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ANIMAL PRODUCTION |
* College of Veterinary Medicine and
and
Department of Animal Science, North Carolina State University, Raleigh 27606
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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Key Words: Beef Cattle • Broiler Litter • Growing Steers • Minerals • Carcass Characteristics • Transport Shrink
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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). The cattle industry in the southeastern United States widely uses recycled poultry bedding (RPB) as a by-product feed (Rankins et al., 2002). Recycled poultry bedding (previously called poultry litter) is the material removed from the floor of poultry houses, particularly broiler houses containing the original bedding material, wasted feed, feathers, and excreta. Before feeding to cattle, RPB should be heat processed, generally by deep stacking, to improve palatability, nutrient availability, and to decrease pathogen load (Pugh et al., 1994b; Rankins et al., 2002). Recycled poultry bedding is relatively low in energy, but it is a good source of CP (40 to 45% of which is nonprotein nitrogen and the rest of which is true protein; Hopkins and Poore, 2001) and contains a very high concentration of some required minerals (Rankins et al., 2002).
Monensin is often included in beef cattle diets to control bloat and improve performance. However, intake and performance are sometimes decreased when monensin is included in diets that contain high concentrations of minerals, such as molasses- (Kunkle et al., 1996) or RPB-based (Poore and Rogers, 1998) diets.
This study was performed to evaluate whether the method of stacking RPB and the addition of monensin to the diet influence performance and liver mineral concentration of beef steers during the growing period, and to assess carcass characteristics and subsequent effects during the finishing period.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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The steers were ranked on live weight (range of 247 to 320 kg) and sorted into five slatted-floor pens (2.74 x 9.14 m) of 12 steers each. Within each pen, two steers were randomly assigned to each of the six treatments. Steers were allowed 2 wk to acclimate to the pens and individual feeders (American Calan, Inc., Northwood, NH) before starting the feeding trial. Steers did not receive growth-promoting implants at any time during the trial.
Dietary Treatments
The experiment was a 2 x 3 factorial design with six dietary groups: control (CON), control + monensin (CON+M); deep-stacked RPB (DS); DS + monensin (DS+M); shallow-stacked RPB (SS); and SS + monensin (SS+M). The CON diets contained 33% corn silage (CS), 33% cottonseed hulls (CSH), 22% corn, and 11% soybean meal (SBM) on a DM basis. A trace mineral supplement was also added to CON diets to provide 0.3 ppm selenium (sodium selenite), 10 ppm copper (sulfate form), 20 ppm zinc (oxide form), and 20 ppm manganese (oxide form) (DM basis). The RPB diets contained 15% CS, 35% corn, 15% CSH, and 35% RPB on a DM basis. No additional mineral supplements were added to RPB treatments. Treatment groups were further subdivided by the presence or absence of monensin (33.3 g/t on a DM basis). Monensin was added to the feed at 22.2 g/t the first week, 27.8 g/t the second week, and at 33.3 g/t the third week through the end of the trial to acclimate calves to diets containing monensin. Diets were formulated to achieve a weight gain of approximately 0.91 kg/d based on beef NRC (1996) requirements. The CON diets were formulated to contain 12% CP and 65% TDN (DM basis). The TDN and CP of the CON diets were based on NRC (1996) requirements. The RPB diets were matched to the control diets in TDN, but had 13.6% CP. All diets had 2,200 IU of vitamin A, 500 IU of vitamin D, and 2.2 IU of vitamin E added per kilogram on a DM basis.
Recycled Poultry Bedding.
Deep-stacked bedding was piled and compressed into a 2.44-m stack and was covered with 6-mil plastic to limit oxygen exposure (Carter and Poore, 1995). Shallow-stacked bedding was loosely piled to a height of 1.07 m and remained uncovered in a commodity shed. Loads of poultry bedding were split upon delivery, and each half was either DS or SS. Thermocouples were inserted throughout the stacks during building to monitor heating. The temperature in the RPB stacks was recorded every 4 to 5 d for 45 d from the time of initial stacking. After the first 45 d, temperatures in the stacks were recorded every 7 d until thermocouple probes were uncovered during feeding.
