During stress, the hypothalamic-pituitary-adrenal (HPA) axis is activated. Hypothalamic neurons within the HPA axis secrete corticotropin-releasing hormone that causes the release of adrenocorticotrophic hormone (ACTH) from the pituitary. The ACTH causes the adrenal gland to secrete cortisol (a stress hormone). Cortisol will initially improve an animal's ability to cope with stress. Long-term increases in cortisol, however, are unhealthy. The hormone responsible for reproduction is gonadotropin-releasing hormone (GnRH). GnRH is released from the hypothalamus and causes the release of luteinizing hormone (LH) from the pituitary. The release of LH stimulates ovarian follicular growth and ovulation (essential processes for reproduction). Physiological and psychological stressors will typically inhibit GnRH and LH release. Some stressors activate the HPA axis (i.e., increase cortisol) and inhibit GnRH and LH through an HPA-dependent mechanism. Other stressors including postpartum negative energy balance have inhibitory effects on GnRH and LH but act through pathways that do not involve the HPA axis. A certain amount of management-induced stress is unavoidable. The best reproductive performance will be achieved when stress is minimized so that the endocrine components of the reproductive axis can function at their maximal level.

This chapter contains sections titled: Introduction The Definitions of Heat and Temperature Stress of the Environment versus Strain on the System Homeostasis and Maintenance of a Constant State Control of the Thermoregulatory System Modes of Heat Exchange and the Thermoregulatory Profile References

Heat stress studies are often conducted using controlled laboratory exposures or field exposures. Each approach has limitations and provides a partial understanding of complex interactions between simultaneous environmental stressors. The question is how similar the responses are in each situation. Several physiological measures of thermal status were used to compare heat stress responses of cattle in controlled chamber stress tests and fluctuating field conditions. Angus steers (N=23; 318±8kgBW) were first placed on either endophyte-infected or -uninfected tall fescue pastures for the field exposure, followed by a controlled heat challenge, which exacerbates the condition known as fescue toxicosis. During the controlled heat challenge, steers were assigned to diets of either 0 or 40μg ergovaline/kg/d to maintain the treatment states. Respiration rate (RR) was measured via flank counting and telemetric temperature transmitters in the rumen of each animal monitored core temperature (Trum). Linear regression fit models for RR, Trum, and air temperature (Ta) were utilized to compare relationships between field and chamber exposure. Correlation coefficients for RR were similar during both chamber (R=0.69) and field exposures (R=0.72). Respiration rate showed greater responsiveness to change in Ta under field conditions having twice the slope (4.40 versus 1.75bpm/°C) and a lower Y-intercept (−42.14 versus +30.97bpm) compared to the chamber run. Ruminal temperature was consistent between exposures showing a similar slope (0.04 versus 0.03°C Trum/°C Ta) and Y-intercept (38.40 versus 39.30°C) for its relationship with Ta. Despite respiration rate being the more sensitive indicator of heat stress, ruminal temperature proved to be the most consistent between environments.

Little is known about the heat adaptive ability of bovine raised in different thermal environments. Heat-sensitive Angus steers (ANG) from Missouri (MO;n=5) and Oklahoma (OK;n=5) were tested with the heat-tolerant Romosinuano breed (RO;n=5) from Florida in environmental chambers at the University of Missouri. Steers were maintained at thermoneutrality (TN; 19–22°C) through Day 8, followed by 2 weeks of cycling heat stress (HS; 26–36 and 30–40 °C for ANG and RO, respectively). RO were kept at a higher ambient temperature (Ta) to incur a similar level of HS as ANG. Respiration rate (RR), and both rectal (Tre) and skin temperatures, were measured every hour for 24 hours at TN, Early Heat (EH), and Late Heat (LH). All animals increased (p<0.05) RR and Tre from TN to HS. MO decreased (p<0.05) Tre and RR from EH to LH by 0.18°C and 9.5 bpm, while OK exhibited no differences (p>0.05). RO had similarities with OK by showing no difference in Tre (p>0.05) from EH to LH. However, RO decreased RR (p<0.05) from EH (102.2 bpm) to LH (84.3 bpm) similar to MO. Likewise, there were temporal skin temperature differences during HS for ANG but not for RO. Results suggest that there are regional differences in bovine response to heat stress that are related to their climatically diverse environments. It is possible to identify phenotypic markers that characterize adaptation levels to heat stress across breeds. USDA Agreement No. 58-6227-3-016

