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Plasma Cortisol concentration as estimated by CPB radioassay increased significantly in cattle during 4 hr of acute thermal exposure while plasma progestins did not. Changes in plasma Cortisol were gradual with the onset of heat exposure and correlated significantly with rectal and environmental temperatures although the correlation was greater with rectal temperature. However, the rise and decline in plasma Cortisol levels following, respectively, the onset and cessation of thermal exposure preceded changes in body temperature. Adrenocortical function in steers and the possible significance of plasma Cortisol secretion during acute thermal stress are discussed.

Seventy crossbred heifers were allotted randomly to 10 treatment groups. Treatments consisted of active immunization against ovalbumin (OV) conjugates of luteinizing hormone-releasing hormone (LHRH), human chorionic gonadotropin (hCG) and bovine luteinizing hormone (bLH) with each of three adjuvants. The adjuvants were complete Freund's adjuvant (CFA), M1036and 6VR6. Control animals were immunized against OV alone using CFA. Bulls were placed with the heifers following immunization to allow comparison of pregnancy rates between groups. Blood samples were collected weekly for 14 wk to determine antibody concentrations. Significant levels of circulating LH or LHRH antibodies were detected in heifers immunized with each of the hormone conjugates. Complete Freund's adjuvant was the most effective for stimulating antibody response to these antigens; however, M103 was equally effective when used with bLH or hCG conjugates. None of the heifers in the bLH-OV-CFA, bLH-OV-M103 or LHRH-OV-CFA immunization groups was pregnant at slaughter, whereas 71% of the OV-CFA control heifers were pregnant. Fertility suppression may be achieved in the bovine by active immunization against any of these three hormone conjugates. However, the duration of this study (8 wk after immunization) does not allow evaluation of the duration of effectiveness of each of the treatments.

Six lactating Holstein cows were used to determine whether a serum luteinizing hormone (LH) surge induced by luteinizing hormone-releasing hormone (LHRH) could be detected in milk. A double antibody radioimmunoassay was evaluated for measuring LH in whole milk. Cows (d 10 of the estrous cycle) were injected with saline (time zero), followed by LHRH 12 h later. Blood samples were collected hourly for 12 h via jugular cannula following each injection; milk removal was accomplished every 2 h by a portable milking machine. On d 10 of the next estrous cycle, treatment order was switched, with the same cows receiving LHRH at time zero and saline 12 h later. Approximately 2 h following LHRH treatment, serum LH levels peaked at 29 ng/ml and remained elevated for 5 h. There was no corresponding change in milk LH detected during the 12-h to 24-h period following the induced serum LH surge. Our conclusion is that the measurement of LH in the milk of cows shows little promise for predicting ovulation time in the cow.

Diet and ambient temperature-humidity are major factors in an animal's environment. Little information is available on the effect of ambient temperature and relative humidity on ruminal pH, oxidation reduction potential (ORP), ammonia (NH3) and lactic acid. Information concerning the interaction of diet and temperature-humidity should be useful m the feeding management of cattle in different climates.Increasing the proportion of concentrates or adding starch and soluble carbohydrates to rations of cattle has been observed to decrease ruminal pH with a concomitant rise in volatile fatty acids (VFA) (Cowsert and Montgomery, 1969; Sutton, 1968; Eadie, Hobson and Mann, 1967; Slanina and Cabadog, 1967). Similarly, high grain rations, and the addition of soluble carbohydrates has been reported to decrease ruminal pH with a simultaneous increase in lactic acid (Brawner et al., 1969; Sutton, 1968; Bruno and Moore, 1962; Waldo and Schultz, 1956). In general, the postfeeding or post treatment ruminal lactic acid peaked in 2 hours.

Six Holstein cows were used to study the use of polyethylene glycol (mol. wt. 3,000 to 3,700) as a marker to estimate rumen volume and weight of rumen contents. The data indicate that approximate fill and weight of rumen contents can be determined using PEG as a reference substance. Its use in digestion studies as used in this experiment is doubtful and a possible explanation is that polyethylene glycol travels through the tract in the liquid phase and not in the solid phase.

