The Professional Animal Scientist 25 ( 2009 ):109–117
E ffects of Tasco in Alleviation of Heat Stress in Beef Cattle J. E. Williams ,*1 PAS, D. E. Spiers ,* L. N. Thompson-Golden ,2 T. J. Hackman ,* M. R. Ellersieck ,* L. Wax ,* D. P. Colling ,† J. B. Corners ,‡ and P. A. Lancaster ,§ PAS * Department of Animal Sciences, University of Missouri, Columbia 65211; † Sweetlix, Parkville, MO 64152; ‡ ADM Alliance Nutrition Inc., Quincy, IL 62301; and § Department of Animal Science, Oklahoma State University, Stillwater 74078
ABSTRACT A study was conducted to evaluate the
influence of Ascophyllum nodosum (Tas-co) on rectal temperature, respiration rates, and in situ NDF disappearance in cattle exposed to elevated ambient tem-perature. Twenty-four Angus crossbred steers (average BW = 300.6 kg) were randomly assigned to treatments using a 2 × 2 factorial arrangement: 1% Tasco vs. no Tasco (control), and thermoneu-tral (TN) vs. heat load (HL) conditions. Three steers in each room were assigned to the Tasco vs. control treatment. In period 1 (10 d), steers were acclimated to chambers at TN (19°C) conditions. Subsequently, for periods 2 and 3, HL was maintained at a daytime high of 36°C and a nighttime low of 19 and 31°C, respectively. For period 4, tem-perature conditions (TN and HL treat-ments) were reversed and period 2 HL conditions were imposed. In periods 2 (P < 0.09) and 4 (P < 0.05), Tasco lowered DMI. In period 2, the control steers had greater (P < 0.0001) differences between maximum and minimum rectal tempera-tures than steers in the Tasco treatment; this difference occurred between d 4 and 8 for the HL group. In periods 2 and 3, the maximum and minimum differences
in respiration rate values were greater (P < 0.05) under HL conditions for the Tasco vs. control treatment. In period 4, the maximum respiration rate response to HL tended to be greater (P = 0.07) under HL conditions for the control vs. Tasco treatment. In period 2, a trend ex-isted for steers in the Tasco treatment to have a greater (P < 0.1) rate of in situ NDF disappearance than control steers. In conclusion, Tasco appeared to reduce rectal temperature for 3 to 4 d.
Key words: Tasco , beef cattle , heat stress
INTRODUCTION A significant loss in farm income is
attributed to heat stress, even after decades of study to alleviate the prob-lem. The economic impact of heat load (HL) on cattle performance has been estimated at $28 and $40 million for Nebraska and Iowa, respectively (Smiley, 1996). These values include estimates for the long-term impact on feed intake and growth and the increased incidence of animal mortal-ity. The high rate of mortality has been attributed to the inability of beef cattle to adjust rapidly to the decreased thermal gradient for heat loss from the body during heat stress (Hahn, 1995).
There has been limited success in the development of cost-effective di-
etary products to alleviate heat stress during the summer months. Seaweed extract (Ascophyllum nodosum) from Acadian Seaplant Ltd. (Nova Sco-tia, Canada) has been evaluated for reducing heat stress. Ascophyllum nodosum is processed in a meal form from brown seaweed and marketed as Tasco, whereas soluble seaweed extract is derived from alkaline hydro-lysis and is a more concentrated form of A. nodosum. Eichen et al. (2001) and Spiers et al. (2004) determined that 1% seaweed extract in the diet temporarily lowered internal body temperature in animals consuming en-dophyte-infected tall fescue that were exposed to heat stress. Steers grazing endophyte-infected tall fescue pasture treated with Tasco (at a rate of 3.4 kg/ha) on April 4 and July 6 ap-peared to have an improved immune function and hair coat associated with fescue toxicosis (Saker et al., 2001). A previous study in our laboratory revealed that inclusion of 2% Tasco or 0.5% seaweed extract had no effect on rectal temperatures of steers consum-ing endophyte-infected tall fescue seed and exposed to heat stress (Williams et al., 2001). Earlier studies of the thermoregulatory effects of seaweed treatment on the heat stress response have been short term. The present study examined the long-term effect of Tasco treatment at thermoneutral-ity and different levels of HL for a
1 Correspondencing author: [email protected] 2 Current address: 8910 1st Rd., Mountain Grove, MO 65711.
more complete representation of the natural environment.