Although RPB was positive for Salmonella when in the poultry houses and on arrival at the research farm, no Salmonella was isolated from feed ingredients, total mixed diet, or steer feces at any time during the trial. Results of Salmonella aspects of this trial are reported elsewhere (Capucille et al., 2002).
No odor or color measurements were attempted on the RPB; however, after stacking, the DS-RPB had the cooked odor and brown color typical of DS-RPB, whereas the SS-RPB had an ammonia odor and was the same color it had been when removed from the poultry house.
Feed Samples.
Samples of the total mixed diet for each treatment and of DS-RPB, SS-RPB, CS, CSH, and concentrate mixes were collected weekly throughout the growing period. During the finishing phase, a sample of the total mixed diet was collected weekly. These samples were frozen at –20°C until analysis.
Feed samples were ground to pass a 5-mm screen in a Wiley Mill and thoroughly mixed until the sample appeared homogenous. Weekly samples were pooled on an equal-weight basis to form a monthly sample. After thoroughly mixing the monthly samples, they were divided into three subsamples. Weekly samples of the finishing diet were pooled into one sample to represent the entire finishing phase. One set of subsamples were ground to pass a 1-mm screen in a Wiley Mill and analyzed for nutrient content. The second set of subsamples of the total mixed diet or the growing ration, finishing diet, and the DS and SS-RPB were submitted to Elanco Animal Health (Greenfield, IN) for measurement of monensin levels in the diets. The third set of subsamples was further pooled into one sample per treatment for the entire growing phase and submitted to a commercial laboratory for mineral analysis (Dairy One, DHIA Forage Testing Laboratory, Ithaca, NY).
For nutrient analysis, samples were dried at 105°C overnight in a forced-air oven and then ashed at 450°C in a muffle furnace to determine DM and OM content (AOAC, 1990). Fiber fractions (NDF, ADF, cellulose, and lignin) were determined sequentially as described by Van Soest et al. (1991), with modifications for use in an ANKOM200/220 apparatus (Ankom Technology, Fairport, NY). Analysis of CP was performed using Kjedahl method on the Technicon nitrogen analyzer (AOAC, 1999). Protein fractions were determined as described by Licitra et al. (1996).
Feeding Trial
Growing Trial.
The steers were weighed before feeding on two consecutive days to establish an average BW for d 0 and 84 of the trial. The steers were also weighed once on d 14, 28, and 56 of the growing period.
At the end of the growing period, ruminal fluid, whole blood, and serum samples were collected, and liver biopsies were performed. All samples were collected approximately 2 h after feeding. Ruminal fluid was collected via orogastric tube from one steer per treatment, per pen (total of 30 steers), and the pH of the samples was measured within 5 min of collection. Fluid samples were then placed on ice for transport to the laboratory, where they were frozen until analyzed for VFA and ammonia concentrations. Liver biopsies were collected from the same 30 steers using a Schackleford-Courtney biopsy instrument (Sontec Instruments; Englewood, CO) as described by Rogers et al. (2001), and then submitted for mineral analysis to the Animal Health Diagnostic Laboratory at Michigan State University (Lansing). Magnesium, zinc, manganese, molybdenum, copper, and iron contents were measured on liver samples. Serum and whole blood were collected from all 60 steers. Serum was collected after centrifugation at 7,350 x g for 15 min, and then stored at –20°C until analysis. Analysis of serum for serum urea N concentration was performed using a Technicon auto-analyzer colorimetric assay (Huntington et al., 2001). Whole blood and serum were shipped on ice for determination of Se concentration to the Animal Health Diagnostic Laboratory.
Transport.
The steers were maintained on their allotted treatment diets for 1 wk following the growing trial, at which time, 54 of the steers (nine per treatment group) were transported on a standard livestock truck for 6 h (466.6 km), unloaded, kept overnight in a dry lot with access to water and grass hay, and then returned to the original facility to simulate moving steers to a commercial feeding facility. One steer from each treatment group was not transported because of shipping weight limitations. Upon return to the research farm, steers were weighed to assess transport shrink. This protocol has been shown to mimic transport stress (Kegley et al., 1997).