An experiment was conducted to determine the effect of environmental heat exposure and catecholamine infusion on cutaneous moisture loss, pulse rate and rectal temperature. Five non-lactating Holstein cows housed in climatic chambers were allowed 3 weeks to adjust at 18 C, 50% RH and then exposed to 18 C, 50% RH and 35 C, 50% RH temperature treatments. At each temperature, saline, epinephrine and norepinephrine solutions were infused intravenously for 6 hours. Epinephrine was infused at a dosage rate of 0.25 mcg/kg/ min. At 18 C, epinephrine increased cutaneous moisture loss 300%, pulse rate 29% and rectal temperature 0.8%. At 35 C, epinephrine increased cutaneous moisture loss 55%, pulse rate 20%, but rectal temperature was not affected. The overall analysis indicated the increase in rectal temperature during epinephrine infusion was an environmental temperature effect and the increase in pulse rate a drug effect. The increase in cutaneous moisture loss during heat exposure and epinephrine infusion was explained in terms of the “blood flow-interstitial fluid volume theory”, which was proposed as a mechanism for cutaneous moisture loss in bovines.

The amount of growth hormone (GH) degraded (or synthesized) per day was calculated as the product of (a) the concentration of GH-P, (b) the rate of disappearance rate (K) of GH-P and (c) the volume of the space of distribution of GH (GHDS). The average GH-P level was found to be 15 mug. per ml., the K per day was 44.7 and the GHDS was 31.8 1. per animal. The average GH secretion rate (GHSR) was found to be 19.1 mg./day/animal.Two methods were used for the estimation of GHSR and both methods gave comparable results. It is hoped that this technique can be widely used to gain a better understanding of the role of GH in lactation and bovine endocrinology.

The hypothesis that ruminal heating decreases thyroid function and metabolic rate comparable to the decreases obtained by exposure of the whole animal to heat was tested. During the investigation, four nonlactating cows with rumen fistulas were kept at a thermoneutral temperature. At this temperature, the rumen was heated with hot water circulating through stainless steel coils placed in the rumen viathe fistula. The effect of ruminal heating on thyroid was studied by the thyroxine I131disappearance rate (K) and PBI. Heat production was measured with an indirect calorimeter. Ruminal heating significantly decreased K and heat production. Respiratory rates and rectal temperatures were not affected using the exogenous heat load of 125 kcal./hr. but were increased by the 300 kcal./hr. heat load. Heating the rumen is suggested as exerting its effect by one or both of the following mechanisms: (a) by stimulating the “central warmth detectors” through impulses from the nerve endings in the rumen to depress the TRF; (b) by alteration of the requirement of the tissues for thyroid hormone. The latter may occur indirectly by a modification of some substrate level which in turn decreases thyroid function viathe feed-back mechanism.

WHEN acutely exposed to a moderate heat stress (35 C) cows accustomed to the experimental procedure increased plasma cortisol significantly (P<.05) in the first 20 min. of exposure from 30 to 37 µg/liter. Plasma cortisol continued to rise for 2 hr. (P<.01) and plateaued around 43 µg/liter between 2 and 4 hr. after the onset of heat exposure. The metabolic clearance rate of cortisol remained unchanged over this period. Cortisol turnover rate plateaued between 2 and 4 hr. of heat stress at around 165 µg/min. after a pre-exposure rate of 117 µg/min. (P<.01). After 7 to 10 weeks of chronic, mild heat stress, (35 C) plasma cortisol was significantly (P<.01) depressed from 30 to 25 µg/liter. Metabolic clearance rate remained unchanged. Cortisol turnover rate also fell significantly (P<.05) from 117 to 97 µg/minute. There were no significant changes during the first 4 hr. of return to 18 C after 10 weeks of 35 C, but over the next 9 days the measured variables returned progressively to their pre-exposure values. There was poor agreement between cortisol turnover rate measured by isotope dilution and by continuous infusion; 430 vs.117 µg/min. at 18 C, and 232 vs.97 µg/min. at 35 C, respectively.It is concluded that the short-term response of cortisol to heat stress is part of an acute non-specific reaction which could be exhibited in response to a wide variety of stressors. Under conditions of prolonged heat exposure specific adjustments are made in the turnover rate and the plasma concentration of the calorigenic glucocorticoid hormone.

