Strategic application of convective cooling to maximize the thermal gradient and reduce heat stress response in dairy cows.

This study determined the effectiveness of convective cooling at different times of day when air temperature (Ta) was cycled from day to night. Mid-lactation Holstein cows (n = 12) were placed in 3 environmental chambers (4 cows per chamber) and acclimated to Ta 19.9°C (thermoneutral; TN) for 7 d followed by an incremental increase over 3 d to a heat stress (HS) condition. Conditions were maintained for 11 d at high and low daily Ta of 33 and 23°C, respectively. To determine adaptive HS response, the HS period was divided into early (E: d 11 to 14) and late (L: d 17 to 20) periods. During HS, cows were exposed to continuous fan (convective) cooling (CC), 8-h day fan cooling (1100 to 1900 h; DC), or 8-h night fan cooling (2300 to 0700 h; NC). Compared with DC, the NC treatment maximized the thermal gradient during the convective cooling. Each animal received all treatments within 3 trials using a repeated 3 × 3 Latin square design. Cows were fed a total mixed ration and milked twice daily. Thermal status was assessed by using thermal conductance and average daily values for mean, minimum, and maximum rectal temperature (Tre), skin temperatures, and respiration rate. Percent reduction in dry matter intake from TN to HS was less for CC than DC and NC, with no change from E to L periods. The DC group exhibited the greatest trend for a percent reduction in total milk yield below CC due to the significantly lower morning milk production. All values for total daily milk production decreased from E to L periods, with E to L reductions in both morning and afternoon milk production. Minimum Tre for CC and NC cows was 0.4°C below DC. In contrast, maximum Tre was similar for NC and DC groups, at 0.5 to 0.6°C above the CC group. Skin temperature for CC cows was always less than DC cows. Skin temperature for NC cows was equal to CC for minimum skin temperature, but exceeded both CC and DC cows for maximum skin temperature. Average skin temperature decreased from E to L, which suggested heat adaptation. The thermal advantage of night (lowest Ta and greatest thermal gradient) versus day cooling (greatest Ta and lowest thermal gradient) was increased heat transfer via thermal conductance with NC. The higher thermal strain of DC cows caused a larger percent decrease in morning milk yield than for NC cows. In contrast, use of convective cooling at night in the absence of elevated humidity could sufficiently reduce heat strain beyond DC to maintain milk production at a level that is closer to that of CC cows.

Relationships between ambient conditions, thermal status, and feed intake of cattle during summer heat stress with access to shade.

Heat stress in feedlot cattle is known to reduce their performance. The challenge comes in determining reliable predictors of current and near-future changes in thermal status and performance. A 42-d study, using crossbred (Bos taurus) steers was conducted during summer months (July through August) to identify best environmental determinants of rumen temperature (Trumen) and feed intake (FI) in feedlot cattle with access to shade. A further goal was to define the relationship between Trumen and FI. Shade coverage was approximately 50%, and all animals were provided standard feedlot diets and water ad libitum. Intraruminal telemetric boluses recorded Trumen several times each hour. Ear tags, telemetrically connected to a feed monitoring system, provided FI data using RFID technology. Data loggers recorded ambient conditions in sun and shade, along with black globe temperature. Regression analyses identified daylight black globe and air temperatures in shade, with one hour delays, as the best predictors of Trumen. Prediction of FI was much less reliable. Unexpectedly, Trumen was not superior to ambient variables in predicting FI. Maximum daily temperature humidity index, calculated using BG in sun with a 5-d lag, was the best significant predictor of FI. These results indicate for feedlot cattle that although air temperature alone in the shade may be the best predictor of Trumen in the heat, black globe temperature in the sun may be a better determinant of feed intake over time. Additional studies are needed to verify the delayed FI response which seems unusually long.

Regional differences in the fescue toxicosis response of Bos taurus cattle.