Transition Period, Finishing Period, and Slaughter.
The steers were transitioned to a high-concentrate finishing diet over a 14-d period. On an as-fed basis, the diet started as 54% CS, 13.4% concentrate mix, 22.6% CSH and 10% SBM. The amount of each ingredient was adjusted daily until the diet contained 0% CS, 90% concentrate mix, 10% CSH, and 0% SBM (as-fed basis). All steers were fed the same finishing diet for 120 d after the transition period. The concentrate contained 92.35% corn, 5.45% SBM, 1.1% limestone, 0.55% trace mineral salt, 0.55% urea, 187 g of Rumensin 80 (Elanco Animal Health), a vitamin premix (provided 2,200 IU of vitamin A, 275 IU of vitamin D, and 30 IU of vitamin E per kilogram of diet DM), and a trace mineral mix (as-fed basis during the growing period; Champion’s Choice, Cargill, Inc.). Steers were weighed before feeding on two consecutive days at the beginning and end of the finishing period to calculate ADG for the period.
For slaughter, steers were shipped in two loads because of abattoir capacity limitations. Steers from the two heaviest pens (24 animals) were shipped first. One week later, the remaining three pens (36 animals) of steers were transported. An independent USDA grader recorded carcass traits following slaughter. Dressing percents were calculated from hot carcass weight and weights (unshrunk) taken as steers were loaded for shipment.
Statistical Analyses
Data were analyzed with the GLM procedure of SAS (SAS Inst., Inc., Cary, NC) in a randomized complete block design that contained the main effects of base diet during the growing period (CON, DS-RPB, or SS-RPB), inclusion of monensin in the diet, and interactions between base diet and monensin addition. Although pen effect on measured variables was initially tested, it was not significant and was removed from the model. To account for any possible differences in the finishing period that were a direct result of growing period performance, an additional analysis was done where weight at the start of the finishing period was inserted into the model as a covariate (resulting in adjusted finishing variables).
Unless noted in the results or discussion, interactions between diet and monensin inclusion were not significant and only main effects are presented. Preplanned contrasts were performed for CON vs. RPB and DS vs. SS treatment groups on all measured variables.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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and 2. Crude protein contents were slightly higher in RPB diets than in control diets. The percentages of all fiber components (NDF, ADF, cellulose, and lignin) were lower for RPB diets than for controls. The RPB diets were somewhat higher in CP, ash, and trace minerals than the CON diets due to high levels of these nutrients in the RPB and the high desired inclusion level of RPB in the diets. Even though the RPB diets were not supplemented with minerals, these diets contained higher levels of most minerals (Tables 1 and 2). A separate analysis for RPB is shown in Table 2 to show its contribution to overall diet nutrient levels, particularly minerals, and protein fractions. These diet characteristics are typical of commercial RPB diets.
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Recycled Poultry Bedding
Little historical information on the bedding was available, but the RPB seemed to be wood shaving-based and three flocks of broilers had been raised on it. Recycled poultry bedding remained undisturbed in stacks for 51 d before the start of the growing trial. Temperatures in the core of the DS reached 55°C by d 37 and stayed there for 14 d. Depending on probe location, average temperatures in the DS ranged from 30 to 40°C for most of the stacking period; however, the core of the DS averaged 40 to 50°C during this time. Average temperatures in the SS ranged from 20 to 30°C for most of the stacking period. Nutrient composition in the RPB (Table 2) was in the range normally observed in commercial RPB (Rankins et al., 2002).
The concentration of CP was not different between DS and SS-RPB diets (P = 0.88); however, there was a difference in the fractionation of proteins. Deep-stacked RPB had a lower concentration of true protein and higher nonprotein nitrogen than SS-RPB (P = 0.06 for both). The concentrations of B1 and B3 protein fractions were lower for DS-RPB than SS-RPB (P = 0.01 for both). The B2 and C fractions were higher for DS-RPB than for SS-RPB (P = 0.09 and P < 0.01, respectively). The concentrations of ADF, cellulose and lignin were higher in DS-RPB than SS-RPB.
