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Feeding strategies for managing heat load in feedlot cattle¹

T. L. Mader^*,2, S. M. Holt, G. L. Hahn, M. S. Davis^* and D. E. Spiers

^* University of Nebraska, Northeast Research and Extension Center, Concord, NE 68728; and South Dakota State University, Brookings, SD; and U.S. Meat Animal Research Center,Clay Center, NE 68933; and and University of Missouri, Columbia, MO 65211

² Correspondence:
Haskell Agricultural Laboratory, 57905 866 Road (phone: 402-584-2812; fax: 402-584-2859; E-mail:
tmader{at}unlnotes.unl.edu).

	Abstract

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Eighty-four Bos taurus crossbred steers were used to investigate effects of level and duration of limit-feeding feedlot cattle in a hot environment. Pens (four/treatment) of steers (seven/pen) were fed feedlot finishing diets and randomly assigned to the following treatments: 1) restricted to approximately 75% of feed consumed when offered ad libitum for 21-d duration (RES21); 2) restricted to approximately 75% of ad libitum for 42-d duration (RES42); and 3) feed offered ad libitum (ADLIB). Tympanic temperatures (TT) were measured via thermistors placed in the ear canal and attached to data loggers. Restricting feed intake for both 21- and 42-d reduced tympanic temperature when compared with ADLIB treatment groups under hot environmental conditions. Temperature reductions exceeded 0.5°C (P < 0.05) depending on time of day. The reduced tympanic temperature is likely due to a reduction in metabolic heat load and/or a concurrent reduction in metabolic rate. Within respective periods, no differences (P > 0.05) were found among treatments for panting or bunching score. However, different proportions of cattle were found to be bunching and panting with ADLIB cattle displaying a greater number of bunched steers that were panting when compared with the other groups. When averaged across diet treatments, dark-colored cattle had the greatest percentage of cattle showing moderate to excessive panting, while light-colored cattle displayed the least panting under thermoneutral climatic conditions. Under hot (mean daily temperature-humidity index >74) conditions, dark-colored cattle tended to bunch more (P = 0.073) and pant more (P < 0.01) than light-colored cattle. Mean TT were 0.2 to 0.6°C (P < 0.05) greater for dark- vs light-colored cattle under hot conditions. Limit-feeding feedlot cattle during early summer is a successful tool for enhancing animal comfort by alleviating the combined effects of high climatic and metabolic heat load.

Key Words: Body Temperature • Feed Intake • Feedlots • Heat Stress

	Introduction

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Discomfort experienced by animals during periods of elevated climatic temperatures may result in reduced feed intake, reduced body weight gain, and, in extreme cases, death of cattle (Hahn, 1994; Lefcourt and Adams, 1996; Mader et al., 1997b). Severe heat episodes occurred in 1995, 1997, and 1999 in which individual producers in the Northern Plains and Cornbelt lost in excess of 100 head while total economic impact to the cattle industry exceeded $20 million per episode (Busby and Loy, 1996; Hahn and Mader, 1997; Hubbard et al., 1999). Problems in managing cattle exposed to elevated climatic temperatures may be further complicated if cattle are being fed high-energy diets (Brosh, et al., 1994; Reinhardt and Brandt, 1994; Gaughan et al., 1996), which also contributes to elevated metabolic heat load. Reducing ME intake through feed restriction could lower heat production (Purwanto et al., 1990) and enhance feed conversion (Hicks et al., 1990; Murphy et al., 1994; Murphy and Loerch, 1994).

Regulation of body temperature is essential for surviving excessive heat load and involves both physiological and behavioral changes. Increased respiratory ventilation, known as panting, is associated with heat exposure (Robertshaw, 1985). Panting can be classified as rapid-shallow (first-phase) or open-mouth (second-phase) categories (NRC, 1981). The use of a panting score or index would appear to be an easy-to-use tool for characterizing heat stress of animals and evaluating heat-stress management strategies. Bunching behavior is also influenced by ambient conditions and is thought to reduce radiant heat absorption as animals provide shade for each other (Lefcourt and Schmidtmann, 1989). The positive effects of bunching are unclear, however.

This study was undertaken to investigate effects of level and duration of restricted feeding of feedlot cattle in a hot environment using tympanic temperature, panting score, and bunching activity as measures.