Cattle of the same breed from different regions of the USA may have altered responses to heat stress and fescue toxicosis. Angus steers from Missouri (MO ANG, n = 10, 513.6 ± 13.6 kg BW) and Oklahoma (OK ANG, n = 10, 552.8 ± 12.0 kg BW) were fed a diet containing either endophyte-infected (E+, 30 µg ergovaline/kg BW/day) or endophyte-uninfected (E-, 0 µg ergovaline/kg BW/day) tall fescue seed for 23 days. Diet treatment began on day 2. Animals were maintained at thermoneutrality (TN, 19-22 °C, days 1-8) and then exposed to heat stress (HS, cycling 26-36 °C, days 9-22). On day 23, ambient temperature was returned to TN and used as a recovery day. Feed intake (FI) was measured daily, with rectal and skin temperatures determined six times daily. Feed intake reduction from pretreatment levels was greater (P < 0.01) for E + (13.9 ± 0.9 versus 11.9 ± 0.3 kg/day) compared to E - (12.6 ± 0.9 versus 12.4 ± 0.3 kg/day) steers over the entire TN period, regardless of Angus group. During HS, E + cattle had reduced FI (P < 0.02; 6.9 ± 0.2 versus 8.4 ± 0.2 kg/day) compared to E - animals, independent of region of origin. A greater decrease in FI (P < 0.01) was observed for OK (12.1 ± 0.3 versus 6.2 ± 0.2 kg/day) compared to MO ANG (12.2 ± 0.3 versus 7.9 ± 0.2 kg/day) when ambient temperature was increased from TN to HS. On day 13 and days 15-22, OK ANG (E+) had reduced FI (P < 0.01, -2.21 kg) compared to OK ANG (E-), while there was no effect on MO ANG. From day 12 to day 22 of HS, daily minimum temperatures for ear, rump, and tail skin were less for E + (P < 0.05) when compared with E-treated steers, signifying peripheral vasoconstriction in E + animals. This was supported by reduced shoulder and lower tail temperatures (P < 0.01) for E + compared to E-treated OK ANG on the recovery day. In summary, regional differences in the response to fescue toxicosis exist, with peripheral vasomotor effects becoming most evident when animals are subjected to rapid changes in their environment.

Heat-tolerant versus heat-sensitive Bos taurus cattle: influence of air temperature and breed on the metabolic response to a provocative immune challenge.

The response of the immune and stress systems have been assessed in response to a lipopolysaccharide (LPS) challenge, yet the role of metabolism in mediating energy requirements during the acute phase response has not been sufficiently studied. This study tested heat-tolerant (Romosinuano [RO]) and heat-sensitive (Angus [ANG]) Bos taurus breeds at different ambient temperatures (Ta) to determine differential metabolic responses to LPS challenge. Twenty-one heifers (ANG: n = 11, 306 ± 26 kg BW; RO: n = 10, 313 ± 32 kg BW) were housed in stanchions in 4 temperature-controlled chambers. Initially, Ta in all 4 chambers was cycling at thermoneutrality (TN; 18.5°C-23.5°C) for a 1-wk adjustment period, followed by an increase in 2 chambers to cycling heat stress (HS; 24°C-38°C) for 2 wk. Five ANG and 5 RO heifers were housed at TN, whereas 6 ANG and 5 RO heifers were housed at HS. On day 19, heifers were fitted with jugular catheters. On day 20, heifers were challenged with LPS (0.5 µg/kg BW; 0 h), and blood samples were collected from -2 to 8 h and at 24 h relative to LPS challenge. Serum was analyzed for glucose, insulin, and NEFA concentrations. In addition, feed intake was measured 3 d before and on the day of the challenge. Feed intake decreased over time (P < 0.001) and was decreased in heifers housed at HS compared with heifers housed at TN (P = 0.013). Glucose concentrations before LPS challenge were greater in RO (P = 0.01) than in ANG heifers and greater in TN-housed heifers (P = 0.02) than in HS heifers. Glucose after LPS challenge initially increased before decreasing below baseline concentrations (P < 0.01) in all heifers. In addition, there was a breed by Ta interaction (P < 0.004), such that HS decreased glucose concentrations in ANG heifers compared with ANG heifers housed at TN (P < 0.001), whereas HS did not affect glucose concentrations after LPS challenge in RO heifers (P = 0.941). Nonesterified fatty acid concentrations before LPS challenge were not affected by breed (P = 0.37) or Ta (P = 0.60). Although NEFA concentration after LPS challenge was unaffected by Ta (P = 0.78), there tended to be a breed by Ta interaction (P = 0.07) such that, when housed at HS, RO heifers had greater serum NEFA concentrations after LPS challenge than ANG heifers (P = 0.009). Insulin concentration before LPS challenge was greater in RO heifers than in ANG heifers (P < 0.01). Insulin after LPS challenge increased (P < 0.01), with RO heifers producing a greater insulin response than ANG heifers (P < 0.01). These data suggest that HS decreases the metabolic response of heat-sensitive ANG heifers in response to LPS challenge, thus providing physiological evidence that may explain differences observed in the acute phase response between heat-sensitive ANG and heat-tolerant RO cattle breeds.

Heat-tolerant versus heat-sensitive Bos taurus cattle: influence of air temperature and breed on the acute phase response to a provocative immune challenge.

