Assessing the heat tolerance of 17 beef cattle genotypes.

Cattle production plays a significant role in terms of world food production. Nearly 82% of the world's 1.2 billion cattle can be found in developing countries. An increasing demand for meat in developing countries has seen an increase in intensification of animal industries, and a move to cross-bred animals. Heat tolerance is considered to be one of the most important adaptive aspects for cattle, and the lack of thermally-tolerant breeds is a major constraint on cattle production in many countries. There is a need to not only identify heat tolerant breeds, but also heat tolerant animals within a non-tolerant breed. Identification of heat tolerant animals is not easy under field conditions. In this study, panting score (0 to 4.5 scale where 0 = no stress and 4.5 = extreme stress) and the heat load index (HLI) [HLI(BG<25°C) = 10.66 + 0.28 × rh + 1.30 × BG - WS; and, HLI (BG> 25°C) = 8.62 + 0.38 × rh + 1.55 × BG - 0.5 × WS + e((2.4 - WS)), where BG = black globe temperature ((o)C), rh = relative humidity (decimal form), WS = wind speed (m/s) and e is the base of the natural logarithm] were used to assess the heat tolerance of 17 genotypes (12,757 steers) within 13 Australian feedlots over three summers. The cattle were assessed under natural climatic conditions in which HLI ranged from thermonuetral (HLI < 70) to extreme (HLI > 96; black globe temperature = 40.2°C, relative humidity = 64%, wind speed = 1.58 m/s). When HLI > 96 a greater number (P < 0.001) of pure bred Bos taurus and crosses of Bos taurus cattle had a panting score ≥ 2 compared to Brahman cattle, and Brahman-cross cattle. The heat tolerance of the assessed breeds was verified using panting scores and the HLI. Heat tolerance of cattle can be assessed under field conditions by using panting score and HLI.

Tympanic temperature in confined beef cattle exposed to excessive heat load.

Angus crossbred yearling steers (n = 168) were used to evaluate effects on performance and tympanic temperature (TT) of feeding additional potassium and sodium to steers exposed to excessive heat load (maximum daily ambient temperature exceeded 32°C for three consecutive days) during seasonal summer conditions. Steers were assigned one of four treatments: (1) control; (2) potassium supplemented (diet containing 2.10% KHCO3); (3) sodium supplemented (diet containing 1.10% NaCl); or (4) potassium and sodium supplemented (diet containing 2.10% KHCO3 and 1.10% NaCl). Overall, additional KHCO3 at the 2% level or NaCl at the 1% level did not improve performance or heat stress tolerance with these diet formulations. However, the addition of KHCO3 did enhance water intake. Independent of treatment effects, TT of cattle displaying high, moderate, or low levels of stress suggest that cattle that do not adequately cool down at night are prone to achieving greater body temperatures during a subsequent hot day. Cattle that are prone to get hot but can cool at night can keep average tympanic temperatures at or near those of cattle that tend to consistently maintain lower peak and mean body temperatures. In addition, during cooler and moderately hot periods, cattle change TT in a stair-step or incremental pattern, while under hot conditions, average TT of group-fed cattle moves in conjunction with ambient conditions, indicating that thermoregulatory mechanisms are at or near maximum physiological capacity.

Livestock production system management responses to thermal challenges.

The adaptive capabilities of animals and livestock production systems have been emphasized in this report. Biometeorology has a key role in rational management to meet the challenges of thermal environments. While the focus is primarily on cattle in warm or hot climates, the importance of dynamic animal responses to environmental challenges applies to all species and climates. Methods used to mitigate environmental challenges focus on heat loss/heat production balance. Under cold stress, reduction of heat loss is the key. Under heat stress, reduction of heat load or increasing heat loss are the primary management tools, although heat-tolerant animals are also available. In general, livestock with health problems and the most productive animals (e.g., highest growth rate or milk production) are at greatest risk of heat stress, thereby requiring the most attention. Risk management, by considering perceived thermal challenges, then assessing the potential consequences and acting accordingly, will reduce the impact of such challenges. Appropriate actions include: shade, sprinkling, air movement, or active cooling. Summarizing, the most important element of proactive environmental management to reduce risk is preparation: be informed, develop a strategic plan, observe and recognize animals in distress, and take appropriate tactical action.

