determine flocculant effects on protein quality and to evaluate AA supplied to the animal.
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2 Corresponding author: Sharon_Freeman{at}ncsu.edu
Received for publication October 19, 2006. Accepted for publication September 27, 2007.
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