The difference in the acute phase response of a heat-tolerant and a heat-sensitive Bos taurus breed to a lipopolysaccharide (LPS) challenge when housed at different air temperatures (Ta) was studied. Angus (ANG; heat-sensitive; n = 11; 306 ± 26 kg BW) and Romosinuano (RO; heat-tolerant; n = 10; 313 ± 32 kg BW) heifers were transported from the USDA Agricultural Research Service SubTropical Agricultural Research Station in Florida to the Brody Environmental Chambers at the University of Missouri, Columbia. Heifers were housed in stanchions in 4 temperature-controlled environmental chambers. Initially, Ta in the 4 chambers was cycling at thermoneutrality (TN; 18.5°C-23.5°C) for a 1-wk adjustment period, followed by an increase in 2 of the 4 chambers to cycling heat stress (HS; 24°C-38°C) for 2 wk. On day 19, heifers were fitted with jugular catheters and rectal temperature (RT) recording devices. On day 20, heifers were challenged with LPS (0.5 µg/kg BW; 0 h), sickness behavior scores (SBSs) were recorded, and blood samples were collected at 0.5-h intervals from -2 to 8 h and again at 24 h relative to LPS challenge at 0 h. Serum was isolated and stored at -80°C until analyzed for cortisol and cytokine concentrations. A breed by Ta interaction (P < 0.001) was observed for RT such that the post-LPS average RT in RO heifers housed at TN was lower than the RT of all other treatment groups (P < 0.001), whereas ANG heifers housed at HS had greater post-LPS average RT than all other treatment groups (P < 0.001). In response to LPS, HS increased SBS after LPS in RO heifers compared to RO heifers housed at TN (P < 0.001), whereas HS decreased SBS after LPS in ANG heifers compared to ANG heifers housed at TN (P = 0.014). The cortisol response to LPS was greater in TN than in HS heifers (P < 0.01) and was also greater in RO than in ANG heifers (P = 0.03). A breed by Ta interaction (P < 0.01) was observed for tumor necrosis factor-α (TNF-α) concentration such that HS increased post-LPS serum concentrations of TNF-α in ANG heifers compared to ANG heifers housed at TN (P = 0.041), whereas HS decreased post-LPS concentrations of TNF-α in RO heifers compared to RO heifers housed at TN (P = 0.008). A tendency (P < 0.06) was observed for a breed by Ta interaction for IL-6 concentrations such that RO heifers had greater post-LPS concentrations of IL-6 than ANG heifers when housed at HS (P = 0.020). A breed by Ta interaction was observed for interferon-γ (IFN-γ; P < 0.01) concentrations such that HS decreased post-LPS concentrations of IFN-γ in ANG heifers compared to ANG heifers housed at TN (P < 0.001), and HS increased post-LPS concentrations of IFN-γ in RO heifers compared to RO heifers housed at TN (P = 0.017). These data indicate differences in the acute phase response between the heat-tolerant RO and heat-sensitive ANG heifers under different Ta which may aid in elucidating differences in productivity, disease resistance, and longevity among cattle breeds.

Effects of a controlled heat stress during late gestation, lactation, and after weaning on thermoregulation, metabolism, and reproduction of primiparous sows.

Heat stress (HS) causes seasonal infertility in sows and decreases reproductive efficiency. The objective was to examine thermoregulation, metabolic responses, and reproduction in sows exposed to HS or thermoneutral (TN) conditions during different phases of a production cycle (gestation, lactation, and breeding). Fifty-eight first-parity Landrace (n = 26) or Landrace × Large White F1 (n = 32) sows were rotated through environmental chambers for 57 d beginning in late gestation. The ambient temperature sequences included either TN (18°C to 20°C) or HS (24°C to 30°C) for each production phase with the following treatment groups: TN-TN-TN (n = 15), TN-HS-TN (n = 14), HS-TN-HS (n = 14), and HS-HS-HS (n = 15) for gestation-farrowing-breeding (20, 24, and 13 d, respectively). Regardless of the temperature treatment, rectal temperatures were greater (P < 0.001) during lactation (39.36°C ± 0.01°C) than during the gestation (38.27°C ± 0.01°C) or the breeding period (38.77°C ± 0.01°C). The increase in rectal temperature (P < 0.001) and respiration rate (P < 0.001) in response to the HS was greatest during lactation. There was an effect of day (P < 0.001) on serum IGF-1 and insulin concentrations because both insulin and IGF-1 increased after farrowing. Compared with HS sows, the TN sows had greater feed intake (P < 0.001) and greater serum concentrations of insulin (early lactation; P < 0.05) and IGF-1 (late lactation; P < 0.05) when they were lactating. The effects of HS on sow BW, back fat, and loin eye area were generally not significant. Average BW of individual piglets at weaning was approximately 0.5 kg lighter for the sows in the HS farrowing room (P < 0.05). Weaning-to-estrus interval, percentage sows inseminated after weaning, subsequent farrowing rate, and subsequent total born were not affected by treatment. In summary, regardless of ambient temperature, sows undergo pronounced and sustained changes in rectal temperature when they transition through gestation, lactation, weaning, and rebreeding. The effects of HS on rectal temperature, respiration rate, feed intake, and metabolic hormones were greatest during lactation. The controlled HS that we imposed affected piglet weaning weight, but rebreeding and subsequent farrowing performance were not affected.

