Effect of feed delivery fluctuations and feeding time on ruminal acidosis, growth performance, and feeding behavior of feedlot cattle 1,2

Effect of feed delivery fluctuations and feeding time on ruminal acidosis, growth performance, and feeding behavior of feedlot cattle 1,2 K. S. Schwartzkopf-Genswein* 3, K. A. Beauchemin*, T. A. McAllister*, D. J. Gibb*, M. Streeter, and A. D. Kennedy *Agriculture and Agri-Food Canada, Lethbridge Research Centre, Lethbridge, Alberta T1J 4B1, Canada; Alpharma Inc., Fort Lee, NJ 07024; and University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada ABSTRACT: Research was conducted to determine whether fluctuations in the amount of feed delivered and timing of feeding affect ruminal ph and growth of feedlot cattle. In Exp. 1, the effects of constant (C) vs. fluctuating (F) daily feed delivery on ruminal ph were assessed in a crossover experiment (two 28-d periods) involving six mature, ruminally cannulated steers. The diet consisted of 86.8% barley grain, 4.9% supplement, and 8.3% barley silage (DM basis) and was offered ad libitum for 2 wk to estimate DMI by individual steers. Steers in group C were offered a constant amount of feed daily equal to their predetermined DMI, whereas steers in group F were offered 10% more or less than their predetermined DMI on a rotating 3-d schedule. Ruminal ph of each steer was measured continuously via an indwelling electrode placed in the rumen during the last 6 d of each period. Mean ph tended to be lower (0.10 units) for F than C (5.63 vs. 5.73; P = 0.15), and ruminal ph of steers in group F tended to remain below 5.8 (P = 0.03) or 5.5 (P = 0.14) for greater proportions of the day than steers in group C. Inconsistent delivery of feed lowered ruminal ph, suggesting increased risk of subclinical acidosis. In Exp. 2, a 2 2 factorial was used to study the effects of pattern (C vs. F) and feeding time (morning [0900] vs. evening [2100]) on the feeding behavior and performance of 234 (310 ± 23 kg) Charolais Hereford beef steers during backgrounding and finishing phases over 209 d. One pen per treatment was equipped with a radio frequency identification (GrowSafe Systems Ltd., Airdrie, Canada) system that monitored bunk attendance by each steer throughout the trial. Pattern of feed delivery did not affect (P = 0.16) DMI (7.36 kg/d), ADG (1.23 kg/d), G:F (0.17), or time spent at the bunk (141 min/d), nor were pattern of feed delivery time of feeding interactions observed (P = 0.18). Late feeding increased (P < 0.05) daily DMI (7.48 vs. 7.26 kg), ADG (1.28 vs. 1.00 kg/d), and G:F (0.21 vs. 0.15). These studies indicate that the risk of subclinical acidosis was increased with fluctuating delivery of feed, but the greater risk of acidosis did not impair growth performance by feedlot cattle. Consequently, daily intake fluctuations of 10% DMI or less that do not alter overall intake by feedlot cattle are unlikely to have any negative consequences on growth performance. Key Words: Behavior, Cattle, Intake Fluctuation, Performance, Ruminal ph 2004 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2004. 82:3357 3365 Introduction The relationship between feeding management, intake, and the incidence of subclinical acidosis and its 1 The authors thank the Canada/Alberta Beef Industry Development Fund for providing financial support for this research. We acknowledge B. Baker, R. Wilde, C. Cockwill, B. Farr, and R. Wuerful, as well as the barn staff at the Lethbridge Research Centre, for their assistance. Many thanks to R. Silasi and B. Genswein for assistance with data and statistical analyses. 2 This is LRC Contribution No. 38703035. 3 Correspondence: P.O. Box 3000 (phone: 1-403-317-3354; fax: 1-403-382-3156; e-mail: gensweink@agr.gc.ca). Received August 6, 2003. Accepted July 8, 2004. effect on the growth and feed efficiency of feed yard cattle remains unclear. Nutritionists and feedlot managers commonly associate subclinical acidosis with abnormal or erratic feeding behavior by cattle. Several studies have concluded that large variation in feed intake by cattle fed high-concentrate diets may cause digestive disturbances (Fulton et al., 1979; Britton and Stock, 1987) and decrease growth performance in feedlot cattle (Stock et al., 1995), with this effect being greatest early in the feeding period (Krehbiel et al., 1995; Soto-Navarro et al., 2000). In contrast, Cooper et al. (1999) reported that variation in intake did not increase acidosis or decrease the performance by finishing steers fed ad libitum. In fact, Zinn (1994) found that variation in intake had a slightly positive effect on the performance of limit-fed steers. 3357