	Materials and Methods

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Eighty-four medium-frame, Bos taurus crossbred steers of uniform weight (389 ± 23 kg BW), approximately 15-mo-old with an average body condition score (4.98 ± 0.16; NRC, 1996), were used during a 63-d summer feeding period beginning in late June. Steers were fed in facilities with shelterbelt provided which are described by Mader et al. (1997a). Pens were of uniform length (43 m), width (8.5 m), and slope (4%). Ample pen space (52 m²/steer) and bunk space (1.2 m/steer) were available for each animal. Water was available for all steers throughout the study and was provided through cattle fountains (Challenger 2 model; Ritchie Livestock Fountains, Conrad, IA) shared between adjoining pens (one waterer/two pens of steers). Steers were blocked by weight. Within a block, steers of similar color were randomly assigned to each pen to ensure that black, red, and white coat-colored cattle were equally distributed among pens. For collection of behavior data, the black and red coat-colored cattle were subsequently classified as dark hided (predominantly black) and white coat-colored cattle as light hided. Pens of steers (four/treatment) were fed feedlot finishing diets and randomly assigned to the following treatments. The treatments were: 1) restricted to approximately 75% of feed consumed for cattle fed ad libitum for 21-d duration (RES21); 2) approximately 75% of feed consumed for cattle fed ad libitum for 42-d duration (RES42); and 3) fed ad libitum (ADLIB). Cattle on RES21 and RES42 treatment groups were stepped up over 4 to 6 d to ad libitum following the 21- and 42-d restriction, respectively. The DMI of steers on the RES21 and RES 42 treatment groups were determined prior to starting the study and were based on projected gain and associated daily DMI of comparable cattle offered feed ad libitum using computer software (NRC, 1996), based on breed type, age, body condition, frame size, and diet. Feed was weighed and delivered to each pen daily between 0800 and 1000. A feedlot finishing diet was fed to all treatment groups and contained dry rolled corn, corn gluten feed, corn silage, and dry supplement (Table 1). For the ADLIB group, two of the four pens were fed a diet with dry-rolled corn substituted for the corn gluten feed to compare a diet containing 40% corn gluten feed to a traditional dry rolled corn-based diet when fed during the summer. Overall DMI and tympanic temperature (TT) differences between the two ADLIB treatment groups were not different (P > 0.10). These two groups were considered as one treatment in subsequent analysis. Steers were implanted with Revalor-S (24 mg estradiol and 120 mg trenbolone acetate; Intervet, Inc., Millsboro, DE) at the beginning of the trial. All steers were fed in the morning at approximately 0800.

During each of the three 21-d periods, thermistors were inserted into the ear canal of 12 steers in six pens (two pens/TRT group; two steers/pen) to obtain tympanic temperature on an hourly basis. The TT were obtained during one 4- to 7-d interval within a period in an effort to obtain data during one or more hot days, in which THI was at 74 or above. Steers were selected based on coat color and weight in an attempt to compare similar steers among treatments. Both light and dark coat-colored steers within a pen and treatment were utilized. Procedures used to measure TT were based on similar procedures used by Hahn et al. (1990). Thermistor cables (Model TMC1-1T; Onset Computer Corporation, Pocassatt, MA) were placed into the ear canal, close to the tympanic membrane, to an approximate depth of 12 cm. The flexible thermistor cables were dipped in Nolvasan (Fort Dodge Animal Health, Fort Dodge, IA) antiseptic ointment prior to insertion. Data loggers (Model Stow Away XTI08 37-46; Onset Computer Corporation) were then connected to the thermistor, wrapped with padded gauze, placed on the inside of the ear, and secured to the ear. Prior to wrapping with gauze, dataloggers were thoroughly wrapped with Tartan Brand Tape "3691" (3M Canada, Inc., London, Ont., Canada) and subsequently sealed in vacuum bags (BagVac; FoodSaver, Tilia Inc., San Francisco, CA). The animals were returned to pens once dataloggers were secured with Vetrap (3M, St. Paul, MN). Data loggers were retrieved from the animals after designated collection intervals ended. Data were downloaded to computer using BoxCar Pro software (Onset Computer Corporation). Prior to the study, data loggers and thermistors were checked for accuracy and determined to be fully operational using a Percival Scientific (Boone, IA) incubator (model I-35LVL).

Within each period, behavior data (panting and bunching) were obtained during thermoneutral days (mean daily THI <74) and hot days (mean daily THI 74) at 1600. Panting scores were obtained by visual assessment of flank movements and respiration pattern in individual steers. A score of 1 indicated slight or no panting, while a score of 2 indicated moderate to excessive panting with mouth opened and/or salivation occurring. At the same time, a bunching score was assigned which indicated the proximity of each animal to its nearest neighbor (within a pen). A score of 1 indicated animals were bunched (any part of one animal within 1 m of the midline of any other animal, with midline determined from shoulders to tailhead), while a score of 2 indicated animals were separated from others. Panting and bunching scores were obtained visually at 1600 on days TT were being obtained. In addition, baseline panting and bunching scores were obtained at the initiation of the study under thermoneutral conditions. Data were also collected throughout the study when hot conditions existed. The study was conducted with the approval of the University of Nebraska-Lincoln Institutional Animal Care and Use Committee.