Use of different levels of ground endophyte-infected tall fescue seed during heat stress to separate characteristics of fescue toxicosis.

Studies of fescue toxicosis using whole seed diets show reduced feed intake and thermoregulatory ability, but much of the seed passes undigested through the animal. Cattle were fed ground tall fescue seed at different levels to potentially facilitate digestion and absorption of toxins and identify toxin sensitivity for major characteristics of the condition [i.e., hyperthermia, reduced feed intake (FI), reduced blood prolactin]. Steers (n = 18; 350 kg BW) were housed in the Brody Climatology Laboratory at thermoneutrality (TN; 19°C) and randomly assigned to daily diet treatments with either ground endophyte-infected [E+; low and high doses at 20 and 40 µg ergovaline/(kg BW/d), respectively] or endophyte-free [E-; control at 0 µg ergovaline/(kg BW/d)] tall fescue seed. After 12 d at TN, animals received 2 d of transition to heat stress (HS; 36°C daytime, 25°C nighttime) and maintained for 14 more days. Cattle were fed twice daily at 0800 and 1600 h, with water ad libitum. Feed intake was measured at 0700 h, with skin and rectal temperatures, and respiration rate at 0600, 1100, 1600, and 2100 h. Blood was sampled on selected days for prolactin and leptin determinations. Steers fed ground E+ diet decreased (P ≤ 0.0001) FI below controls at TN, with no dose effect. Maximum FI reduction with E+ treatment was 25% at TN, with an additional 46% decrease during HS (P ≤ 0.05). By the end of HS, E+ FI increased (P > 0.05) to that of E-, suggesting recovery. Prolactin was reduced (P ≤ 0.05) in high E+ cattle below controls at study end. Leptin blood concentrations were unaffected by E+ treatment (P > 0.05) but was reduced (P ≤ 0.05) by the end of HS. Pattern of rectal temperature response to HS showed a more rapid initial increase and decline for both E+ groups compared with controls (P ≤ 0.05). Skin temperature was the only variable that identified E+ dose differences. Although there were no treatment differences at TN, skin temperature was lower (P ≤ 0.05) for high E+ steers compared with controls during HS when air temperature was reduced each day. In general, FI was more responsive to E+ toxins than body temperature or blood prolactin, declining even at TN and exhibiting dynamic activity during HS. Although body temperature response to E+ toxins appears to stabilize during HS, this is misleading as rapid change in air temperature exposes effects on skin temperature.

Influence of environmental temperature on the physiological, endocrine, and immune responses in livestock exposed to a provocative immune challenge.

Although livestock experience many stressors throughout their life, one of the most commonly experienced, and most difficult to control, is stress caused by fluctuations in environmental temperatures that extend beyond the thermoneutral (TN) zone for an animal. In swine, cold stress has long been recognized as a main cause of neonatal morbidity and mortality. A possible explanation for this increased morbidity and mortality may be related to their inability to generate a febrile response. Previously, we reported that the acute phase immune response, including the generation of fever, after exposure to lipopolysaccharide (LPS; Escherichia coli O111: B4; Sigma-Aldrich, St Louis, MO, USA) is substantially altered in neonatal pigs maintained in a cold environment (ie, 18°C). Neonatal pigs that were maintained in a cold environment and administered LPS experienced a period of hypothermia coupled with altered endocrine and proinflammatory cytokine responses that could prove detrimental. In cattle, we previously reported differences in the acute phase immune response of two diverse breeds of Bos taurus cattle (Angus and Romosinuano) when maintained under TN conditions and exposed to LPS. More recently we have reported that differences in the stress and immune responses of Angus and Romosinuano heifers varies, depending on whether the cattle were housed at either TN or heat stress air temperatures. Our data clearly show that even intermittent periods of heat stress similar to that experienced in production environments can have significant effects on the stress and innate immune responses of cattle. Understanding the effect of thermal stress on livestock is critical to developing and implementing alternative management practices to improve their overall health and well-being.

Differential acute phase immune responses by Angus and Romosinuano steers following an endotoxin challenge.