Continuous exposure of cattle to summer heat in the absence of shade results in significant hyperthermia and impairs growth and general health. Reliable predictors of heat strain are needed to identify this condition. A 12-day study was conducted during a moderate summer heat period using 12 Angus x Simmental (Bos taurus) steers (533 ± 12 kg average body weight) to identify animal and ambient determinations of core body temperature (T core) and respiration rate (RR) responses to heat stress. Steers were provided standard diet and water ad libitum, and implanted intraperitoneally with telemetric transmitters to monitor T core hourly. Visual count of flank movement at 0800 and 1500 hours was used for RR. Dataloggers recorded air temperature (T a), and black globe temperatures (T bg) hourly to assess radiant heat load. Analysis was across four periods and 2 consecutive days averaged within each period. Average T a and T bg increased progressively from 21.7 to 30.3°C and 25.3 to 34.0°C, respectively, from the first to fourth periods. A model utilizing a quadratic function of T a explained the most variation in T core (R 2 = 0.56). A delay in response from 1 to 3 h did not significantly improve R 2 for this relationship. Measurements at 0800 and 1500 hours alone are sufficient to predict heat strain. Daily minimum core body temperature and initial 2-h rise in T a were predictors of maximum core temperature and RR. Further studies using continuous monitoring are needed to expand prediction of heat stress impact under different conditions.

Rats were fed diets containing alkaloids (E+), minus alkaloids (E−), or pair‐fed (PF to E+). Groups were further divided into temperature treatments of thermoneutrality (TN; 21°C) or heat stress (HS; 33°C). Rats received diet treatments at TN for 1 week followed by 3 days HS or TN treatment. Feed intake and body weight were reduced by alkaloids, heat stress, and combination of both stressors. Feed intake was reduced 82% from preheat level in E+HS rats compared to 75% in HS rats. Blood samples were collected at trial end and analyzed by flow cytometry. Natural killer (NK) cells increased with all stressors, such that E+HS level was twice that of E‐TN rats (4.2% vs. 2.1%; α=0.05). Caloric restriction (HS and TN) modestly increased NK cells. The combination of stressors dimished B cells by one‐half (α=0.05) compared to E‐TN. Treatment differences in T cell percentage, however, were not statistically significant. In summary, combination of alkaloid intake and heat stress significantly affected the proportion of circulating NK and B cells, with more modest changes in T cells. Caloric restriction (PF) increased NK cells, regardless of environmental conditions. (USDA Agreement No. 58‐6227‐3‐016)

Few studies have quantified the impact of dehydration on bovine thermal status during heat stress. Angus steers (n=8) in trial 1 were dehydrated and rehydrated over 6 days at thermoneutrality (TN; 19–21°C), with 4 month recovery. Trial 2 consisted of 5 days of cyclic heat stress (HS; 26–36°C) followed by water restriction and rehydration as in trial 1. Measurements included respiration rate (RR) and rectal temperature (Tre) taken 6 times daily, with body weight (BW), feed intake (FI), and sweat rate (SR) determined at selected times. Dehydration‐induced reductions in FI (70–75%), BW (4–9%), and RR (~15bpm) were similar in magnitude regardless of ambient temperature. FI and BW quickly returned to baselines after 1 day rehydration, while RR took 4 days. SR approached 100 g/m2h during HS, declined to 30 g/m2h with dehydration, and did not recover until after 3 days rehydration. Tre exhibited no significant change with dehydration, due to reduction in FI that counterbalanced decreases in RR and SR. Rehydration decreased Tre (~0.5°C), with slow recovery over 4 days. Surprisingly, dehydration of cattle did not result in a change in thermal status during heat stress. USDA Agreement No. 58‐6227‐3‐016