3358 Schwartzkopf-Genswein et al. Some studies evaluating the effects of time of daily feed delivery indicated that cattle fed in the afternoon or evening hours had improved ADG and feed efficiency compared with those fed in the morning (Reinhardt and Brandt, 1994; Pritchard and Knutsen, 1995), whereas other studies reported that the time of feed delivery did not affect performance of limit-fed cattle (Soto-Navarro et al., 2000). None of the studies cited compared the effects of feeding management on feeding behavior, which may also play a role in the manifestation of metabolic disorders and subsequent performance (Galyean and Eng, 1998). To date, no studies have examined the effect of intake patterns on subclinical acidosis or growth performance in feedlot cattle. The objective of this study was to further investigate the effects of intake fluctuation and time of feed delivery on DMI, ADG, feed efficiency, and feeding behavior. Continuous monitoring of ruminal ph was used to assess the effect of these management practices on the occurrence of subclinical acidosis. Materials and Methods All cattle used in this study were cared for under the guidelines established by the Canadian Council on Animal Care (1993). Experiment 1 Six mature (mean BW = 797 kg) ruminally cannulated steers were used in a crossover design with two 28-d periods to investigate whether fluctuations in the quantity of feed delivered affects the incidence of ruminal acidosis as assessed by ruminal ph variables. Steers were fitted with ruminal cannulas (Bar Diamond, Parma, ID) several months before the start of the study. During this time, the steers received a diet containing 80% hay (DM basis). Twenty-eight days before the experiment, the steers were adapted to a high-grain experimental diet. During the adaptation period, feed was offered ad libitum (10% feed refusals) and the DMI of each steer was estimated over a 2-wk period. Two feeding schedules were employed: constant (C), with each steer offered their estimated ad libitum intake on a daily basis, and fluctuating (F), with each steer fed at 110% of ad libitum DMI for 3 d followed by 90% of ad libitum DMI for 3 d. The cattle were fed a finishing diet comprised (DM basis) 86.8% steam-rolled barley, 8.3% whole crop barley silage, and 4.9% supplement containing vitamins and minerals to meet or exceed NRC (1996) recommendations. No antibiotics or medications were added to the diet. The diet was prepared daily using a feed mixer. Feed was offered once daily at 1000 and feed offered and refused was recorded daily. All orts were removed from each animal on a daily basis. Samples of barley silage and concentrate were collected weekly and DM was determined and used to adjust the silage to concentrate ratio of the diet as necessary. Samples of the diet and orts were collected during the last 7 d of each period and composited by animal and dried (0.5 kg) at 55 C. Dry matter intake by each steer was calculated based on the feed DM offered minus DM refused. Variation in intake was calculated as the difference in intake between consecutive days. Continuous Measurement of Ruminal ph. Steers were housed in individual stalls (152 cm 203 cm) equipped with rubber mats. Test periods consisted of 28 d, with ruminal ph measured every 5 s for 24 h the last 6 d of each period with indwelling ph electrodes (model PHCN-37, Omega Engineering, Stamford, CT) inserted into the rumen. A weight was attached to the electrode to ensure that it remained in the ventral sac. In addition, a protective shield with large openings that allowed ruminal fluid to percolate freely was placed around the electrode to prevent it from coming in contact with the ruminal epithelium. The electrodes were removed from the rumen approximately 1 h before feeding each day and calibrated with ph 4.0 and 7.0 standards. The ph measurements were averaged over 15- min intervals and recorded using a data logger for approximately 23 h out of each day. Ruminal ph data were summarized daily for each steer in each period as daily mean ph, maximum and minimum ph, amount and percentage of time recorded each day during which ph was below 5.8 or 5.5, and area that the ph curve was below ph 5.8 or 5.5. The area was calculated by adding the absolute value of negative deviations in ph from ph 5.8 or 5.5 for each 15-min interval. The number was expressed as ph units h. The threshold values of 5.8 and 5.5 were chosen because these values have been used previously to indicate subclinical ruminal acidosis (Ghorbani et al., 2002). The duration the ph remained below the threshold indicated the duration of subclinical acidosis, whereas the area between the curve and the ph threshold indicated the severity of subclinical acidosis. Statistical Analyses. Data were analyzed with the mixed model procedure of SAS (SAS Inst., Inc., Cary, NC). The model included the fixed