Our primary objective of this experiment was to evaluate potential genetic differences between two diverse Bos taurus breeds [Angus (AG) and Romosinuano (RO)] in response to an endotoxin challenge. Eighteen steers (n = 9 steers/breed; 299.4 ± 5.2 kg BW) were acclimated to environmentally controlled chambers maintained at thermoneutrality (19.7 °C) and then fitted with indwelling jugular catheters and rectal temperature (RT) recording devices 1 d before the endotoxin challenge. The next day, blood samples were collected at 30-min intervals from -2 to 8 h, and RT was measured continuously at 1-min intervals throughout the study. At time 0, all steers received an intravenous bolus injection of lipopolysaccharide (LPS; 2.5 µg/kg BW). Serum samples were stored at -80 °C until analyzed for cortisol, proinflammatory cytokines [tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1ß), IL-6, and interferon gamma (IFN-γ)], and acute phase proteins (serum amyloid A, acid soluble protein, ceruloplasmin, and α-acid glycoprotein). Rectal temperatures increased in both breeds within 1 h after LPS, with RO producing a greater increase in RT than AG steers (P < 0.001). Serum cortisol and TNF-α increased (P < 0.01) in both breeds within 1 h after the LPS challenge. For cortisol, an overall breed effect (P < 0.02) was detected, such that AG steers had a higher cortisol response than RO steers. A breed × time interaction (P < 0.01) was observed for TNF-α, such that the response was delayed and extended in the RO steers compared with the AG steers. At 2 and 2.5 h after LPS, TNF-α concentrations were greater (P < 0.03) in RO steers than in AG steers. For IL-1ß, a breed × time interaction (P < 0.04) was also observed. At 3 h after LPS, IL-1ß concentrations were greater (P < 0.01) in RO steers than in AG steers. Serum IL-6 and IFN-γ increased (P < 0.01) in a similar manner in both groups after the LPS challenge. These data show differences in the innate immune response between two diverse Bos taurus breeds which may provide insight about differences observed in productivity, heat tolerance, disease resistance, and longevity among cattle breeds.

Evaluation of physiological and blood serum differences in heat-tolerant (Romosinuano) and heat-susceptible (Angus) Bos taurus cattle during controlled heat challenge.

A study was performed to evaluate differences in thermoregulatory ability of 2 Bos taurus breeds with known differences in heat tolerance. Nine Angus (AG; 304 +/- 7 kg of BW) and 9 Romosinuano (RO; 285 +/- 7.5 kg of BW) steers were transported to the Brody Environmental Center at the University of Missouri. Steers were housed for 18 d at thermoneutrality (TN; 21 degrees C) before initiation of heat stress (HS), which consisted of daily cyclic air temperature (26 degrees C, night; 36 degrees C, day) for 14 d. Rectal temperature and respiration rate were measured 6 times daily throughout the study. Sweat rates at shaved skin sites were recorded on specific days. Blood samples were taken once per week. Angus steers maintained rectal temperature 0.5 degrees C greater than RO at TN (P < 0.001). Likewise, respiration and sweat rates were greater (P < 0.001) in AG than RO at TN (P < 0.05). Rectal temperature increased during HS for both breeds with AG maintaining greater temperatures (P < 0.001). Both breeds increased respiration rate during HS, with AG steers exhibiting the greater rate (P < 0.001). Sweat rate increased more than 4-fold during HS (P < 0.001), followed by reduction after 7 d. Even after HS acclimation, AG exhibited the greater sweat rate (P < 0.001). Breed differences for serum leptin, creatinine, and cholesterol were found throughout the study with AG being greater than RO. Although there were no breed differences (P = 0.21) at TN, only AG steers exhibited a HS-induced increase (P < 0.05) in prolactin, creatinine, and cholesterol concentrations to suggest that an increase in rectal temperature is required for this effect. Use of rectal temperature along with endocrine markers, such as prolactin, may aid in the identification of B. taurus sensitivity to heat.

Effects of short-term heat stress on endophytic ergot alkaloid-induced alterations in rat hepatic gene expression.

Exposure to ergot alkaloids in endophyte-infected fescue (E+) is associated with impaired animal productivity, especially during heat stress, which is commonly referred to as fescue toxicosis. To elucidate the pathogenesis of this condition, the effects of short-term heat stress (HS) on hepatic gene expression in rats exposed to endophytic ergot alkaloids were evaluated. Rats implanted with telemetric transmitters to continuously measure core temperature were fed an E+ diet and maintained under thermoneutral (TN) conditions (21 degrees C) for 5 d, followed by TN or 31 degrees C (HS) conditions for 3 d. Feed intake (FI) and BW were monitored daily. The E+ and HS-induced alterations in hepatic genes were evaluated using DNA microarrays and PCR analyses. Hepatic antioxidant enzyme activities, as well as the incidence of apoptosis, were determined. As expected, intake of E+ reduced FI and BW from pretreatment levels under TN conditions, with greater reductions during short-term HS. Genes involved in gluconeogenesis and apoptosis were upregulated, whereas genes associated with oxidative phosphorylation, xenobiotic metabolism, antioxidative mechanisms, immune function, cellular proliferation, and chaperone activity were all downregulated with short-term HS. Hepatocytic apoptosis was increased and antioxidant enzyme activity decreased in the livers of rats exposed to HS. The hypothesized, exacerbating effects of HS on the direct, endophytic toxin-related and indirect, reduced caloric intake-associated alterations in hepatic gene expression were clearly demonstrated in rats and may help to elucidate the pathogenesis of fescue toxicosis in various animal species.