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.

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.

Shade and wind barrier effects on summertime feedlot cattle performance.

In each of three summertime trials conducted over consecutive years, approximately 110 predominantly black and black-white-face steers were blocked by weight and randomly allotted to one of 16 pens in a 2x2 factorial arrangement of treatments. Factors consisted of cattle being fed in facilities with or without wind barriers and with or without shade. Steers were fed dry-rolled corn-based diets (1.43 Mcal/ kg, NEg). Mean starting date and days on feed were June 26 and 79, respectively. In unshaded areas, temperature and humidity averaged 21.6 degrees C and 77.9%, and the blackglobe-humidity index (BGHI) at 1500 averaged between 84.0 and 89.1. Each of four 6.1-x6.1-m structures (mean height = 3.4 m) with white steel roofs provided shade (2.65 m2/steer) for two pens. In facilities with wind barriers provided, airflow was reduced from the north and northwest by a 25-m-wide shelterbelt containing six rows of trees. For cattle fed in pens with wind barriers, shade increased (P<.05) gain from 0 to 56 d and decreased (P<.05) DMI/ADG from 0 to 28 d. Differences (P<.05) in performance were not found between shaded and unshaded cattle in any portion of the feeding period for cattle fed in the pens without wind barriers and over the entire feeding period in either type of facility. The shade response in pens with wind barriers seemed to be greater the 1st yr than in subsequent years. Differences in weather patterns among years, especially air temperature, humidity, and solar radiation, may partially explain this interaction. Also, in yr 1, cattle tended to have greater fat thickness at finish than in yr 2 and 3. Correlations between BGHI and DMI tended to be greater during the early portion of the trial (0 to 28 d) than over the entire trial. Correlations between the difference in BGHI under shade vs no shade and percentage of shade use had the greatest magnitude and were significant only in the first 28 d vs over the entire feeding period. Although no heat-related cattle deaths occurred in this study, results suggest that shade improves cattle performance in the summer when they are fed in facilities with winter wind protection available and have not become acclimated to hot conditions. Once cattle are acclimated or hot conditions subside, compensation by unshaded cattle offsets much of the initial benefits of providing shade.

Quantifying livestock responses for heat stress management: a review.

Hot weather challenges livestock production but technology exists to offset the challenge if producers have made appropriate strategic decisions. Key issues include understanding the hazards of heat stress, being prepared to offer relief from the heat, recognizing when an animal is in danger, and taking appropriate action. This paper describes our efforts to develop biological response functions; assesses climatic probabilities and performs associated risk analyses; provides inputs for computer models used to make environmental management decisions; and evaluates threshold temperatures as estimates of critical temperature limits for swine, cattle and sheep.

Dynamic responses of cattle to thermal heat loads.

The focal point of this limited review is bioenergetic research conducted in the Biological Engineering Research Unit at the U.S. Meat Animal Research Center (MARC), using recently developed instrumentation and analytical techniques. The dynamics of observed thermoregulatory responses in cattle to thermal heat load challenges are explored, with an emphasis on physiological and behavioral parameters of body temperature, respiration rate, and feed intake. Observations of body temperature, especially tympanic temperature, have shown hot environments to cause phase shifts, increased amplitude, and increased means for diurnal rhythms. Fractal analysis of body temperature records obtained at 2- to 10-min intervals has been found to be robust for objectively differentiating among responses of cattle in cool to hot environments, and it indicates a stress threshold of approximately 25 degrees C (coincident with declining feed intake). Other analyses determined a 21 degrees C threshold for increased respiration rate. The reported observations and analyses provide further understanding of how and why the animals respond to environmental challenges, an understanding that is necessary for refining performance models and developing energetic and thermoregulatory models. The dynamic responses are discussed in the context of establishing criteria for proactive environmental management for cattle during hot weather, using heat waves as an example.

Effect of booster vaccination with a multivalent clostridial bacterin-toxoid on sudden death syndrome mortality rate among feedlot cattle.