Statistical Analysis.
Dry matter intake and mean TT were analyzed using the GLM procedures of SAS (SAS Inst. Inc., Cary, NC) for a randomized complete block design with the model including the fixed effects of TRT and block. Tympanic temperatures, taken over time, were analyzed using repeated measures, with day and coat color added to the model. Pen was used as the experimental unit for DMI analysis, while animal was used as the experimental unit for TT analysis. Behavior data were analyzed using chi-square test with a mean panting and bunching score determined for each treatment.

	Results

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For the duration of the study, THI averaged 71.5 and ranged from a daily average of 64.2 to 79.4 (Table 2). Respectively, mean THI were 72.8, 70.3, and 71.6 for the three 21-d feeding periods. Although the second period had the lowest average temperatures, the hottest days(s) were found in that period. That period also had the greatest variation in maximum temperature. In general, climatic conditions were comparable with normal summer climatic conditions previously recorded (Mader et al., 1999a).

View larger version (24K): [in this window] [in a new window]   Figure 1. Tympanic temperatures taken between 22 and 42 d for steers restricted in feed intake for the first 21 d (RES21) and 42 d (RES42), respectively, or ad libitum fed (ADLIB) the entire feeding period. Vertical lines indicate SE. Means differ between RES42 and other treatments for 1800 through 0100 (P < 0.05).

View larger version (19K): [in this window] [in a new window]   Figure 2. Tympanic temperatures for dark vs light coat-colored cattle. Vertical lines indicate SE. Means differ for 1200 through 2100 (P < 0.05).

Coat color was found to have a significant (P < 0.01; Table 7) effect on panting score. When averaged across diet treatments, dark-colored cattle had the greatest percentage of cattle showing moderate to excessive panting, while light-colored cattle displayed the least panting, under thermoneutral climatic conditions. A similar pattern was seen under hot climatic conditions. The percentage of cattle showing moderate to excessive panting increases approximately 30% from thermoneutral to hot conditions. Only when cattle were exposed to hot climatic conditions did trends in bunching become apparent. Under hot conditions, dark-colored cattle bunched more (P < 0.07) than light-colored cattle. Since cattle of different coat colors were in the same pens, it would appear that the light-colored cattle tend to stay away from the dark-colored cattle. Whether they are not bunching because they are cooler, having fewer problems with flies than dark-colored cattle, or sense heat coming from the dark-colored animals, is unknown. Although the data are not shown, observed effects of coat color on bunching tended to diminish over time, particularly from period 2 (P < 0.03) to period 3 (P < 0.14). Thus, the percentage of light-colored animals bunching appears to increase over time, as body condition and days of feed increase. It would appear that as light-colored cattle put on more body condition they tend to behave more like the dark-colored cattle under hot conditions.

	Discussion

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Restricted feeding techniques are one form of dietary manipulation in which feed intake is restricted to a percentage of the intake of a control pen of cattle offered feed ad libitum or restricted to a percentage of projected ad libitum intake. Other restricted feeding techniques include limiting maximum intake, stair-step or plateau feeding, utilization of intake inhibiting feed additives, and time restriction (Owens et al., 1995). Restricted feeding increases the efficiency with which cattle convert feed into gain (Plegge, 1987; Hicks et al., 1990; Murphy and Loerch, 1994); however, restricted feeding also tends to decrease ADG. Consequently, hot carcass weights are reduced or if animals are fed to similar final weights, days on feed are increased (Plegge, 1987; Hicks et al., 1990; Murphy and Loerch, 1994). The lower DMI in period 3 (Table 3) for steers previously restricted in DMI indicates that DMI continues to be depressed after the restricted feeding period, and that compensatory intake would likely not offset performance reductions associated with the restricted feeding period. Also, during the hot conditions (Table 4), ADLIB cattle DMI was depressed in periods 2 and 3, but not in period 1 when compared with their DMI over the entire study (Table 3). The largest differences in TT between ADLIB and restricted feeding treatment were found in period 1. Data suggest that one mechanism by which cattle reduce heat load themselves is through DMI reduction. This would appear to be a behavior change that would only occur with prior exposure and associated discomfort. The results of such studies are presented here to lend support for the use of restricted feeding programs in summer feeding situations without adversely affecting animal performance, particularly if used on a short-term (21 to 42 d) basis to manage heat stress.