Regional differences in sweat rate response of steers to short-term heat stress.

Six Angus steers (319 +/- 8.5 kg) were assigned to one of two groups (hot or cold exposure) of three steers each, and placed into two environmental chambers initially maintained at 16.5-18.8 degrees C air temperature (Ta). Cold chamber Ta was lowered to 8.4 degrees C, while Ta within the hot chamber was increased to 32.7 degrees C over a 24-h time period. Measurements included respiration rate, and air and body (rectal and skin) temperatures. Skin temperature was measured at shoulder and rump locations, with determination of sweat rate using a calibrated moisture sensor. Rectal temperature did not change in cold or hot chambers. However, respiration rate nearly doubled in the heat (P < 0.05), increasing when Ta was above 24 degrees C. Skin temperatures at the two locations were highly correlated (P < 0.05) with each other and with Ta. In contrast, sweat rate showed differences at rump and shoulder sites. Sweat rate of the rump exhibited only a small increase with Ta. However, sweat rate at the shoulder increased more than four-fold with increasing Ta. Increased sweat rate in this region is supported by an earlier report of a higher density of sweat glands in the shoulder compared to rump regions. Sweat rate was correlated with several thermal measurements to determine the best predictor. Fourth-order polynomial expressions of short-term rectal and skin temperature responses to hot and cold exposures produced r values of 0.60, 0.84, and 0.98, respectively. These results suggest that thermal inputs other than just rectal or skin temperature drive the sweat response in cattle.

Relationship of thermal status to productivity in heat-stressed dairy cows given recombinant bovine somatotropin.

The responses of lactating Holstein cows to daily administration of bovine somatotropin (bST) were measured at thermoneutrality (Tn) and under both constant and cycled heat-stress conditions to determine the relationship between thermal status and bST-induced shifts in milk production. All tests included a 5-d acclimation period at Tn (18 degrees C), followed by a 2-d increase in ambient temperature to 28.5 degrees C. After d 3, ambient temperature was cycled between 28.5 (day) and 25.5 degrees C (night) for 4 d. Daily injections with either 31 mg of bST or saline began on d 1 of the experiment. Milk production, feed intake, and respiratory rate (RR) were measured daily. Intraperitoneal, telemetric temperature transmitters were used for a continuous measure of core body temperature (T(core)). Blood samples were collected during each phase to evaluate the changes in serum chemistry in response to bST and heat stress. Following a 15-d recovery, cows were switched across injection treatments and the study was repeated. Milk production decreased by approximately 18.4% below the initial yield at Tn by the end of 7 d of heat challenge. Although a reduction in milk production occurred during heat stress in both groups, milk production was higher in bST-treated cows compared with control cows during periods of constant and cyclic heat. Likewise, bST treatment during the entire period increased the milk-to-feed ratio over the control level by approximately 11.3%. Plasma insulin-like growth factor 1 and serum nonesterified fatty acids accompanied the increased growth hormone level with bST treatment (approximately 122.0 and 88.8%, respectively), whereas plasma urea nitrogen was reduced by approximately 13.3% to reflect the shift to lipid metabolism. There was no difference in T(core) of the treatment and control groups at Tn. Both bST and control cows increased RR and T(core) above the Tn level by approximately 94.8 and 2.9%, respectively, during constant heat, with a greater increase in T(core) of bST-treated compared with control cows (approximately 0.6%). The increase in RR during heat stress preceded T(core) by 1 d for both groups. During cyclic heat, T(core) decreased by approximately 0.4% compared with constant heat in both the control and bST-treated groups. Bovine somatotropin treatment increased milk production similarly during the Tn and heat-stress periods, approximately 8.3% over the control; however, the bST-induced increase in milk-to-feed ratio was greatest during the continuous and cyclic heat-stress phases, approximately 16.2%. This increase occurred together with the elevation in T(core).

Effect of fescue toxicosis on hepatic gene expression in mice.