MATERIALS AND METHODSTwenty-four Angus crossbred steers
(average BW = 300.6 ± 7 kg) were assigned in a factorial arrangement to determine the effects of Tasco on the performance, rectal temperature (Tre), and respiration rate (RR) of heat-stressed steers. Eight of the steers were fitted with ruminal cannulae (Bar Diamond, Parma, ID) to evalu-ate the influence of Tasco on in situ NDF disappearance when steers were exposed to heat stress. The surger-ies and study were approved by the University of Missouri Animal Care and Use Committee. Steers were al-lowed to recover for 3 wk before the study. During recovery and adapta-tion, animals were housed in open pens (4 × 14 m) on concrete partially covered with a roof. Because the hair coat of each steer varied in length, it was clipped to approximately 2 cm to create a uniform surface.
All animals were housed in the Brody Environmental Center at the University of Missouri in 1 of 4 cham-bers (6 steers/chamber with 3/treat-ment). The daily ration contained either 1% Tasco or no Tasco, serving as the control. Two cannulated steers representing Tasco and the control were assigned to each chamber. Lights remained on between 0600 and 2200 h and were dimmed from 2200 to 0600 h. Steers were tethered by a neck har-ness and were individually housed in stanchions equipped with feed bunks and automatic waterers. Environ-mental conditions were continuously measured using data loggers (Hobo, Onset Computer Corp., Bourne, MA) to record air temperature (Ta) and percentage relative humidity. Before the study, steers were acclimated to surroundings at 19°C for 7 d. At this time, steers began their dietary treatments (Tasco or control). Rela-tive humidity was maintained at 50% or lower throughout the study to focus primarily on the thermal effects. During period 1 (10 d), steers were maintained at thermoneutral (TN)
conditions (Yousef, 1985) to estab-lish baseline levels for all measured variables. The temperature-humidity index (THI) was calculated according to Thom (1959). Ambient tempera-ture within the 4 chambers ranged from 18.5 to 19.5°C (maximum THI = 63 to 65) during this period (Figure 1). For period 2 (12 d), 2 chambers were maintained at TN (20.5°C; maxi-mum THI = 66), and the 2 HL cham-bers were increased during the day to an average Ta of 36.0°C (THI = 86) and decreased at night to an average of 25.0°C (THI = 72). Additional heat stress was then applied during period 3 (11 d) in the HL rooms, with day-time high Ta maintained at approxi-mately the previous level (35.5°C; THI = 86) but the nighttime low being decreased to 31.0°C (THI = 80) to reduce the ability of the animals to dissipate heat at night. During period 4 (13 d), the daytime high Ta and nighttime low Ta were approxi-mately the same as in period 2, but the TN (19.5°C; THI = 65) and HL (day: 36.0°C, THI = 86; night: 25.5°C, THI = 73) treatments were reversed, so that steers previously maintained at TN were heat stressed and those under HL conditions were switched to TN. The purpose for reversing the temperature conditions was to determine whether length of exposure to Tasco under TN conditions would have an effect on the initial response to heat stress.
Animals were fed at 0700 h daily, with orts removed and weighed for each steer, followed by feeding of the presupplemental diet. The presupple-mental diet, which consisted of 0.05 kg ground alfalfa and 0.05 kg ground corn along with 1% Tasco or with-out Tasco, was fed before feeding the main ration to ensure that steers con-sumed the daily allotment of Tasco. The basal diet consisted of cracked corn, cottonseed hulls, and a supple-ment (Table 1). The steers were fed this diet twice daily at approximately 0700 and 1600 h to ensure that they had access to fresh feed.
Samples were taken from each load of feed for chemical analysis. The diet was analyzed for DM (official method
978.10; AOAC, 1995), N by combus-tion analysis (Leco Corp., St. Joseph, MI; official method 990.03, AOAC, 1995), and NDF content (Van Soest et al., 1991).