Experiments were conducted to measure the plasma levels of growth hormone [GH] and its clearance rates (k) in cattle following prolonged exposure to thermoneutral (18 C, 50% RH) and high ambient temperatures (35 C, 50% RH). Plasma levels of GH were measured by coated charcoal radioimmunoassay and the clearance from circulation was determined by the labeled GH disappearance method, while the GH distribution space (GHDS) was obtained by extrapolation technique. The GH secretion rate was estimated as the product of these three parameters.As compared to the respective values at thermoneutrality, all the components of GHSR changed in heat exposed animals. The GHDS increased from 23.2 to 25.9 liters which partially was responsible for a decline in [GH] from 18.2 to 13.5 ng/ml plasma. The drastic decline in GHSR from 16.04 mg to 9.09 mg/cow/day was due to a major decline in k from 38.4 to 26.3 pools/day. The observed decrease in the GHSR have been viewed as a compensatory hormonal response of the homeotherm to prolonged environmental heat stress which may be a factor in depressed milk production in the tropics.

The serum biochemical and hematological measurements of heat tolerant (Zebu) and cold tolerant (Scotch Highland) cattle maintained under identical environmental conditions were compared using three purebred yearling heifers of each breed. There were differences between the mean serum concentrations, or activities, of the two breeds in glucose, cholesterol, urea nitrogen, total protein, creatinine, lactic dehydrogenase, alkaline phosphatase and glutamic-oxaloacetic transaminase. There were no differences between the two breeds in the mean serum concentrations of sodium, chloride, calcium, inorganic phosphorus, potassium, or total bilirubin. When the hematological measurements of the two breeds were compared, there were found to be differences in erythrocyte count, packed cell volume, hemoglobin concentration, mean corpuscular volume, mean corpuscular hemoglobin, leucocyte count, neutrophil percent and eosinophil percent.

Twelve Friesian calves 6 months old were exposed in the climatic chamber for 3 days at 18.3 C and 50% relative humidity followed by 3 days at 32.2 C and 50% relative humidity. Total body water was determined during each exposure using the tritiated water and extrapolation technique. Total body water was subtracted from the average body weight to obtain the total body solids. Whole body 40K, nitrogen retention and rectal temperature were determined on the third day of each thermal exposure. Heat caused a 15% decrease (P<.01) in total body solids. The percentage decrease in the latter, as a measure of heat stress, varied among the animals from 24.5% to 0.2%. Nitrogen retention decreased (P<.01) but the animals were in a positive balance indicating no lean mass loss with short-term heat exposure. This was confirmed by a lack of significant loss in whole body 40K. Thus the body solids loss was attributed to a loss in body fat. Total body water increased 10.1 1 (P<.01), which replaced the 9.9 kg loss in body solids thus resulting in no significant change in body weight. Body water turnover rate increased (P<.01) from 3.8 to 6.4 ml/kg/hr., thus preventing the increase in rectal temperature (P<.01) from reaching a lethal level.

Three Scotch Highland and three Zebu heifers, 18 months of age, were subjected to ambient temperature-relative humidity (RH) treatments, 2 weeks each, of 10 C-50% RH and 31 C-5O% RH.Respiration rates and rectal temperatures increased significantly for both breeds at 31 C, with the increase being much greater for the Scotch Highlands.Total ruminal volatile fatty acid (VFA) concentration, mEq/liter, significantly decreased for both breeds at 31 C. This may have been the result of a significant decrease in feed consumption and increase in water consumption for both breeds at the higher temperature.The Scotch Highlands had significantly greater total ruminal VFA concentration than the Zebus at 10 C. At this temperature there was no breed difference in feed consumption, although the Scotch Highlands drank significantly more water.There was no significant difference in total ruminal VFA concentration between the two breeds at 31 C, even though the Scotch Highlands drank significantly more water and consumed significantly less feed than the Zebus at this temperature.