Steer Performance
Table 3 contains a summary of ADG, DMI, and G:F by steers during each period of the study. Results will be described by period.
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Transport Shrink.
Steers were weighed after transport to determine transport shrink. Steers fed DS diets shrank more than the steers fed CON or SS diets (6.26% for DS vs. 5.61% for CON and 5.56% for SS; P < 0.07 and P < 0.05, respectively).
Transition Period.
Steers fed the SS diet gained more during the transition period than steers fed CON or DS diets (P < 0.01 and P < 0.02, respectively; Table 3). Dry matter intake during the transition period tended to be lower in steers fed CON diets than in steers fed the SS diet (P = 0.08). Gain:feed in the transition period followed the pattern of ADG, with steers fed SS diets performing more efficiently than steers fed CON and DS diets (P < 0.01 and P = 0.01, respectively). Gain:feed for steers fed CON and DS diets did not differ (P = 0.39). Monensin inclusion in the growing diets continued to show an effect in the transition period, with steers from diets containing monensin in the growing period gaining more (P = 0.03) and converting feed to gain more efficiently (P = 0.03) than steers not fed monensin.
Finishing Period.
Steers fed the SS diet during the growing period had significantly higher DMI during the finishing period than did steers fed CON diets (P = 0.05; Table 3). Steers fed the CON diet were significantly heavier than the steers fed the SS diet at both the beginning and end of the finishing period (P < 0.01 and P = 0.05, respectively). Finishing period starting BW also was different between steers fed DS and CON diets (P = 0.02). Ending BW was not different for steers fed DS and CON diets (P = 0.48).
Finishing period starting weight was a significant factor in the analysis for finishing period end weight (P < 0.01; Table 3), and tended to affect ADG and G:F during the finishing period (P = 0.09 and P = 0.11, respectively). With weight at the beginning of the finishing period as a covariate, effects of diet on average BW at the end of the finishing period were not significant (P = 0.26). Weight at the start of the finishing period was also used as a covariate in all other analyses of finishing period performance (adjusted performance measures). Steers that had been fed the DS diet during the growing phase had slightly higher adjusted ADG during the finishing phase than steers fed CON diets (P = 0.07). Daily DMI was lower in steers fed CON diets than for RPB steers (P < 0.01). These differences were reflected in G:F, which tended to be lower in steers fed SS diets than in either other diet group (CON and DS; P = 0.07).
Carcass Characteristics
Marbling score; LM area; backfat; kidney, pelvic, heart fat; and dressing percent did not differ by growing period diet (P = 0.37, P = 0.68, P = 0.71, P = 0.92, and P = 0.63, respectively; Table 4). Carcass quality grade for the steers fed SS diets was lower than for steers fed CON diets (P = 0.06). Yield grade for steers fed DS diets was lower than for steers fed CON diets (P = 0.08). Hot carcass weight was lower for steers fed SS diets than for steers fed CON diets (P = 0.03).
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Blood Variables
Steers in the CON group had higher whole blood selenium than steers fed DS diets (P < 0.01; Table 5). Monensin inclusion in the diet decreased whole blood selenium (P = 0.04). Serum Se concentration in steers fed CON diets was higher than steers fed either RPB diet (P < 0.01).
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Ruminal pH and Volatile Fatty Acids
The pH of the ruminal fluid was higher for steers fed DS diets than for steers fed SS or CON diets (P < 0.01 and P = 0.02, respectively; Table 5). The presence of monensin in the diet did not affect the average ruminal fluid pH (P = 0.58).
Ruminal ammonia concentration was lower for the steers fed CON diets than for either RPB diet steers (P < 0.01). Diets containing monensin resulted in a lower ruminal ammonia concentration than diets without monensin (P = 0.02).