Steers were weighed before the 0700-h feeding at the beginning and end of each period. Thermal status measurements were taken daily at 0800, 1200, 1600, and 2000 h. Ther-mal status included Tre and RR. The Tre was recorded using a stainless steel thermistor probe (Model 8110-20; Cole-Palmer Instrument Co., Ver-non Hills, IL). Respiration rates were made by enumeration of abdominal movements for 30 s. The daily maxi-mum and minimum values for Tre and RR were determined for each animal, potentially to identify extremes in response to Ta as opposed to an aver-age of the daily response. Ultimately, these values were averaged across days and periods.
110 Williams et al.
Figure 1. Air temperature (°C) in the rooms housing the steers exposed to thermoneutral (TN) and heat load (HL) conditions during the 4 periods. (A) Ambient temperature for the 2 groups exposed to periods of HL followed by TN conditions. (B) Ambient temperature for the 2 groups exposed to TN conditions followed by HL conditions. The space between parallel, vertical lines was not included in the analysis of data.
In Situ Study
On d 7 of each of the 4 periods, Dacron bags containing 6.5 g ground bromegrass hay were placed in the rumen of each steer to measure in situ NDF disappearance. Brome hay was previously used in other studies to evaluate in situ NDF disappear-ance (Wen, 2001). It provided a more consistent source of NDF as compared with cottonseed hulls. Triplicate bags of bromegrass were incubated for 0, 3, 6, 12, 24, 48, and 72 h. Bags were placed in the rumen in the reverse order and were removed at the same time. Bags were washed according to the procedure of Nocek (1985). The DM content of the residue in the bags (official method 978.10; AOAC 1995), and NDF content (Van Soest et al., 1991) were determined. The in situ NDF disappearance was expressed as potential extent of degradation (Fd), rate of disappearance (Kd), and discrete lag time before the onset of degradation.
Statistical Analysis
The DMI, RR, and Tre data were analyzed as a repeated measures design, as outlined by Littell et al. (1998). The BW, total BW gain, and ADG data were analyzed as a ran-domized complete block design. The linear model contained the main plot effects of HL, Tasco, and the inter-action of HL × Tasco. The subplot contained the effects of time with the main plot effects. Mean differences were determined using Fisher’s least significant difference. This was ac-complished using the MIXED proce-dure (SAS Institute Inc., Cary, NC). In Figures 1, 2, 3, and 4, the space between parallel, vertical lines and representing the transition period was not included in the analysis of the data. Animal was the experimental unit. A value of P < 0.05 was used for establishing significant differences. For the BW, total BW gain, and ADG, no HL × Tasco interactions (P > 0.20) were found.
For the in situ study, all degrada-tion data were expressed as a per-centage of disappearance. Data were corrected for insoluble NDF washed from the bag by using the equation of Weisbjerg et al. (1990).
The NLIN procedure (SAS Insti-tute, Inc.) was used to fit degradation data to a first-order exponential model with a discrete lag time (McDonald, 1981):
Y b 1 kt e dt( ) = --( )( ,[ ]t where Y(t) is
disappearance (%), b is the potential extent of degradation (%), kd is degradation rate (%/h), t is time (h), and τ is the discrete lag time before the onset of degradation (h). Note that an intercept term (commonly referred to as a) was not included because the correction for NDF disappearance forced the condition Y(0) = 0. Values of b and τ were restricted between 0 and 100% and
111Tasco and heat stress in beef cattle
Table 1. Diet composition
ItemAmount, % of DM
Ingredient Cottonseed hulls 50.60 Cracked corn 37.00 Soybean meal 2.60 Urea 1.30 Limestone 0.60 Salt 0.84 Cane molasses 2.50 Trace mineral1 0.16 Dicalcium phosphate 0.33 Tallow 0.71 Dehydrated alfalfa 3.30 Rumensin 802 0.02 Vitamins A, D, and E3 0.04Chemical analysis DM 90.5 CP 12.6 ADF 30.91Contained 10% Fe (minimum), 10% Mn (minimum), 10% Zn (minimum), 2% Cu (minimum), 500 ppm Co, 1,000 ppm I, 1,500 ppm Se.2Included at 0.18 kg/ton and supplied by Nutrablend (Neosho, MO).3Contained 1,818 kIU vitamin A, 364 kIU vitamin D, and 546 IU vitamin E per kg.