effects of feed delivery, day, and their interaction. Period and steer were considered random effects, day was considered a repeated measure, and the restricted maximum likelihood function was used to estimate the variance components. Treatment effects were declared significant at P < 0.05, and trends were discussed at P < 0.15, unless otherwise noted. Experiment 2 Animals, Feed, and Housing. A total of 234 crossbred beef steers (initial BW 310 ± 23 kg) was used to determine the effects of amount (C vs. F) and time (morning [M] vs. evening [E]) of feed delivery on feeding behavior and animal growth. Steers were fed for a 209-d feeding period commencing December 7, 1999 until July 5, 2000. Average evening temperatures (1800 to 0500) are

Behavior and acidosis with bunk management 3359 Table 1. Average (±SE) evening (2000 to 0500) temperatures ( C) by month over a 209-d feeding period for steers fed according to a morning/evening or constant/fluctuating feed delivery schedule Month Temperature, C SE January 9.36 0.32 February 6.24 0.32 March 0.11 0.32 April 3.40 0.32 May 8.65 0.32 June 11.71 0.32 July 15.83 0.32 presented in Table 1. On entry into the feedlot, all steers were weighed and tagged and 60 steers were also fitted with radio frequency transponders (Allflex, Dallas Ft. Worth, TX). Animals were given 25 ml of Dectomax pour-on (Pfizer, London, Ontario, Canada) for internal parasites, 2 ml of Somnustar ph (Novartis, Mississauga, Ontario, Canada) for hemophalus, 2 ml of Barvac 3 (Boringer-Engelheim, Burlington, Ontario, Canada) for bovine viral diarrhea, and 5 ml of Fortress-7 s.c. (Bayer, Toronto, Ontario, Canada) for clostridial related diseases. Boosters with Somnustar ph and Barvac 3 were given 28-d later. During the first 35 d of a 56-d backgrounding period, cattle were fed a diet consisting of 57.48% steam-rolled barley, 37.62% whole crop barley silage, and 4.9% supplement. During the last 21 d of this period, cattle were adapted to a finishing diet containing 86% steam-rolled barley, 9% whole crop barley silage, and 5% supplement (DM basis) using three transition diets fed for a total of 21 d. The third transition diet hereafter is referred to as the finishing diet. Supplement was included at 5% of DM in all diets and included trace minerals and vitamins to meet or exceed NRC (1996) recommended levels, 15% protein (from all plant sources), and calcium carbonate to provide a minimum of 0.62% calcium in the diet. No antibiotic or medications were included in the diet. Steers in the C group were fed a constant amount of feed to meet ad libitum intake by ensuring that there was always a small amount of feed remaining in the bunk before feeding. Steers in group F were fed 10% more or 10% less than cattle in the C group on a rotating 3-d schedule as described in Exp. 1. Feed was delivered once per day at either 0900 for calves in the M group, or at 2100 for calves in the E group. Main factors were arranged in a 2 2 factorial to yield CM, CE, FM, and FE treatments. Steers were blocked by BW and assigned randomly to one of 16 pens with 15 animals per pen and four pens per treatment. Animals were housed in pens measuring 21 m 27 m with 15 m of concrete bunk, and were provided with a continuous supply of fresh water and a bedded area away from the feed bunk. Linear bunk space and pen space available for each steer were 25.4 cm and 12.6 m 2, respectively. Cattle were allowed 10 d of adaptation to their new pens before the commencement of data collection. Four of the 16 pens were equipped with an electronic feed bunk monitoring system (GrowSafe Systems Ltd., Airdrie, AB), as previously described by Gibb et al. (1998) and Schwartzkopf-Genswein et al. (1999). Bunk Attendance, Intake, and Performance. Frequency and duration of attendance at the feed bunk were obtained using the feed bunk monitoring system, and data were collected for 60 steers for 24 h/d throughout the 209-d trial. The system was checked every 2 d throughout the trial to ensure that all cells within the mat were operational (Schwartzkopf-Genswein et al., 2001). Frequent system monitoring indicated that there were no inoperable cells during the trial. All occasions when animals were removed from their pens were recorded, and data for that day and animal were discarded from the behavioral data set. To isolate distinct feeding events, a visit was defined as attendance at the feed bunk after an absence of at least 5 min; a return within 5 min was considered to be continuous attendance. The definition of a visit was based on survival analysis theory (de Haer et al., 1992), as well as work by Gibb and McAllister (1999) and Sowell et al. (1998) in beef cattle. Feed intake per pen was determined by recording feed offered daily and weekly orts. Dry