OBJECTIVE: To determine whether booster vaccination with a multivalent clostridial bacterin-toxoid would affect the sudden death syndrome (SDS) mortality rate among feedlot cattle. DESIGN: Field trial. ANIMALS: 83, 115 cattle at a Nebraska feedlot. PROCEDURE: Cattle arriving at the feedlot underwent routine processing according to established protocol. All cattle received a sequentially numbered ear tag and a 2-ml dose of a multivalent bacterin-toxoid designed to protect cattle against Clostridium chauvoei, C speticum, C novyi, C sordellii, and C perfringens types C and D. Approximately 90 days prior to slaughter, growth promotants were implanted in all cattle, and cattle were allocated to a treatment or control group on the basis of the last digits of their ear tag numbers. Cattle in the treatment group received a second 2-ml dose of clostridial bacterintoxoid; control cattle did not. RESULTS: Significant differences between groups in regard to crude, feeding pen, or SDS mortality rates were not detected. Sudden death syndrome mortality rate across both groups was 0.24%. If the SDS mortality rate in midwestern feedlot cattle was reduced > or = 40% by booster vaccination with a multivalent clostridial bacterin-toxoid, this experiment included enough animals to have a 90% probability of detecting that difference. CLINICAL IMPLICATIONS: Booster vaccination with a multivalent clostridial bacterin-toxoid does not affect SDS mortality rate among feedlot cattle.

Comparison of calf housing types and tympanic temperature rhythms in Holstein calves.

During fall, 30 Holstein calves were randomly assigned to three housing treatments from birth to weaning: conventional wooden hutches, enclosed molded polyethylene domes, and thermomolded opaque polymer hutches with ridge-top ventilation systems. The wooden and polymer hutches had outdoor pens. Fifteen calves, 5 in each housing type, were fitted with portable data loggers to record ambient (calf microclimate) and tympanic temperatures. Additional data collected included weekly girth, BW, and feed intake; blood samples were collected within 24 h of birth and at weaning (8 wk); and behavioral observations were made at 1, 4, and 7 wk of age. Polyethylene domes had the warmest microclimate, followed by wooden and polymer hutches. Feed intake, growth measurements, blood physiology, and behavior were unaffected by housing type. Diurnal tympanic temperature rhythms of neonatal dairy calves in this study were monophasic: maximums were at 1200 to 1700 h, and minimums were at 0600 to 0900 h. Computed fractal dimensions of tympanic temperature by week indicated a gradual diminishing of stress as the calves became older and acclimated to their environment. This objective characterization provides a basis for further evaluation of physiological stress and a means of improving environmental management.

Measuring physiological responses of animals to environmental stressors using a microcomputer-based portable datalogger.

The microcomputer-based datalogger system described in this report provides excellent flexibility for accurate recording of signals from a variety of sensors at programmable intervals. The small size of the unit permits applications in many research situations in which physiological and microclimate records are required from unrestrained animals. Data presented from two experiments with cattle with ad libitum access to feed demonstrate the utility of the system. The high resolution and frequent measurements illustrate the fine detail of thermoregulatory responses of cattle (as represented by tympanic temperature) to sequential moderate and hot environments, the association of feeding activities with tympanic temperature at moderate conditions, and the disruptions in thermoregulatory function and feeding activities in hot conditions. A comparison of four anatomical sites (two internal, two sub-dermal) for measuring body temperature also illustrates shortcomings in using sub-dermal measures to reflect responses to dynamic ambient environments.

Adrenal and behavioral responses of swine restricted to varying degrees of mobility.

Adrenal and behavioral responses in swine restricted to varying degrees of mobility were examined to determine what component may impose chronic stress. The components of restriction considered were the inability to turn around and/or move freely. For 5 to 6 weeks 32 pigs were restrained such that pigs A) could move freely; B) could not turn around or move freely; C) could not turn around but could move back and forth the same distance as pigs in A; or D) could turn around but not move freely. After 4 weeks no significant differences due to degree of restraint were found in the profile of serum cortisol concentrations, neutrophil:lymphocyte ratio and ambulatory abilities; however overall cortisol concentrations were higher in pigs that could turn around but not move freely. Increases in serum cortisol due to ACTH injection were higher in pigs that could turn around but not move freely. No alterations in the manner in which gilts stood up and layed down could be observed. Collectively these data suggest that swine adapt when mobility is restricted to not turning around and not moving freely but cannot when animals can turn around but not move freely.