Figure 2. Dry matter intake of steers on the control and Tasco treatments exposed to thermoneutral (TN) and heat load (HL) conditions during the 4 periods of the study. During period 4, the steers exposed to TN and HL conditions were reversed. The space between parallel, vertical lines was not included in the analysis of data. LSD = least significant difference.
>0 h, their theoretical ranges. No HL × Tasco interactions were found (P > 0.20) and therefore are not presented.
RESULTSResults from the study are present-
ed by period because the study was designed as a 2 × 2 factorial arrange-
ment to determine steer response to Tasco under different HL condi-tions that existed in periods 2 and 3. Finally, in period 4, the long-term effects of Tasco were examined by switching steers assigned to the TN and HL conditions in periods 2 and 3; the HL group became TN and the TN treatment became HL.
In period 1, steers in the Tasco and control treatments remained at TN conditions to acclimate them to the climatic chambers before exposing them to elevated Ta. No differences (P > 0.05) existed in DMI, ADG, Tre, or RR, or between the maximum and minimum RR between treatments. However, as shown in Figure 3, steers in the Tasco treatment had lesser dif-ferences (P < 0.05) between maxi-mum and minimum Tre than control steers (0.62 vs. 0.76°C, respectively). This difference between maximum and minimum Tre occurred primar-ily on d 6 and was attributed to the adjustment period.
Effect of Tasco During Progressively Higher Levels of Heat Stress
In periods 2 and 3, DMI was af-fected by HL (P < 0.05), with an average of 10.7 vs. 8.4 kg/d and 11.7 vs. 8.0 kg/d for TN and HL, respec-tively (Figure 2). There was a Tasco × period interaction (P < 0.0001) for DMI, with a trend for lower (P < 0.09) values for the Tasco treatment as compared with the control treat-ment in period 2. In periods 2 and 3, HL reduced (P < 0.05) the final BW, total BW gain, and ADG, but Tasco had no effect (P > 0.10) on these variables (Table 2).
The RR for steers in the Tasco and control treatments was not different (P > 0.10) throughout periods 2 and 3, regardless of whether steers were in the TN or HL group (Figures 3a and 4a, respectively). The shift from peri-ods 2 to 3 at TN did not change the RR, with only a slight 15% downward shift, likely attributed to adaptation to the enclosure. In period 3, it was surprising that RR was not increased above the period 2 level, even with a reduced level of nighttime cooling. Al-though RR was not affected by Tasco and HL, a HL × period interaction (P < 0.0001) existed. The difference (P < 0.05) between maximum and minimum values for RR contrasted for steers in the control [42.0 and 27.9 breaths/min (bpm)] and Tasco (48.5 and 34.2 bpm) treatments during
112 Williams et al.
Figure 3. Maximum and minimum respiration rate (A) and rectal temperature (B) throughout the entire study for steers in the 2 treatment groups exposed to periods of thermoneutral (TN) followed by heat load (HL) conditions. The LSD bars signify least significant differences for maximum (Max) and minimum (Min) respiration rate and rectal temperature. The space between parallel, vertical lines was not included in the analysis of data. bpm = breaths per minute.
periods 2 and 3, respectively. The maximum RR for steers in the Tasco treatment during these periods was only 1.5 to 5.1 bpm above that of control steers. It is unlikely that such a small difference in RR accounted for the early differences in Tre (Figure 4b).
The average Tre did not change (P > 0.05) more than 0.1°C with contin-
uous TN exposure throughout periods 2 and 3 (Figure 3b). In contrast, the average Tre of steers exposed to the HL environment increased (P < 0.05) by 1.0°C during period 2 (Figure 4b). After 4 d of HL, the Tre peaked in this environment. Near the end of period 2, there was a large decrease in Tre of more than 1.0°C as an indica-tion of adaptation to heat exposure.