matter intake was calculated using the DM content of the feed and orts, which was determined by drying samples (0.5 kg) at 55 C. The ADG was determined by weighing the animals at 21-d intervals throughout the trial and dividing weight gain by the number of days. Feed efficiency was calculated as G:F (kg of gain/kg of DMI) on a pen basis. Statistical Analyses. Data were analyzed as a completely randomized design using the mixed model procedure in SAS. Pen was considered the unit of analysis for DMI, ADG, and G:F, whereas individual animals were the unit of analysis for bunk attendance duration and frequency. The model included the fixed effects of the consistency in amount of feed delivered, time of feed delivery, and their interactions. Treatment effects were declared significant at P < 0.05. Experiment 1 Results and Discussion The mean ruminal ph of 5.73 for steers in group C was similar to values reported previously for feedlot cattle fed high-grain diets comprised mainly of barley (Table 2). Beauchemin et al. (2001) reported mean ph values ranging between 5.79 and 6.06, and minimum ph values ranging from 5.21 to 5.56, depending on the extent to which the grain was processed. Similarly, Ghorbani et al. (2001) reported a mean ruminal ph of 5.71, with a minimum ph of 5.17, and Koenig et al. (2003) reported a mean ph of 5.76, and a minimum ph of 5.14 for cattle fed diets similar to those used in the current study. From these studies, it is clear that feedlot cattle fed barley grain-based diets regularly experience subclinical ruminal acidosis, characterized by pro-

3360 Schwartzkopf-Genswein et al. Table 2. Effects of feeding program on dry matter intake and ruminal ph variables in feedlot cattle (n = 6) fed a high-concentrate diet a P-value Treatment Day Item Fluctuating Constant SEM (T) (D) T D DMI, kg/d 8.81 8.66 0.38 0.37 NA NA DMI, kg/d as % of feed offered 94.10 88.90 2.60 0.13 NA NA Mean ph 5.63 5.73 0.12 0.15 0.06 0.87 Minimum ph 5.04 5.17 0.10 0.15 0.14 0.86 Maximum ph 6.53 6.45 0.08 0.21 0.49 0.50 Time ph < 5.8, h 14.70 12.50 2.20 0.09 0.04 0.89 Percentage of the day ph < 5.8 63.70 54.30 9.50 0.09 0.03 0.85 AUC for ph 5.8, ph h 7.67 6.23 1.38 0.18 0.13 0.82 Time ph < 5.5, h 11.20 9.00 2.00 0.18 0.15 0.79 Percentage of the day ph < 5.5 48.40 38.70 8.60 0.17 0.14 0.78 AUC for ph 5.5, ph h 3.77 2.98 0.82 0.28 0.24 0.76 a NA = not applicable; AUC = area under the curve. b Fluctuating = each steer was fed at 110% of ad libitum DMI for 3 d followed by 90% of ad libitum DMI for 3 d; Constant = each steer was offered its estimated ad libitum intake on a daily basis. longed periods each day during which ruminal ph is low (<5.8; Owens et al., 1998). Although the ph associated with the feeding of barley grain-based diets fits the definition of subclinical acidosis, this condition seems to arise primarily as a result of the accumulation of VFA because lactate is typically detected at concentrations of less than 10 mm (Ghorbani et al., 2002). Mean ruminal ph tended to be 0.10 units lower (P = 0.15) for steers in group F than for steers in group C (Table 2). The lower ph of steers in group F occurred both during the 3 d in which feed was allocated above and the 3 d in which feed was allocated below ad libitum intake (Figure 1). Furthermore, ruminal ph of cattle fed F tended to remain below 5.8 for a longer duration each day (P = 0.09) and for a greater portion of the day (P = 0.05) than those fed C (Table 2). A similar numerical trend was observed for the time ruminal ph remained below 5.5. These data indicate that conditions in the rumen of feedlot cattle fed high-grain diets are not favorable for fiber digestion, and inconsistent delivery of feed aggravates the situation. In vitro and in vivo studies indicate that fiber digestion in the rumen can be severely inhibited by even modest declines in ruminal ph (Russell and Wilson, 1996; Yang et al., 2002) due to decreased bacterial adhesion at low ph (Mouriño et al., 2001). Inconsistent delivery of feed increased the severity of subclinical acidosis and possibly fiber digestion in steers fed barley-grain diets. The significance of low fiber digestion for feedlot cattle in terms of production efficiency is unknown; however, considering that the NDF content of barley is between 18 and 15% (NRC, 1996), and the NDF content of barley silage is typically between 50 and 60% (NRC, 1996), approximately 25% of the DMI of steers in this study was fiber. Therefore, a severe decrease in fiber digestion due to low ph could substantially decrease feed conversion efficiency. The increased severity of subclinical acidosis caused by fluctuating feed delivery in this study was likely not directly related to increased intake or increased daily variations in intake. There was