Restricting feed intake significantly (P < 0.05) lowered TT. Purwanto et al. (1990), suggested that total heat production is dependent, in part, on feed intake. The lowered production is likely a result of decreased maintenance heat production. Mader et al. (1999b) fed one of the three diets to steers housed in controlled climatic conditions. Diets fed consisted of a high-energy (1.36 Mcal/kg NEg) diet fed ad libitum (HE), a low-energy (1.15 Mcal/kg NEg) diet fed ad libitum (LE), or the high-energy diet fed at 90% ad libitum (RE). Pulse rate, respiration rate, and rectal temperature were monitored under both thermoneutral and hot conditions as were DMI and water intake. Pulse rate of RE and LE steers was reduced (P < 0.05) under both thermoneutral and hot environmental conditions compared with HE steers and was concluded to be more indicative of level of ME intake rather than heat stress. The decrease in pulse rate is indicative of a decrease in heat production (Brosh et al., 1998) which is common to animals on restricted energy intake regimens (Carstens et al., 1991). Also, the decrease in heat production was supported by rectal temperatures obtained from these animals with cattle on the low ME intake feeding regimens having lower (P < 0.05) rectal temperatures than HE steers at all times sampled.

Changes in organ size and metabolic rate also likely contributed to the differences in TT and resultant reduced susceptibility to heat stress. Differences in maintenance requirements as a result of different planes of nutrition have been routinely associated in changes in metabolism and(or) size of metabolically active organs such as the liver, gastrointestinal tract, heart, spleen, and kidney (Koong, et al., 1985; Burrin et al., 1990; Freetly et al., 1995). Debate remains as to whether improvements in overall energetics of animals is due to changes in relative organ size or actual changes in metabolic activity per unit of organ (Burrin et al., 1990). Furthermore, reductions in fasting heat production have been shown to occur in response to decreased feed intake (Graham and Searle, 1972; Graham et al., 1974).

In this study, expected carryover of the effects of restricted feeding into the ad libitum feeding period was not observed at least in TT measurements. A lower TT in periods following restricted feeding would be indicative of a change in organ mass and/or lowered metabolic activity due to feed restriction. Carryover effects on TT have been observed in other studies. Davis, et al. (2001) reported differences in TT of 0.5°C or more between cattle previously restricted in DMI and ADLIB cattle, once both groups were full-fed with the ADLIB group having greater TT. In this study, cattle previously restricted in DMI had TT equal to control (ADLIB) cattle when both were full-fed. It should be noted, however, that mean TT remained constant, as opposed to increasing, for both the RES21 (39.3°C) and the RES42 (39.1°C) when they went from restricted feeding to full-fed.

In regard to coat color, differences in TT of steers as a result of coat support findings of Finch et al. (1984) and Arp et al. (1983). Finch et al. (1984) reported that rectal temperature averaged 0.3°C higher in dark-red vs white B. taurus cattle. This difference was attributed to the greater heat flux present at the skin of the darker-haired animals (159 vs 115 W/m²). Additionally, Arp et al. (1983) found that black-haired steers in commercial feedlots had body surface temperatures as much as 21°C greater than white-haired contemporaries. The fact that coat color influenced TT of dark coat-colored animals lends support to commercial feedlot surveys reporting increased susceptibility of black-haired animals during times of heat stress (Busby and Loy, 1996; Mader et al., 2001). Also, it would appear that as animals become more conditioned, their ability to detect radiant heat from other animals diminishes. Although, in the present study, light coat-colored animals under hot conditions were less likely to bunch than dark coat-colored animals, possibly to avoid additional radiant heat from other animals that may contribute to their already high heat load. Previously learned behavioral patterns may be useful to help animals avoid stress.

	Implications

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Programmed or restricted feeding systems would be useful during periods of hot weather by reducing body temperature, possibly through reductions in metabolic heat and a concurrent reduction in metabolic rate. Lowered and/or stable body temperatures appear to carry over into the initial period that ad libitum feeding takes place. Diurnal changes or shifts in body temperature, regardless of coat color, may be a further effect of limiting feed intake. Restricting intake may be an appropriate management tool to shield cattle from potential heat stress events. Utilizing programmed or restrictive feeding systems from the first to the middle portion of summer would appear to be sufficient to cover most heat waves.

	Footnotes

 
¹ Published as Journal series no. 13524, Agric. Res. Div., University of Nebraska. Partial research support was provided through USDA NRI Competitive grant No. 9803525.

Received for publication November 8, 2001. Accepted for publication May 10, 2002.

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