The largest decrease was in daily maximum Tre, with less change in the daily minimum Tre. A Tasco × HL × period interaction (P = 0.06) existed for the difference between maximum and minimum Tre. During this period under HL conditions, steers in the control treatment had greater (P < 0.0001) differences between maximum and minimum Tre (1.25°C) than those in the Tasco treatment (1.0°C). This difference was primarily between d 4 and 8 for HL.
Effect of Tasco on a Shift Between Thermoneutral and Heat Stress Conditions
Steers fed the Tasco or control diet for 3 periods under TN conditions were switched to HL conditions in period 4, with an opposite switch for those maintained under HL condi-tions. This was done to determine whether the length of feeding Tasco affected the response to a change in Ta conditions. The reversal of Ta conditions from TN to HL decreased (P < 0.05) DMI more for steers in the Tasco than the control treatment, with a reduction of 12.0 to 9.6 kg (20%) for control steers and 11.4 to 8.4 kg (27%) for steers fed Tasco (Fig-ure 2). In period 4, the difference (P < 0.001) in total BW gain and ADG between steers exposed to HL and TN conditions continued (Table 2).
A switch in thermal exposure of steers from HL to TN conditions resulted in an immediate reduction (Figure 4a) in RR. The percentage reduction was 51 to 53% (i.e., 115 to 120 bpm reduced to 56 bpm) for both treatment groups. Comparison of steers in the Tasco and control treatments within a thermal environ-ment showed no difference (P > 0.10) in mean RR or maximum RR, and no differences between maximum and minimum RR during period 4. The exposure of animals in the TN condi-tion to the HL condition resulted in an increase in RR to 116 to 122 bpm, or 75 and 60% increases in RR for the control and Tasco treatments, respec-tively (Figure 3a). It is important to note that the increase in RR of steers
113Tasco and heat stress in beef cattle
Figure 4. Maximum and minimum respiration rate (A) and rectal temperature (B) throughout the entire study for steers in the 2 treatment groups exposed to periods of heat load (HL) followed by thermoneutral (TN) conditions. The LSD bars signify least significant differences for minimum (Min) and maximum (Max) respiration rate and rectal temperature. The space between parallel, vertical lines was not included in the analysis of data. bpm = breaths per minute.
in the control and Tasco treatments from periods 1 to 2 was 93 and 109%, respectively. Therefore, increased duration in the study reduced the RR response to HL, with steers in the Tasco treatment showing the greater reduction. Maximum RR tended to be lower (P = 0.07) for steers fed the Tasco (131 bpm) vs. control (141 bpm) diet. The Tasco × HL × period interaction (P < 0.05) for maximum RR was also attributed to the trend for greater (P = 0.07) maximum RR for the control vs. Tasco-fed steers under HL conditions. Finally, the difference between maximum and minimum RR was less (P < 0.01) for Tasco-fed (32 bpm) compared with control steers (42 bpm).
The Tre was affected by the change in thermal environment and the Tasco diet (Figures 3b and 4b). After 23 d of heat exposure, the TN environ-ment resulted in a 1.5°C decrease in Tre for steers in both the Tasco and control treatments. The Tre was not different for steers in the Tasco and control groups (P > 0.10) in the TN
environment. The shift from the TN to the HL environment resulted in an increase of 1.0 and 1.3°C in Tre for the control and Tasco treatment, respectively. Initial exposure to HL in period 2 resulted in a 1.0°C increase for steers in the Tasco and control treatments, suggesting that the 1.3°C increase for steers in the Tasco treat-ment in period 4 may be attributed to the longer exposure to Tasco. The absolute Tre value of steers in the Tasco treatment in period 4 was also higher than the control in the HL environment (P < 0.01). Likewise, the maximum and minimum differences in Tre were greater (P < 0.01) for steers in the Tasco treatment, suggesting a greater sensitivity to heat stress. This difference existed only for the first portion of HL exposure and was not present during the last 4 d (Figure 3b). An interaction of Tasco × HL × period was observed for Tre (P < 0.05) and minimum Tre (P < 0.05), and was attributed to the effects of HL on Tasco-fed and control steers.