A rapid method for estimation of thyroxine secretion rate (TSR) in cattle was described in detail. The TSR was calculated as the product of the plasma thyroxine (PT), the thyroxine disappearance rate (K) and the thyroxine distribution space (TDS).Different methods were used for the estimation of the fraction K. Results indicated that using plasma for the estimation of K is satisfactory for all practical purposes. The K values obtained using 2 days were similar to the values obtained using 4 days. Thus, 2 days were recommended as sufficient to estimate the TSR of cattle instead of using the thyroxine replacement methods which require at least 2 weeks for one determination.The average TSR at 18° C. temperature in non-lactating Holstein cows was found to be 1.58 mg. per day per cow or 0.316 mg./100 kg. bodyweight. The average TDS was 81.5 1. per cow or 13.7 1./100 kg. bodyweight. These values are similar to those reported by the chemical thyroxine turnover method.The method reported in this paper has numerous advantages over the replacement method in short-term experiments.

Five Holstein cows were used to determine the effect of hydrocortisone on calorigenesis at 18° and 35° C. The objective of the study was to determine the calorigenic action, latent period and biological effective time of hydrocortisone at different environmental temperatures. The intravenous injection of hydrocortisone significantly increased resting or standing heat production and in pulse rate in animals maintained at either 18° or 35° C. The latent period was found to be 3 to 4 hr. at 18° C. and 2 to 3 hr. at 35° C. The biological effective time was estimated to be 3 and 5 hr. at the comfort temperature (18° C.) and under exposure to heat (35° C), respectively. High temperature (35° C.) decreased pulse rate, heat production and feed intake, and increased respiration rate and rectal temperature, indicating a disturbance in thermal equilibrium.

Studies were conducted on the effects of humidity level (85, 30 or 85-30% alternating) in a warm growing environment (29° C.) on the quantitative and qualitative traits of Landrace carcasses. The rate of gain and composition of the carcasses were not altered markedly by environmental humidity level. Color and gross morphology mean ratings and means of processing properties of the muscles were not significantly different among the treatment groups, although pigs reared in high humidity tended to have superior l. dorsicolor and gross morphology. Humidity level in the growing environment influenced the direction of association of quantitative and qualitative carcass traits. The experiment indicated that high (85%) environmental humidity in combination with a warm temperature does not have detrimental effects on the growth, carcass composition and muscle quality of Landrace pigs.

Studies were conducted to determine physiological responses during heat-stress and postmortem muscle properties of pigs which had been acclimated to 29 or 18° C. and those subjected to fluctuating temperatures (29-18° C. alternating). Elevation of body temperature and respiratory rate during a 4-hr, period of heat-stress (37-40° C.) was pronounced in the pigs previously acclimated to 18° C. However, after recovery from the heat-stress and after subsequent exsanguination, these heat-stress-susceptible animals exhibited postmortem muscle properties that were similar to those of the other groups of pigs. Correlation analyses revealed that physiological responses during heat-stress were highly correlated with postmortem muscle properties in temperature-acclimated pigs and poorly correlated in those subjected to fluctuating temperatures. The data indicated that prediction of postmortem muscle properties of animals previously acclimated to a constant-temperature environment would be possible by measuring their body temperature and respiratory rate prior to and during a short period of heat-stress.