no treatment effect (P = 0.23) on mean DMI consumed daily during the period (F vs. C; 8.81 kg/d ± 0.38 vs. 8.66 ± 0.38). Furthermore, the standard deviation associated with DMI was similar for both groups (F vs. C; 1.008 vs. 0.839 kg/d; P = 0.48); however, the fluctuating pattern of feed delivery changed the day-to-day pattern of intake. For steers in the F group, the pattern of intake over time closely resembled the 3-d fluctuating pattern of feed delivery. However, the DMI of steers in group C also fluctuated from day to day, but the fluctuations were erratic. Thus, allocating a constant amount of feed daily did not eliminate day-to-day variations in intake. It seems that dayto-day fluctuations in intake are normal for cattle fed to appetite when confined in metabolism stalls, even when the amount allocated each day is constant. In this study, 99% of the day-to-day intake variation (three times the SD) was less than 2.5 kg/d of DM, or approximately 29% of DMI. Imposing variation in intake by fluctuating feed delivery by 10% every 3 d was within the normal variation in intake demonstrated by control cattle. This day-to-day fluctuation has also been observed in penned feedlot cattle (Schwartzkopf-Genswein et al., 2003). Day affected mean ph (P = 0.06), time ph < 5.8 (P = 0.04), and the percentage of the day ph < 5.8 (P = 0.03) for both groups indicating the importance of measuring ruminal ph over several days (Figure 1). The observed day-to-day variation in ruminal ph was similar to that reported previously for cattle fed a high-grain barley based diet (Ghorbani et al., 2001). The day feed delivery schedule interaction was not significant for these ph variables, indicating that the day-to-day variation in ph was similar for both groups of cattle. For both groups, mean ruminal ph fluctuated daily, partly due to daily fluctuations in intake (Figure 2). Overall, mean daily ph was moderately negatively correlated to daily DMI (ruminal ph = 7.03 0.154 DMI, kg; r = 0.57;

Behavior and acidosis with bunk management 3361 Figure 2. Dry matter intake on the days of ph measurement by feedlot steers (n = 6) fed a high-grain finishing diet offered once daily. Steers received feed delivered in a constant amount equal to their previously determined ad libitum intake (black bars) or at 110% of ad libitum for d 1 to 3, followed by 90% of ad libitum intake for d 4 to 6 (gray bars). Error bars represent one SD. Figure 1. Mean (upper graph) and average minimum (lower graph) ruminal ph recorded in feedlot steers (n = 6) fed a high-grain finishing diet offered once daily. Steers received feed delivered in a constant amount equal to their previously determined ad libitum intake (black bars) or at 110% of ad libitum for d 1 to 3, followed by 90% of ad libitum intake for d 4 to 6(gray bars). Mean ph values are averages of values recorded at 15-min intervals using indwelling ph probes. A diurnal pattern of fluctuating ph was observed for steers fed both diets (Figure 3). Typically, ph was high before feeding, dropped afterwards, and recovered during the night. The diurnal fluctuation was greater for steers fed F because minimum ph tended to be lower (P = 0.15) for these animals, whereas maximum ph was similar for both groups (Table 2). In a previous study with limit-fed cattle (80% of ad libitum intake; 90% concentrate diet), imposed day-to-day feed delivery variations of 0.7 and 1.4 kg/d increased (P < 0.05) the risk of subclinical acidosis as measured by the area of ruminal ph below 5.6 (Cooper et al., 1999). However, P = 0.001), indicating that a decrease in intake was associated with an increase in ruminal ph because less substrate was available in the rumen for fermentation. Furthermore, cattle with the highest intakes had the lowest ruminal ph; however, ruminal ph is not solely a function of the amount of substrate consumed, for if this were the case, one would expect the ph to be higher for cattle fed F than those fed C on d 4, 5, and 6, which was not the case. Although cattle fed F were only allocated 90% of their normal intake on d 4, 5, and 6, their ph remained lower than for cattle fed C (Figure 1). Thus, for animals fed F, there was a carryover effect from the period of above ad libitum feeding to the period of below ad libitum feeding that could be related to a number of factors, including saliva secretion, buffering capacity of saliva, ruminal dilution rate, and acid absorption. The implication of this finding is that animals adapt slowly to eating a highly fermentable diet and thus have less acidosis. Figure 3. Mean diurnal pattern of ruminal ph for cattle fed a constant amount of feed (bold solid line) or in a fluctuating 3-d cycle of 10% above (solid line) followed by 10% below (dotted line) ad libitum intake.