Total VFA concentration was lower for steers fed DS diets than for steers fed CON or SS diets (P <0.01). The molar proportion of acetate tended to be higher for steers fed DS diets than for steers fed SS diets (P = 0.12). Molar proportion of propionate was higher in the steers fed SS diets than in steers fed CON and DS diets (P = 0.02 and P = 0.06, respectively). The acetate:propionate ratio was lower for steers fed SS diets than for steers fed CON and DS diets (P = 0.04). The molar proportion of butyric acid was higher in steers fed CON diets than for those fed either RPB diet (P < 0.01). The presence of monensin in the diet decreased acetate (P < 0.01), increased propionate (P < 0.01), decreased the acetate:propionate ratio (P < 0.01), and decreased butyric acid (P < 0.01).
Liver Mineral Analysis
Liver Cu concentrations were higher in steers fed RPB than in steers fed CON diets (P < 0.01, Table 5). Both diet and monensin affected liver Zn concentration, with a significant interaction occurring between the variables (P = 0.02). Steers fed the CON diet had higher liver Zn concentrations than did steers fed DS and SS diets (P = 0.04 and P < 0.01, respectively). Monensin inclusion decreased the liver Zn concentration (P = 0.04). The steers fed DS and CON diets had lower Zn concentrations when diet included monensin (84 ppm for DS+M vs. 94 for DS without M; P = 0.05; 89 ppm for CON+M vs. 105 ppm for CON without M; P < 0.01). Liver Mg concentration in the steers fed CON diets was higher than in the steers fed SS diets (P = 0.04). Although analysis of the presence of monensin on liver Mg concentration showed no difference overall (P = 0.77), there was a significant interaction with diet (P = 0.01). There was a difference in steers fed CON diets (491 ppm for CON+M vs. 551 ppm for CON without M; P = 0.01) and for steers fed SS diets (514 ppm for SS+M vs. 459 ppm for SS without M; P = 0.02). Steers fed the RPB diets had increased liver Mn concentrations compared with CON (P < 0.01).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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; Jeffery et al., 1998; Martin et al., 1998) as well as components of spilled and/or undigested feed, which could include antibiotics, heavy metals, and restricted feed ingredients, such as ruminant meat and bone meal (Rankins et al., 2002). The primary focus of our study was to determine the potential for spread of Salmonella from contaminated RPB to cattle fed a diet containing it (Capucille et al., 2000, 2002). Despite evidence that feeding RPB is generally safe, some countries have chosen to ban this practice altogether. All local, state, or federal regulations should be considered before using RPB as a feed ingredient.
Typically, bedding is allowed to build up for 5 to 10 flocks of broilers before removal from the poultry house. In this study, we used bedding that had a smaller number of flocks (three) because the primary focus was Salmonella contamination of bedding and potential transmission to steers fed the material (Capucille et al., 2002). Although the number of flocks raised on the RPB was not considered typical, nutrient composition (Table 2) was in the range normally observed in commercial RPB (Rankins et al., 2002). Also, the protein fractions recorded in the RPB were similar to those described by Hopkins and Poore (2001), except that ADIN (Fraction C) in DS-RPB was not as high as in their report (18%). Recycled poultry bedding is higher in nonprotein nitrogen than in many other plant protein sources (NRC, 1996). Differences noted in the protein and fiber fractions are attributed to the heating of the DS-RPB. The heating process increased some fiber fractions and decreased the availability of some of the nutrients in the feed. However, the apparent increase in palatability (as evidenced by higher DMI) offset this small decrease in nutrient availability.
Steer Performance
Growing Period.
Deep stacking is generally recommended to heat RPB and to decrease the number of viable pathogens present (McCaskey et al., 1991; Rankins et al., 2002). Another reason to encourage deep stacking of RPB is that the steers in this study had higher intakes and slightly better ADG when fed DS diets compared with SS diets.