Fescue toxicosis affects wild and domestic animals grazing fescue pasture infected with the endophytic fungus Neotyphodium coenophialum. Signs of fescue toxicosis include increased core body temperature and respiration rate and decreased milk yield and reproductive performance. Laboratory mice also exhibit symptoms of fescue toxicosis, as indicated by reduced growth rate and reproductive performance. Mice were used to study the effects of fescue toxicosis on hepatic gene expression. Twenty-seven mice were randomly allocated to a diet containing either 50% endophyte-infected (E+; 6 ppm ergovaline) or endophyte-free (E-) fescue seed for 2 wk under thermoneutral conditions. Liver genes differentially expressed due to fescue toxicosis were identified using DNA microarray. A 2-stage ANOVA of microarray data identified 36 differentially expressed genes between mice fed E+ and E- diets. Another analysis method, significance analysis of microarray, identified 9 genes as differentially expressed between treatment groups, and some genes overlapped with genes identified by ANOVA. Hierarchical clustering of 36 genes identified by ANOVA clearly separated the mice by diet, with 100% confidence as computed by bootstrap analysis. Expression of 11 genes was verified using quantitative real-time PCR. The E+ diet resulted in downregulation of genes involved in the sex-steroid metabolism pathway and genes involved in cholesterol and lipid metabolism. Genes coding for ribosomes and protein synthesis were upregulated by the E+ diet. Genes identified in the present analysis indicate some of the mechanisms by which fescue toxicosis occurs in animals.

Genomic analysis of the impact of fescue toxicosis on hepatic function.

Fescue toxicosis is caused by consumption of toxins produced by an endophytic fungus, Neotyphodium coenophialum, in tall fescue [Lolium arundinaceum (Schreb.) Darbysh]. Microarray analysis was used to identify shifts in genetic expression associated with the affected physiological processes to identify potential targets for future pharmacological/toxicological intervention. Male rats (n = 24) were implanted with temperature transmitters, which measure core temperature every 5 min. After an 8-d recovery, the rats were fed an endophyte-free diet for 5 d. During the following 5-d treatment period, rats were fed either an endophyte-free or an endophyte-infected (91.5 microg of ergovaline.kg of BW(-1).d(-1)) diet. At the end of treatment, rats were euthanized and a sample of the liver was obtained. Feed conversion efficiency was calculated for both treatment groups. Serum prolactin concentrations were measured using ELISA. Liver tissue RNA was reverse transcribed and hybridized to an oligonucleotide microarray chip. Microarray data were analyzed using a 2-step ANOVA model and validated by quantitative real-time PCR. Significant reductions in mean core temperature, feed intake, feed conversion efficiency, BW, liver weight per unit of BW, and serum prolactin concentrations were observed in endophyte-infected rats. There was downregulation (P < 0.05) of various genes associated with energy metabolism, growth and development, and antioxidant protection, as well as an upregulation of genes associated with gluconeogenesis, detoxification, and biotransformation. This study demonstrated that even short-term exposure of rats to tall fescue endophytic toxins under thermoneutral conditions can result in physiological responses associated with altered gene expression within the liver.

A model of fescue toxicosis: responses of rats to intake of endophyte-infected tall fescue.

A study was conducted to develop a model for fescue toxicosis using rats fed a diet containing endophyte-infected tall fescue seed (E+). Rats implanted with telemetric transmitters to continuously monitor core body temperature (Tc) and activity were housed at thermoneutrality (21 degrees C) and were fed a diet containing endophyte-free fescue seed (E-). After 2 wk, they were assigned to either E+ or E- diets and initially maintained at thermoneutrality (preheat) for 8 d. They were then exposed to heat stress (31 degrees C) for 22 d, followed by 1 wk of recovery at thermoneutrality (post-heat). Body weight and feed intake were measured daily. Rats receiving the E+ diet showed decreased feed intake (P = 0.001) and weight gains (P = 0.003) during the preheat period. The decrease in Tc from the pre-treatment level was greater in E+ than in E- rats during the preheat (P = 0.001) and postheat (P = 0.001) periods. With heat stress, both groups showed parallel decreases in feed intake. The increase in Tc from pre-heat to heat conditions was greater in E+ vs. E- rats (P = 0.001). Activity level was lower in E+ than in E-rats during heat stress (P = 0.009) and postheat (P = 0.037) periods. These results show that the rat model for fescue toxicosis is extremely useful because many of the observed responses to E+ diet are similar to those noted for cattle, and additional variables that are difficult to measure in cattle, such as activity, can be easily evaluated.

Analyses of thermoregulatory responses of feeder cattle exposed to simulated heat waves.