Effect of Heat Stress and Tasco on In Situ NDF Disappearance
No HL effects existed for Fd, Kd, and lag time during any period. In period 2, a trend existed for greater (P < 0.10) Kd (3.58 vs. 2.33) for Tasco-fed vs. control steers (Table 3). In period 3, Kd values for the control steers were 63% greater than those for Tasco-fed steers, but the SEM was large, precluding a difference between treatments. In period 4, Fd appeared to be greater (P < 0.06) for control than Tasco-fed steers.
DISCUSSIONThe DMI for control and Tasco-fed
steers in periods 2 and 3 decreased at approximately 4 d after the initial in-crease in Ta. This decrease in DMI in control and Tasco-fed steers paralleled the lower total BW gain and ADG for these treatments. Others (Hahn and Mader, 1997; Hahn, 1999; Nienaber et al., 2001) have similarly reported that a heat-induced reduction in feed
114 Williams et al.
Table 2. Performance of steers fed Tasco and exposed to elevated ambient temperature1
Item
HL TN
SE
P <
Control Tasco Control Tasco Tasco2 HL
Period 1 Initial BW, kg 306.4 294.8 306.4 294.8 4.75 0.66 0.55 Final BW, kg 329.6 321.4 329.6 321.4 5.56 0.31 0.85 Total BW gain, kg 23.2 26.6 23.2 26.6 1.96 0.24 0.80 ADG, kg/d 1.66 1.9 1.66 1.9 0.14 0.24 0.80Period 2 Initial BW, kg 329.7 313.3 329.5 329.5 7.87 0.85 0.80 Final BW, kg 342.8 321.7 356.4 353.4 8.28 0.16 <0.02 Total BW gain, kg 13.1 8.4 26.8 23.9 3.02 0.22 <0.001 ADG, kg/d 0.86 0.56 1.79 1.59 0.20 0.22 <0.001Period 3 Initial BW, kg 342.8 321.7 356.4 353.4 8.28 0.50 0.42 Final BW, kg 343.4 329.8 367.9 367.0 8.40 0.40 <0.01 Total BW gain, kg 0.61 8.1 11.5 13.6 2.34 0.06 <0.01 ADG, kg/d 0.06 0.81 1.15 1.36 0.23 0.06 <0.01Period 4 Initial BW, kg 343.4 329.8 367.9 367.0 8.40 0.75 0.81 Final BW, kg 378.3 364.5 368.49 365.4 8.15 0.31 0.59 Total BW gain, kg 34.9 34.6 0.6 −1.59 3.57 0.73 <0.001 ADG, kg/d 2.68 2.66 0.05 −0.13 0.27 0.73 <0.0011HL = heat load; TN = thermoneutral; Tasco = Ascophyllum nodosum; Control = no Tasco.2The Tasco × treatment interaction was not significant for any period; for periods 1 and 4, P > 0.70, and for periods 2 and 3, P > 0.20.
intake occurs after 3 to 4 d following the onset of heat exposure. New to the present study is that exposure to a greater HL after 9 d of heat stress did not result in any greater reduction in intake. Also interesting is the fact that DMI exhibited a partial rebound after 7 to 8 d following an increase in HL (periods 2 and 3). Recovery of DMI for control steers after return to thermoneutrality began after several days, with complete return after 10 d. In fact, DMI of the control steers shifted from HL to TN surpassed that of the animals at thermoneutrality in period 3. Likewise, the steers shifted from TN to HL conditions in period 4 exhibited a reduction in DMI similar to that noted for the other group in period 2. These results show that the effect of exposure to HL and TN con-ditions is repeatable and consistent over time.