Monensin inclusion in this study decreased DMI of steers compared with those fed diets without monensin (P < 0.01); however, the improvement in G:F offset this decrease, and no significant difference in growing ADG was evident (P > 0.80). Other studies have also suggested that intake of diets containing by-products high in minerals (e.g., molasses and caged layer waste) was suppressed by the addition of monensin, but ADG was not significantly affected (Vijchulata et al., 1980a,b; Kunkle et al., 1996). Conversely, Poore and Rogers (1998) reported both DMI and ADG were decreased (to 84 and 90% of CON, respectively) when monensin was added to RPB diets compared with bambermycins or lasalosid, which showed the same DMI and ADG as control.
Transport.
Average shrink noted in this study was comparable to other studies where calves were fed diets that did not contain RPB (7 to 7.6%, 6.5%, and 7.6 to 9.1%; Cole et al., 1988; Kegley et al., 1997; Self and Gay, 1972, respectively) and showed that the steers had experienced the stress expected after a period of transport. No other studies indicating the amount of shrink in diets containing RPB were found. Although statistically significant, the difference in the amount of transport shrink between the steers fed DS diets and those fed the other diets was not considered biologically or economically significant.
Transition Period.
Caswell et al. (1977) inferred palatability based on DMI in a feeding trial, concluding that palatability of RPB diets is lower than in non-RPB diets, but that mixing the product with other feed ingredients improves intake/palatability (Caswell et al., 1977). The increased ADG, DMI, and G:F in the transition period in steers previously fed RPB was attributed to increased palatability of the transition diet. Perhaps the abrupt addition of monensin to the transition diet of steers that had not been acclimated to it during the growing period caused lower performance during the transition period.
Finishing Period.
Although all steers were fed the same finishing diet, carryover effects of the growing diet were assessed to determine whether RPB diets influenced the ability of the steers to perform during the finishing period. Although the steers fed RPB started the finishing period lighter than the steers fed CON diets, steers from the RPB groups tended to gain and consume more during the finishing period when an adjustment was made for this initial BW difference. Assuming steers with similar genetic backgrounds are compared, lighter/leaner cattle gain more efficiently than heavier cattle because more nutrients are required to produce fat (Ensminger et al., 1990). All steers completed the finishing period at typical slaughter weights and carcass quality. This is consistent with other reports of steers finished on RPB diets (Cullison et al., 1976; Westing et al., 1985).
Steers fed the DS diets during the growing period tended to perform better than steers fed SS diets. The reason for this difference in performance is not explained by the data collected, but it supports the argument for deep stacking before feeding RPB.
Overall, performance was good as compared with industry expectations. On average, these steers gained faster during the growing phase and slower during the finishing phase than is usually seen in the industry.
Carcass Characteristics
The differences in carcass characteristics between CON and SS-RPB were significant but slight. Part of this difference was due to the fact that the steers fed the SS diet were lighter at slaughter than steers in the other groups. This emphasizes the need to properly process RPB before feeding, as increased length of time on feed increases costs and decreases profitability. Although direct comparisons should not be made because of the differences in study design, steers finished on processed RPB have been shown to perform as well as steers finished on traditional diets (Cullison et al., 1976; Westing et al., 1985).
Blood Variables
Selenium is deficient in the soils of North Carolina and much of the southeastern United States, and therefore must be supplemented in most cattle diets. Although the Se content was higher in the RPB diets than in the CON diets, the concentration of Se in the blood of RPB steers was lower than in steers fed CON diets. It is known that Se and S interact (Greene, 1999). Either the high concentration of sulfur in the RPB diets inhibited the absorption of Se or Se in RPB has a lower bioavailability than Se from sodium selenite. Enough Se was available to maintain the steers above deficiency levels during the growing period.
Ruminal Fluid
The DS diets resulted in a ruminal environment that had a higher pH than the CON or SS diets. All RPB diets led to higher ammonia concentrations in the ruminal fluid than the CON diets. The difference in diet fiber and protein fractions would result in different microbial populations and therefore fermentation products, leading to differences in the pH and ammonia concentrations (Merchen, 1988; Owens and Goetsch, 1988).