Heat stress in feedlot cattle causes reduced performance, and in the most severe cases, death of the animals, thus causing the loss of millions of dollars in revenue to the cattle industry. A study was designed to evaluate dynamics of thermoregulation and feeding activities when feeder cattle were exposed to simulated heat waves, in comparison with repeated sinusoidal hot and thermoneutral environments. Nine beef steers were randomly assigned to an individual pen in one of three environmental chambers. Each chamber was subjected to each of three temperature regimes (heatwave simulation from Rockport, Mo., 1995, heatwave simulation from Columbia, Mo., 1999, and Controlled heat stress treatment of 32+/-7 degrees C) for a period of 18 days, according to a Latin square treatment design, with a 10-day thermoneutral period (18+/-7 degrees C) separating treatment periods. Respiration rate, core body temperature, heat production, feed intake, and feeding behavior were measured on each animal for the duration of the experiment. Differences were found in all treatments for all parameters except feeding behavior. It was shown that the two simulated heat waves elicited very different thermoregulatory responses. Based on these results the heat wave centered at Rockport, Mo. in 1995 was devastating because the animals were not acclimated to hot conditions, thus causing an acute response to heat stress. The responses of cattle to conditions at Columbia, Mo. showed some acclimation to heat prior to the peak stress days, and therefore a dampened response was seen. It appears the extreme conditions at Columbia, Mo., 1999 were made severe by environmental conditions not simulated during this study (low wind speed and intensive solar radiation). Overall, it was determined while a cyclic heat stress treatment is a representative model to test heat stress in cattle, further heat stress experiments should be conducted in an actual feedlot.

Effects of feeding ergovaline on lamb performance in a heat stress environment.

Lambs exposed to a heat-stressed environment (33 degrees C, 50% relative humidity) were used in three experiments to determine whether ergovaline (EV) is the primary toxin involved in fescue toxicosis. The first study evaluated the effects of feeding diets containing increasing levels of endophyte-infected tall fescue seed (E+) and decreasing levels of endophyte-free tall fescue seed (E-). The second and third study evaluated the response to a diet that contained synthetic EV added to an E- diet and the response to a diet containing endophyte-infected ryegrass seed (R+) with an elevated concentration of EV. In Exp. 1, lambs were fed diets of: 1) 10% E- and 0% E+, 2) 5% E- and 5% E+, or 3) 0% E- and 10% E+. Increasing the percentage of E+ in the diet resulted in a linear decrease (P < 0.01) in feed intake (as-fed basis), skin temperature, thermocirculation index (TCI), and serum prolactin. Body weight gain also decreased (P < 0.06). Respiratory rate and core body temperature were not affected by the 5 or 10% E+ diets. In Exp. 2, lambs were fed diets that contained: 1) 10% E-, 2) 10% E- with synthetic EV added at a level equivalent to the 10% E+ diet, or 3) 10% E+. Feed intake (as-fed basis), body weight gain, and skin temperature did not differ for lambs fed the E- and EV diets. The EV diet elicited a decrease (P < 0.05) in TCI and prolactin compared with the E- diet. The TCI for lambs fed EV did not differ (P > 0.10) from the E+ lambs; however, serum prolactin was lower (P < 0.05) for lambs on the E+ diet than for those fed EV. Core body temperature was not affected (P > 0.10) by feeding EV or E+ fescue seed in Exp. 2. In Exp. 3, lambs were fed diets that contained: 1) 10% E-, 2) 3.24% R+ and 6.76% E-, which added an equivalent amount of EV to E+ diets but reduced concentrations of other ergot alkaloids, or 3) 10% E+. Lambs fed the E+ diet and maintained at 33 degrees C had lowered feed intake (as-fed basis), skin temperature, and TCI compared with lambs fed the E- or R+ diets (P < 0.05). Lambs fed the E+ diet had increased rectal temperatures and lowered serum prolactin compared with lambs on the R+ diet (P < 0.05). Lambs on the R+ diet had a greater rectal temperature and lower serum prolactin than lambs on the E- diet (P < 0.05). These results suggest that EV is a fescue toxin; however, other alkaloids might work synergistically with EV, causing the full expression of fescue toxicosis.

Feeding strategies for managing heat load in feedlot cattle.

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 degrees 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 degrees 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.

Use of a rat model to evaluate tall fescue seed infected with introduced strains of Neotyphodium coenophialum.

Experimental cultivars of the pasture grass tall fescue are infected with unique strains of the fungal endophyte Neotyphodium coenophialum, which produce low concentrations of ergot alkaloids. A rat model was evaluated as a tool for rapid, initial screening of experimental cultivars considered to be nontoxic. Rats were fed diets that included seed from experimental cultivars of tall fescue with introduced strains of N. coenophialum and a toxic control diet containing seed of the cultivar Kentucky 31 (KY31), with its endemic strain of N. coenophialum. Rats were preconditioned to a nontoxic diet and then fed treatment diets for 13 days with 5 days at thermoneutrality (21 degrees C) followed by 8 days under heat stress (31 degrees C). For most of the 13-day treatment period, rats fed KY31 exhibited depressed daily intake compared to those fed diets of cultivars with introduced endophytes (P < 0.05). In addition, rats fed KY31 exhibited significantly less weight than rats on other diets after heat treatment was imposed. For all initial trials and repeated trials, total intake and total gain calculated at the end of each trial were the most consistent indicators of toxicity.