Dry matter intake appeared to be lower for steers in the Tasco treat-ment under HL conditions for peri-ods 2 and 4. In period 4, the greater reduction in DMI under both HL and TN conditions may be attributed to the length of time steers were fed Tas-co. Steers had been fed Tasco for 36 d
at the beginning of period 4. Spiers et al. (2004) observed that 1.0% Tasco reduced DMI, whereas 0 and 0.5% Tasco had no effect. From previous communications (D. P. Colling), it is thought that the 1% Tasco inclusion rate is the upper limit of the toler-able level. Therefore, there may be a negative effect of long-term treat-ment with Tasco at thermoneutral-ity. Additional studies are needed to verify this. Other reports (Allen et al., 2001; Saker et al., 2001) revealed no effect on BW gain when steers grazed Tasco-treated pastures and were sub-sequently finished in the feedlot.
The increase in RR during heat exposure has been reported to occur at a lower Ta than core body tempera-ture (Hahn et al., 1992, 1997; Hahn, 1999; Brown-Brandl et al., 2005). The critical Ta threshold is 21.3°C (Hahn et al., 1997), compared with 24 to 25°C for Tre. The RR should increase first because it is an effec-tor mechanism designed to increase heat loss, and if successful, should prevent any increase in Tre. As previ-ously observed, RR increased before the increase in Tre in periods 2, 3, and 4. More importantly, RR began
to decrease within 1 d after the peak value was obtained in all HL periods. This suggests RR is not only a rapid indicator of heat strain, but possibly also is a more sensitive marker of adaptation to the stress. Maximum RR did not increase to a higher level from periods 2 to 3, which agrees with the fact that there was no greater reduction in DMI across these peri-ods, even though the HL increased. Return to TN (period 4) from HL (period 3) resulted in a return of RR to near the initial TN level in period 1. Once again, this agreed with the return of DMI for the control group to near the initial TN level in period 1. In contrast, Tre decreased to below the preheat level in period 1. Settivari et al. (2007) noted a similar response in dairy cows (i.e., close return of RR to the TN level and undershooting of Tre) following return to thermoneu-trality after heat exposure. They suggested this response was possibly an indication of adaptation to heat stress in the bovine. This suggests RR responds rapidly to increased HL and appears to parallel the change in DMI. Furthermore, these physiologi-cal responses to HL suggest it might be a useful predictor of a change in production.
The Tre increased from period 1 to 2 when Ta rose above 26°C. The Tre for steers exposed to TN conditions did not change across periods 2 and 3. In contrast, steers under HL condi-tions had increased Tre after 2 to 3 d of exposure and peaked at 4 d. This indication of thermal strain occurred earlier than the 4-d delay in a change in DMI, which supports the fact that Tre is a more sensitive indicator of a heat effect. Once the peak Tre was obtained in periods 2, 3, and 4, there was a delay of several days before a trend downward began. Although minimum Tre increased with HL in periods 2 and 3, it exhibited little change. The increase in nighttime Ta in period 3 increased only the daily minimum Tre, as expected, with no influence on the daily maximum Tre. It should be noted that the decreases in Tre and RR in the HL group on d 21 were artifacts caused by a short
115Tasco and heat stress in beef cattle
Table 3. Effect of Tasco on rate of in situ NDF disappearance in steers exposed to head load treatment (HL)1
Item2 Control Tasco SEM3
P <
Tasco4 HL
Period 24
Fd, % 64.8 53.0 9.5 0.43 0.35 Kd, %/h 2.33 3.58 0.42 0.10 0.36 Lag, h 2.93 4.44 0.96 0.33 0.74Period 3 Fd, % 49.0 52.0 2.50 0.14 0.30 Kd, %/h 6.17 3.20 2.29 0.26 0.25 Lag, h 3.16 2.46 0.88 0.41 0.35Period 4 Fd, % 53.5 48.5 1.16 0.06 0.42 Kd, %/h 4.40 5.00 0.57 0.66 0.19 Lag, h 2.92 3.72 1.85 0.78 0.551Tasco = Ascophyllum nododum; control = No Tasco.2Fd = potential extent of degradation; Kd = rate of disappearance; Lag = discrete lag time before onset of degradation. 3SEM = standard of mean for 2 observations per treatment4The Tasco × HL interaction was not significant (P > 0.2) for any period.
malfunction in the HL chamber, re-sulting in a 2 to 3°C lower Ta at night and a delay in heat exposure on the following day. The result was a cool-ing effect, with a reduction in heat content that was restored during the transition period into period 3.