Total VFA in steers fed DS diets was significantly lower than in steers fed CON or SS diets. A difference in the CON vs. RPB diets was not unexpected. The difference in the DS- and SS-fed steers was unexpected, especially as the VFA proportions in steers fed SS diets were not different from steers fed CON diets. A feed mixing error did not seem to be the problem because the concentrate mixes used for the both the DS and SS diets were the same and they differed between diets with and without monensin.
Monensin is a carboxylic ionophore, whose function is to alter the population distribution of ruminal organisms, thereby altering ruminal fermentation end products, to increase propionate production, which is then utilized more efficiently (Pressman and Fahim, 1982; Oehme and Pickrell, 1999). The total VFA concentration was unchanged in this study by including monensin. However, propionate concentration was higher and acetate, isobutyric, butyric, and valeric acid concentrations were lower in diets containing monensin. The acetate:propionate ratio was decreased due to the increase in propionate and concurrent decrease in acetate. In this trial, monensin was effective in increasing the G:F ratio, presumably by decreasing the acetate:propionate ratio. The effects of monensin are similar to those observed by others (Vijchulata et al. 1980a,b).
It is important to note that ruminal fluid samples were collected at only one time point in this study. A more accurate indication of the ruminal environment would be reflected by analysis of multiple samples collected at regular intervals after feeding. This would allow for more definitive conclusions to be drawn on the affect of stacking method on the ruminal environment.
Liver Mineral Analysis
The concentration of Cu in RPB is high due to copper sulfate feeding to poultry for disease control and improved performance (Banton et al., 1987), resulting in higher liver Cu concentrations in RPB steers than in CON-fed steers. The liver Cu concentrations were in the high end of the normal range reported by the laboratory (90 to 540 ppm on a dry basis) in steers fed RPB diets, but they did not reach concentrations considered toxic in cattle (Kincaid, 1999). If RPB was fed at these levels as a part of the diet for an indefinite period, it could result in liver Cu concentrations that would be toxic to the animal. Fontenot and Webb (1975) demonstrated that with over-winter feeding of RPB for several consecutive years, cows did not develop Cu toxicity and were able to excrete the excess Cu during the remainder of the year when RPB was not fed.
Recycled poultry bedding is generally high in other minerals as well, which can affect the gastrointestinal absorption of other minerals, either through competition for binding sites or complexing within the intestine and inhibiting absorption. Molybdenum is known to complex with Cu in the gastrointestinal tract, which could lead to a decrease in Mo in cattle consuming high-Cu rations (Kincaid, 1999). There was a slight decrease seen in liver Mo in RPB steers during this study, when liver Cu concentration was included as a covariate in the analysis (P < 0.10). Zinc absorption was decreased by increased concentrations of Ca, Cu, and Fe in the diet (Greene, 1999). It is logical that steers fed diets containing RPB, which is high in these minerals, would have lower Zn absorption and therefore lower liver Zn concentrations, as our study demonstrated. Whereas there was less Mg available to the steers fed CON diets, they tended to have higher liver concentrations of Mg than RPB steers. It is well recognized that Mg absorption is inhibited by the presence of high concentrations of potassium and nitrogen (Greene, 1999), such as are found in RPB diets. This fact should be considered carefully when feeding late-gestation or early-lactation cows diets containing RPB, as they are more susceptible to hypomagnesemia associated with rapid fetal growth and milk production (Pugh et al., 1994a). The differences in liver mineral profiles of steers fed RPB diets in this study are consistent with those found by Westing et al. (1985), except for the concentration of Mg. They found a significant increase in liver Mg concentration for steers fed RPB diets, whereas we observed a decrease. The Westing et al. (1985) study was approximately twice that of the current study, which may have allowed equilibration of Mg in the liver. Measured liver mineral concentrations deviated from CON values, but not to a degree that would cause concern for the length of time growing diets are generally fed.
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2 Correspondence: 4700 Hillsborough Street (phone: 919-513-6244; fax: 919-513-6464; e-mail: dawn_capucille{at}ncsu.edu).
Received for publication September 16, 2003. Accepted for publication June 3, 2004.
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