3362 Schwartzkopf-Genswein et al. Table 3. Effects on feed intake fluctuation and time of feed delivery (0900 vs. 2100) on performance and feeding behavior of feedlot steers fed a barley-grain and barley-silage diet over a 56-d backgrounding period (values shown are mean ± SE) Treatment a Item CE FE CM FM All cattle b No. of steers (pens) 59 (4) 59 (4) 56 (4) 60 (4) ADG, kg 1.27 ± 0.04 1.24 ± 0.04 1.10 ± 0.04 1.10 ± 0.04 Daily DMI, kg/steer 7.12 ± 0.23 7.05 ± 0.23 6.70 ± 0.23 6.89 ± 0.23 G:F 0.182 ± 0.01 0.189 ± 0.01 0.189 ± 0.01 0.177 ± 0.01 Behavior cattle c No. of steers (pens) 15 (1) 15 (1) 15 (1) 15 (1) ADG, kg 1.93 ± 0.05 1.52 ± 0.05 1.14 ± 0.05 1.07 ± 0.05 Daily DMI, kg/steer 7.38 ± 0.11 6.93 ± 0.11 6.59 ± 0.11 6.73 ± 0.11 G:F 0.166 ± 0.01 0.180 ± 0.01 0.171 ± 0.01 0.172 ± 0.01 Bunk attendance, min/d 156.57 ± 4.6 159.38 ± 4.6 161.87 ± 4.6 158.52 ± 4.6 Frequency, visits/d 10.12 ± 0.22 10.21 ± 0.22 10.23 ± 0.22 10.21 ± 0.22 a CE = constant feed delivery offered in the evening (2100); FE = fluctuating feed delivery offered in the evening (2100); CM = constant feed delivery offered in the morning (0900); FM = fluctuating feed delivery offered in the morning (0900). Once daily, steers were fed a constant (C) amount of feed or in a fluctuating (F) 3-d cycle of 10% above followed by 10% below ad libitum intake. b Includes all steers in the trial (n = 234). c Steers fed in pens containing a feed bunk monitoring system (n = 60). in the same study, steers fed at ad libitum levels of intake did not experience increased acidosis with imposed intake variations of up to 1.8 kg/d. The results from the Cooper et al. (1999) study do not concur with our observations for cattle fed for ad libitum intake, indicating that the negative effects of day-to-day variations in feed delivery on ruminal acidosis do apply to animals fed to appetite. The reason for the apparent discrepancy between the two studies is not clear, but may relate to the length of the imposed variations in intake. In our study, feed fluctuations were made every 3 d, whereas in the Cooper et al. (1999) study, the fluctuations were made daily. They speculated that subjecting steers to alternating days of intake variation allowed the steers to build buffering capacity or baseexcess on the days of decreased feed offered, so that upon overconsumption the following day, acidosis was not induced. The proposed mechanism was not observed in our study. Experiment 2 Intake and Performance. Over the entire trial average feed intake was 7.36 ± 0.16 kg/d (DM basis), ADG was 1.23 ± 0.17 kg/d, and G:F was 0.175 ± 0.05, which are low compared with commercial industry levels; however, values were within the range reported for feedlot cattle over a feeding period that included backgrounding and finishing phases (Cooper et al., 1999; Soto-Navarro et al., 2000). No interaction effects between feed fluctuation and time of feed delivery were observed (P = 0.17) for DMI, ADG, or G:F over the entire feeding period or for the backgrounding and finishing periods alone (Tables 3, 4, and 5). A primary symptom of subclinical acidosis is believed to be decreased and erratic feed intake; however, the relationship between cause and effect are still unknown (Cooper et al., 1999). In this study, there was no difference between the intakes of C- and F-fed steers (P = 0.26) in either the backgrounding, finishing, or entire feeding period (Tables 3, 4, and 5), indicating that imposing feed fluctuations of up to 2 to 3 kg over a 3-d period did not change the overall intake of C and F steers, regardless of feeding phase. Hickman et al. (2002) reported that animals fed ad libitum could fluctuate their intakes as much as 2 to 3 kg/d with no adverse affects on health and performance. In the current study, the imposed fluctuations in feed delivery were of a similar magnitude to the natural intake fluctuations observed in feed yard cattle fed barley-based finishing diets (Hickman et al., 2002). In addition, ADG and feed efficiency did not differ (P > 0.15) between C and F steers in either backgrounding or finishing phases or over the entire feeding period (Tables 3, 4, and 5). These results are consistent with the intake data and are in agreement with several other studies documenting the effects of intake fluctuation on performance. Soto-Navarro et al. (2000) reported similar results in limit-fed cattle, indicating that a daily fluctuation in intake of 10% did not affect ADG or G:F over an 84-d feeding period. Zinn (1994) also reported that a daily fluctuation in feed intake of 20% (1.4 kg/d) was not sufficient to adversely affect ruminal and total tract digestion during the late finishing phase. Cooper et al. (1999) found that intake variation of up to 1.8 kg/d did not decrease performance by finishing steers fed ad libitum intake. It was suggested that alternating feed delivery between excess and inadequate amounts of feed allowed the cattle to adjust their intake in a manner such that subclinical acidosis was not induced. Results of our study suggest that subclinical acidosis was present in these animals, based on the results of the