Spiers et al. (2004) observed that a 1% Tasco inclusion rate in a cot-tonseed hulls- and corn-based diet lowered the Tre and RR of steers exposed to a heat challenge. This suggested that under these condi-tions, Tasco might be used to reduce hyperthermia associated with fescue toxicosis. The cause of the reduction in Tre is unknown at present.
During period 2, neither the Tasco treatment nor HL had an effect on Tre. However, for the first few days, Tasco appeared to lower Tre in steers exposed to HL; after this time, as steers became acclimated to the HL with a decrease in daily maximum Tre, Tasco had no further effect on Tre. The acclimation to the HL carried through period 3, with no apparent effect of Tasco on lower-ing Tre. However, in period 2, the difference between maximum and minimum Tre was 0.24°C lower for the Tasco-fed steers than the control steers exposed to HL. During period 2, the Tasco-fed steers exposed to HL had a greater difference be-tween maximum and minimum RR than the control animals (48.5 vs. 42.0 bpm, respectively). In period 3, the same trend was observed for HL conditions (34.2 vs. 27.9 bpm, respectively). One may infer that Tasco was affecting Tre by increasing the daytime RR under HL condi-tions.
In period 4, the Tasco-fed steers exposed to a HL appeared to have greater maximum and minimum Tre than the control steers during the early phase of this period. The maximum RR was 10 bpm more for the control steers as compared with the Tasco-fed steers. This suggested that animals in the control group were respiring at a greater rate to dissipate heat as compared with those in the Tasco group. In looking at the difference between maximum
and minimum RR, the control group had 9.2 more bpm as compared with the Tasco group. This suggested that the mechanism controlling core body temperature during HL exposure in period 4 was different from that in period 2, where the Tasco-fed animals had a lower Tre and a greater RR response. The lower maximum and minimum Tre for the control group appeared to be attributed to the greater maximum RR for the control vs. Tasco groups. Perhaps the length of time steers were fed Tasco (36 d) may have contributed to the inability of Tasco to lower Tre, as occurred in period 2. A lower dose of Tasco or fewer days of feeding Tasco may alleviate this effect. These findings warrant further research to determine the mechanisms of action.
Heat load had no effect on the rate of in situ NDF disappearance in the rumen of steers. However, there ap-peared to be a greater rate of in situ NDF disappearance for steers fed Tasco in period 2. However, in peri-ods 3 and 4, the rates of in situ NDF disappearance were not influenced by Tasco. The large SEM for the rate of in situ NDF disappearance may have precluded detecting treatment effects in period 3. In addition, one may infer that Tasco-treated steers had a greater rate of in situ NDF disappearance during the initial HL challenge. After adjustment to the el-evated Ta, continued exposure to the HL had no further effect on rate of in situ NDF disappearance or potential in situ NDF disappearance.
Leupp et al. (2005) examined the impact of seaweed extract in com-pressed blocks on in situ DM, NDF, and CP degradability. The in situ disappearance rate of DM, NDF, and ADF was not affected by addition of seaweed extract to compressed blocks. However, the seaweed block increased the slowly degraded CP fraction and extent of CP degrada-tion compared with the positive control. In addition, inclusion of sea-weed meal to the compressed block increased total tract digestion of OM and CP.
IMPLICATIONSFeeding Tasco at 1% of DMI low-
ered the DMI of steers exposed to heat stress. For those steers exposed to a TN environment for 36 d fol-lowed by a heat stress period, feeding Tasco lowered DMI under the TN and heat stress conditions. Tasco ap-peared to have a short-term effect (3 to 4 d) in lowering the Tre of steers during early exposure to heat stress. Tasco had no effect on Tre when steers were exposed to a higher level of heat stress. When comparing the responses across the heat-stressed periods, there was an indication that Tasco altered RR, resulting in a shift in Tre. These findings warrant the need for further research establishing the level and length of time for feeding Tasco.
ACKNOWLEDGMENTSThe authors greatly appreciate the
partial support from Land O’Lakes and Acadian Seaplants for this re-search project.
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