Our objective was to characterize further the acute-phase response following endotoxin (i.e. lipopolysaccharide; LPS) exposure in the bovine. Nine pure-bred Angus castrated males (i.e. steers; average body weight=299+/-5 kg) were used in a randomized complete block design in environmentally controlled chambers, set at thermoneutral level, to characterize the acute physiological, endocrine, immune, and acute-phase protein responses following an i.v. bolus administration of 2.5 microg of LPS/kg body weight. One day before administration of LPS, all steers were fitted with an indwelling jugular vein catheter for serial blood collection. Blood samples were collected at 30-min intervals from -2 h to 8 h relative to the LPS challenge (time 0), and serum was harvested and stored at -80 degrees C until analyzed for concentrations of cortisol, pro-inflammatory cytokines, and acute-phase proteins. Indicators of thermal status (i.e. rectal temperature, ruminal temperature, respiration rate, sweat rate, and skin temperatures) were measured at 30-min intervals from -1 h to 6 h relative to the challenge. Endotoxin exposure increased (P<0.05) serum concentrations of cortisol, tumor necrosis factor-alpha (TNF-alpha), interleukin 1-beta (IL-1beta), IL-6, interferon-gamma (IFN-gamma), and serum amyloid A. Respiration rate, rectal temperature, and rump skin temperature also were increased (P<0.05) following LPS administration. Endotoxin exposure dramatically decreased ear skin temperature (P=0.002), but tended to increase (P<0.10) ruminal temperature, shoulder skin temperature, and shoulder sweat rate. Serum concentrations of acid soluble protein, alpha-acid glycoprotein, IL-4 and IL-2, and rump sweat rate were not altered (P>0.24) by the challenge. To our knowledge, this report is the most complete characterization of the bovine acute-phase response to a bolus-dose endotoxin challenge conducted under thermoneutral conditions and should provide foundation data for future research.

Fescue toxicosis affects wild and domestic animals consuming ergot alkaloids contained in tall fescue forage infected with the endophytic fungus, Neotyphodium coenophialum. When animals are consuming infected fescue (E+) forage during periods of elevated ambient temperatures (summer), a range of phenotypic disorders collectively called summer slump is observed. It is characterized by hyperthermia, with an accompanying decrease in feed intake, growth, milk yield, and reproductive fitness. Laboratory mice also exhibit symptoms of fescue toxicosis at thermo-neutral (TN) temperature, as indicated by reduced growth rate and reproductive fitness. Our goal was to characterize the differences in gene expression in liver of mice exposed to summer-type heat stress (HS) and E+ when compared to mice fed E+ at TN temperature. Mice were fed E+ diet under HS (34 +/- 1 degrees C; n = 13; E+HS) or TN conditions (24 +/- 1 degrees C; n = 14; E+TN) for a period of 2 weeks between 47 and 60 days of age. Genes differentially expressed between E+HS versus E+TN were identified using DNA microarrays. Forty-one genes were differentially expressed between treatment groups. Expressions of eight genes were measured using quantitative real-time PCR. Genes coding for phase I detoxification enzymes were upregulated in E+HS mouse liver. This detoxification pathway is known to produce reactive oxidative species. We observed an upregulation of genes involved in the protection against reactive oxidative species. Key genes involved in de novo lipogenesis and lipid transport were also upregulated. Finally, genes involved in DNA damage control and unfolded protein responses were downregulated.