Behavior and acidosis with bunk management 3363 Table 4. Effects on feed intake fluctuation and time of feed delivery (0900 vs. 2100) on performance and feeding behavior of feedlot steers fed a barley-grain and barley-silage diet over a 153-d finishing period (values shown are mean ± SE) Treatment a Item CE FE CM FM All cattle b No. of steers (pens) 59 (4) 59 (4) 56 (4) 60 (4) ADG, kg 1.29 ± 0.04 1.21 ± 0.04 1.24 ± 0.04 1.25 ± 0.04 Daily DMI, kg/steer 8.31 ± 0.33 8.24 ± 0.33 8.19 ± 0.33 8.21 ± 0.33 G:F 0.157 ± 0.01 0.152 ± 0.01 0.158 ± 0.01 0.160 ± 0.01 Behavior cattle c No. of steers (pens) 15 (1) 15 (1) 15 (1) 15 (1) ADG, kg 1.22 ± 0.05 1.28 ± 0.05 1.25 ± 0.05 1.23 ± 0.05 Daily DMI, kg/steer 8.51 ± 0.11 8.23 ± 0.11 8.72 ± 0.11 8.24 ± 0.11 G:F 0.145 ± 0.01 0.142 ± 0.01 0.141 ± 0.01 0.162 ± 0.01 Bunk attendance, min/d 105.19 ± 4.68 105.97 ± 4.72 105.24 ± 4.72 107.45 ± 4.68 Frequency, visits/d 8.72 ± 0.22 8.67 ± 0.22 8.80 ± 0.22 8.95 ± 0.22 a CE = constant feed delivery offered in the evening (2100); FE = fluctuating feed delivery offered in the evening (2100); CM = constant feed delivery offered in the morning (0900); FM = fluctuating feed delivery offered in the morning (0900). Once daily, steers were fed a constant (C) amount of feed or in a fluctuating (F) 3-d cycle of 10% above followed by 10% below ad libitum intake. b Includes all steers in the trial (n = 234). c Steers fed in pens containing a feed bunk monitoring system (n = 60). metabolism study, but that it did not have a negative effect on animal performance. In the Cooper et al. (1999) study, mean ruminal ph and area ph <5.6 tended to be lower for steers offered fluctuating compared with constant feed delivery, but with only four animals per treatment, differences were not statistically significant. Intakes were higher (P < 0.05) for E than for M cattle (7.48 ± 0.16 vs. 7.26 ± 0.16 kg/d, respectively), and when compared by feeding phase intakes, intakes were lower (P < 0.001) during the backgrounding than during the finishing period (6.29 ± 0.07 vs. 8.23 ± 0.07; Table 6). No interaction effects between time of feeding, feed fluctuation, or feeding phase were observed for feed intake. Cattle fed late in the day gained marginally (P = 0.07) more weight than cattle fed in the morning (1.24 ± Table 5. Effects on feed intake fluctuation and time of feed delivery (0900 vs. 2100) on performance and feeding behavior of feedlot steers fed a barley-grain and barley-silage diet over a 209-d feeding period (values shown are mean ± SE) Treatment a Item CL FL CE FE All cattle b No. of steers (pens) 59 (4) 59 (4) 56 (4) 60 (4) Initial BW, kg 310.7 ± 3.1 310.0 ± 3.1 310.0 ± 3.1 310.6 ± 3.1 Final BW, kg 599.7 ± 5.6 588.9 ± 5.6 584.3 ± 5.7 586.5 ± 5.5 ADG, kg 1.27 ± 0.02 1.23 ± 0.02 1.22 ± 0.02 1.22 ± 0.02 Daily DMI, kg/steer 7.52 ± 0.09 7.45 ± 0.09 7.20 ± 0.09 7.33 ± 0.09 G:F 0.178 ± 0.01 0.177 ± 0.01 0.174 ± 0.01 0.173 ± 0.01 Behavior cattle c No. of steers (pens) 15 (1) 15 (1) 15 (1) 15 (1) Initial BW, kg 310.0 ± 7.4 309.7 ± 7.4 309.8 ± 7.4 307.9 ± 7.4 Final BW, kg 607.2 ± 12.3 590.7 ± 12.7 582.2 ± 12.3 582.9 ± 12.3 ADG, kg 1.31 ± 0.02 1.24 ± 0.02 1.21 ± 0.02 1.22 ± 0.02 Daily DMI, kg/steer 7.66 ± 0.02 7.23 ± 0.02 7.16 ± 0.02 7.13 ± 0.02 G:F 0.181 ± 0.01 0.182 ± 0.01 0.174 ± 0.01 0.184 ± 0.01 Bunk attendance, min/d 137.7 ± 3.2 140.7 ± 3.2 141.9 ± 3.2 142.2 ± 3.2 Frequency, visits/d 9.7 ± 0.2 9.6 ± 0.2 9.7 ± 0.2 9.6 ± 0.2 a CE = constant feed delivery offered in the evening (2100); FE = fluctuating feed delivery offered in the evening (2100); CM = constant feed delivery offered in the morning (0900); FM = fluctuating feed delivery offered in the morning (0900). Once daily, steers were fed a constant (C) amount of feed or in a fluctuating (F) 3-d cycle of 10% above followed by 10% below ad libitum intake. b Includes all steers in the trial (n = 234). c Steers fed in pens containing a feed bunk monitoring system (n = 60).