Four Holstein cows were used to determine the influence of growth hormone on calorigenesis at 18 and 38° C. The objective of the study was to determine the calorigenic action, latent period, biological effective time and peak metabolic response of growth hormone at different environmental temperatures. The injection of GH at 18 and at 38° C. increased resting or standing metabolism and pulse rate. The latent period was 10 hr. at 18° C. and 4 hr. at 38° C. The biological effective time was 40 and 50 hr. at the comfort zone (18° C.) and under exposure to heat, respectively. Growth hormone increased thyroid activity and decreased the RQ. These results suggest that the calorigenic action of GH may be due to a synergetic reaction with thyroxine or to the increase in fat metabolism. High temperature (38° C.) decreased pulse rate, oxygen consumption and feed intake and increased respiration rate and rectal temperature, indicating a serious disturbance in thermal equilibrium.

Six nonlactating Holstein cows were fasted and subjected to different environmental temperatures. The effect of fasting for 8 and 24 hr. on thyroxine I131disappearance rate and the plasma PBI was recorded. The thyroxine I131disappearance rate increased significantly after 8 hr. of fasting, but after 24 hr. of fasting it was similar to that of the fed animals. The plasma PBI was not affected by fasting. The data indicated an inverse relationship between the thyroid activity and environmental temperature in fasting cows. The increase in thyroid activity in fasting animals at 1° C. compared with 18° C. suggests that thyroid function is dependent mainly on environmental temperature and not on feed intake. Raising the environmental temperature above 18° C. caused a depression in thyroid activity. It is also suggested that increasing the period of fasting for several days may result in a depression in thyroid activity. The possible mechanisms involved in thyroid changes under fasting are discussed, and a convenient laboratory method for fasting the bovine has been described.

Six nonlactating Holstein cows were fitted with rumen cannulas to permit the control of feed intake. The animals were exposed to sudden changes in temperatures of 1, 18, 38 and 18° C, consecutively, and were kept at the changed temperature for at least a week. Thyroxine I131disappearance rate (K), PBI, O2consumption and rectal temperature were measured daily. At 38° C. the K values were depressed significantly (P<.05) only after 60 hr. of exposure. Plasma PBI increased at 12 hr. and decreased significantly at 60 hr. Rectal temperature and thyroid indices (K and PBI) displayed inverse trends during exposure to heat. Metabolic rate tended to follow the trends of the K and PBI values during the hot period. At 1° C. the K values were significantly increased at 36 hr. and then gradually declined toward the 18° C. values. After 60 hr. of 1 ° C. exposure, plasma PBI had increased significantly. Rectal temperatures and metabolic rates were not significantly affected. These data provide evidence that thyroid function and metabolic rate are involved in the compensation stage of acclimation to the new environment.

Six lactating Holstein dairy cattle were used to determine the effects of environmental temperature and controlled feeding on the blood thyroxine I131disappearance rates. Environmental temperature of 88° F. significantly depressed blood thyroxine I131disappearance rates in ad libitumand control-fed animals. Temperature differences in the thyroxine I131disappearance rate were greater (P<.10) for the controlled-fed animals than the ad libitumgroup. There was a highly significant difference in the thyroxine I131disappearance rate due to environmental temperature for all animals.These results suggest that thyroxine I131disappearance rates are depressed directly by high environmental and body temperatures rather than by depressed feed intakes.

Four Holstein cows were used to determine the influence of thyroxine on calorigenesis. The objective of this study was to determine the calorigenic action, latent period, biological effective time and peak metabolic response at different environmental temperatures. Injection of L-thyroxine at 18° and at 32° C. increased “resting or standing” metabolism, pulse rate and lactation. The latent period was 2 to 3 days and 1 to 2 days at 18° and 32° C, respectively. The biological effective time was found to be 4 to 5 days and 5 to 6 days at the comfort zone and under exposure to heat, respectively. The rate of thyroxine metabolism is slow in cattle and decreases progressively under hot conditions. The results suggest that estimates of thyroxine secretion rate are too low when calculated by blood methods and too high when the in vivothyroid measurement technique is used. High temperature (32° C.) decreased pulse rate, oxygen consumption, milk production and feed intake and increased rectal temperature and respiration rate, indicating a serious disturbance in the cow's thermal equilibrium.