Intake of ergot alkaloids found in endophyte-infected tall fescue grass is associated with decreased feed intake and reduction in body weight gain. The liver is one of the target organs of fescue toxicosis with upregulation of genes involved in xenobiotic metabolism and downregulation of genes associated with antioxidant pathways. It was hypothesized that short-term exposure of rats to ergot alkaloids would change hepatic cytochrome P450 (CYP) and antioxidant expression, as well as reduce antioxidant enzyme activity and hepatocellular proliferation rates. Hepatic gene expression of various CYPs, selected nuclear receptors associated with the CYP induction, and antioxidant enzymes were measured using real-time PCR. Hepatic expression of CYP, antioxidant and proliferating cell nuclear antigen (PCNA) proteins were measured using Western blots. The CYP3A1 protein expression was evaluated using primary rat hepatocellular cultures treated with ergovaline, one of the major ergot alkaloids produced by fescue endophyte, in order to assess the direct role of ergot alkaloids in CYP induction. The enzyme activities of selected antioxidants were assayed spectrophotometrically. While hepatic CYP and nuclear receptor expression were increased in ergot alkaloid-exposed rats, the expression and activity of antioxidant enzymes were reduced. This could potentially lead to increased oxidative stress, which might be responsible for the decrease in hepatocellular proliferation after ergot alkaloid exposure. This study demonstrated that even short-term exposure to ergot alkaloids can potentially induce hepatic oxidative stress which can contribute to the pathogenesis of fescue toxicosis.

This chapter contains section titled: RELATIONSHIP BETWEEN FESCUE TOXICOSIS AND ENVIRONMENTAL CONDITIONS Thermoregulatory Terminology Animal Models and Control System Terminology Application of Animal Models to Fescue Toxicosis Short-Term Responses: Bovine and Rodent Models Long-Term Responses: Bovine and Rodent Models Use of Bovine and Rodent Models to Evaluate New Approaches to Reduce the Impact of Fescue Toxicosis on Thermoregulatory Ability Summary References

OBJECTIVE To determine whether cattle exposed to heat stress alone or heat stress while consuming endophyte-infected fescue (EIF) have lower whole-blood (WB) concentrations of glutathione (GSH). ANIMALS 10 Simmental cows. PROCEDURE Cows were sequentially exposed to thermoneutral (TN; 2 weeks; 18 C, 50% relative humidity [RH]), heat stress (HS; 2 weeks; alternating 4-hour intervals at 26 and 33 C; 50% RH), and heat stress while consuming EIF (10 microg of ergovaline/kg/d; 2 weeks, HS + EIF). Blood samples were collected after each period and tested for GSH and oxidized glutathione (GSSG) concentrations. RESULTS Feed consumption was similar when data were analyzed for time points at which WB concentrations of GSH or GSSG were determined. However, significant effects of treatment, cow, days exposed to heat, cow-by-treatment interaction, and treatment-by-days exposed to heat interaction were detected when data were considered simultaneously. Mean +/- SD hematocrit for TN, HS, and HS + EIF were 35.3+/-3, 33.3+/-2, and 37.1+/-3%, respectively. Mean WBGSH concentrations for TN, HS, and HS + EIF were 3.2+/-0.65, 2.7+/-0.62, and 2.4+/-0.56 mmol/L of RBC, respectively. Reduced WBGSH concentrations were associated with reduced feed intake during the later part of each heat period. CONCLUSIONS AND CLINICAL RELEVANCE Decreased GSH and increased GSSG concentrations were evident during heat stress, especially when cattle consumed EIF These were associated with reduced feed intake during heat stress. Heat stress, reductions in feed intake, and thermoregulatory effects of EIF may induce oxidative stress in cattle.

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.

A model, termed the PET model, is used to estimate body temperature in cattle challenged by hot cyclic chamber temperatures. The model is based on Newton's law of cooling, driven by an estimated sinusoidal function. In practice, it is often difficult to maintain hot sinusoidal fluctuations in chamber temperatures. However, it is possible to model cyclic chamber temperatures using a discrete Fourier series. By increasing the precision in estimating the cyclic temperature driving function, we can more precisely estimate the parameters in the PET model. Simulation studies were performed to investigate the effect of under- and over-parameterization on accuracy of estimates, performance of a number of model selection criteria, and on nonlinear behavior such as intrinsic and parameter-effects curvature, bias, excess variance, and skewness. Our results will help researchers decide how to model ambient temperatures producing heat stress in cattle and improve estimates for evaluating management strategies.