3364 Schwartzkopf-Genswein et al. Table 6. Effects of time of feed delivery (0900 vs. 2100) and feeding phase (backgrounding vs. finishing) on performance and feeding behavior of feedlot steers (n = 234) fed a barleygrain and barley-silage diet over a 209-d feeding period Treatment a Item EB EF MB MF No. of steers (pens) 59 (4) 59 (4) 56 (4) 60 (4) ADG, kg 1.28 ± 0.04 b 1.25 ± 0.04 b 1.00 ± 0.04 c 1.25 ± 0.04 b G:F 0.211 ± 0.01 b 0.151 ± 0.01 c 0.174 ± 0.01 c 0.153 ± 0.01 c a EB = evening-fed (2100) backgrounding phase; EF = evening-fed (2100) finishing phase; MB = morningfed (0900) backgrounding phase; MF = morning-fed (0900) finishing phase. b,c Within a row and variable, values with different superscripts differ, P < 0.001. Values shown are means ± SE. 0.02 vs. 1.18 ± 0.02 kg/d), which supports the results observed for feed intake. Similar findings have been reported by Reinhardt and Brandt (1994) and Pritchard and Knutsen (1995), showing higher gains by cattle fed in the evening vs. the morning. In contrast, Soto- Navarro et al. (2000) reported no differences in the gains of morning- and evening-fed animals, stating that differences observed by other researchers may have been related to gastrointestinal fill instead of growth performance. An interaction (P < 0.005) for ADG and feed efficiency was observed between time of feed delivery and whether the cattle were in the backgrounding or finishing phase of the experiment (Table 6). The lowest ADG was recorded for M steers during the backgrounding phase, whereas the highest ADG was recorded for E steers during the backgrounding period (1.00 ± 0.04 vs. 1.28 ± 0.04 kg/d), but these differences were not evident in the finishing phase. These data indicate that evening-fed performed better than morning-fed cattle. In the backgrounding period, E steers were also more efficient (P < 0.005) than M steers. Morning-fed cattle in the backgrounding phase were more efficient than E steers during finishing (Table 6). Cattle fed in the evening also had higher (P < 0.05) daily DMI than M-fed cattle (7.48 ± 0.06 vs. 7.26 ± 0.06 kg/d, respectively), indicating it may be beneficial to feed in the evening from a cold climate thermodynamics perspective because the heat produced during fermentation and metabolism is shifted to the evening when cold stress is more likely to occur. Likewise, in regions where heat stress is a concern, evening feeding would help decrease any additional heat load that could occur if the animals were fed during the warmest part of the day. Bunk Attendance. Over the entire feeding period steers (n = 60) in the feed bunk monitoring pens spent an average of (141 min/d) at the bunk over an average of 10 visits/d. These bunk attendance patterns are similar to those reported for steers during a feeding trial fed diets similar to those in this experiment that included both backgrounding and finishing phases (Schwartzkopf-Genswein et al., 1999, 2001). There was no difference in the total daily bunk attendance of either F or C fed steers, M or E steers, or their interactions (Tables 3, 4, and 5). Fluctuations in feed delivery did not cause animals to compensate by altering their duration of feed consumption. One possible reason for this finding is that the steers may have increased or decreased their rate of consumption; however, this is only speculation because measurements of rates of consumption per meal were not made in this study. The results of the current study are in contrast with the findings of Cooper et al. (1999), who reported that the total time spent eating per day was lower (P < 0.05) for cattle fed a fluctuating amount of feed (0.9 kg/d change in feed delivery) compared with cattle that were fed ad libitum. However, differences in eating rate could strongly affect the relationship between bunk attendance and intake and should be taken into consideration when interpreting these kinds of data (Schwartzkopf-Genswein et al., 2001). There was also no effect of fluctuation or time of feed delivery on the number of visits per day to the feed bunk (Tables 3, 4, and 5). Thus, it seems that feeding duration and the number of meals were not affected by changes in feeding management. A more detailed study looking at consumption rates may better explain the relationship between bunk attendance and bunk management. Literature Cited Beauchemin, K. A., W. Z. Yang, and L. M. Rode. 2001. Effects of barley grain processing on the site and extent of digestion of beef feedlot finishing diets. J. Anim. 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