Effect of fasting and transportation on various physiological parameters and meat quality of slaughter hogs.

Forty-nine slaughter hogs were transported and fasted or fasted with no transport for 0 (no transport), 24, 48 and 72 h. Blood samples were taken before treatments were imposed and again prior to slaughter. Soft tissues of the ham were chemically analyzed and loin samples were evaluated by a sensory panel. Fasting and transportation decreased plasma triiodothyronine and hematocrit and increased plasma osmolality. Preslaughter treatment did not affect juiciness or desirability of cooked chops or percentage of fat in the ham soft tissues. These data suggest that although fasting and transporting of slaughter hogs may disrupt homeostasis of pigs, such physiological changes do not negatively affect meat quality.

Plasma adrenocorticotropic hormone and cortisol in pigs: effects of time of day on basal and stressor-altered concentrations.

An initial study was conducted to establish the presence in plasma of diurnal rhythms of immunoreactive porcine adrenocorticotropic hormone (pACTH) and cortisol in castrated male pigs (barrows). Fourteen barrows with jugular catheters were bled at 6-hr intervals for 24 hr. Significant changes in plasma pACTH were evident with peak levels (61 +/- 6 pg/ml) at 0100-0700 hr and a trough (38 +/- 4 pg/ml) at 1900 hr. Changes (P less than 0.05) in plasma cortisol were also present in barrows with a peak (44 +/- 6 ng/ml) at 0700 hr and a trough (21 +/- 5 ng/ml) at 1900 hr. Plasma norepinephrine and epinephrine were measured at the same time intervals and did not differ among hours. In these unstressed pigs the ratio cortisol/log10pACTH at 0700 hr (25.3 +/- 3.0) was greater than the ratio at 1900 hr (12.9 +/- 2.7). Sequential blood samples were subsequently taken on four of the barrows 12 and 26 days later. Plasma pACTH was variable among pigs and did not differ among hours. Plasma cortisol on both dates was greater (P less than 0.05) in the morning (0100 or 0700 hr) than at 1900 hr. The ratio cortisol/log10pACTH at 0700 hr was repeatedly greater than at 1900 hr. A second study was conducted to determine whether plasma pACTH and cortisol responses to mild (32 degrees C for 2 hr) or strong (20-min restraint) stressors were dependent on the time of day of stressor application (0800 hr, AM; 1600 hr, PM). Response-associated parameters (maximum concentration, maximum incremental concentration, and integrated response) for pACTH and cortisol did not differ between AM and PM. However, a qualitative difference existed between the AM and PM plasma pACTH responses to restraint +32 degrees C wherein the AM response consisted of a single prolonged surge, and the PM response of an initial major peak followed by a second significant minor peak. A suggested explanation is that the initial 20-min restraint stressor potentiated the hypothalamic-hypophyseal response to 32 degrees C. These studies are the first direct measurements which suggest the presence of diurnal changes in plasma ACTH and cortisol in barrows. The studies also indicate for barrows an absence of diurnal changes in plasma epinephrine and norepinephrine. The responsiveness of the pituitary-adrenocortical axis to stressors did not exhibit quantitative diurnal changes at the time periods measured. However, it is hypothesized that the repeatable AM-PM difference in the ratio cortisol/log10ACTH reflects a diurnal change in adrenal responsiveness to ACTH in unstressed pigs.

Serving capacity of crossbred yearling beef bulls. II. Summer grazing activity and body temperature patterns during average and heavy mating loads at pasture.

Ten low (LSC) and 10 high (HSC) serving capacity yearling bulls were exposed individually to 25 naturally cyclic (N) cows for 3 d (average mating load) and subsequently to 9 estrus-synchronized (S) cows for 1 d (heavy mating load) in a randomized complete block design consisting of 10, 4-d blocks. Bulls were fitted with vibracorders and temperature acquisition modules to record grazing activity and body temperature (BT), respectively. During the N cow treatment, LSC bulls had fewer (P less than .05) services per cow and a higher mount to service ratio than HSC bulls, and LSC bulls tended (P = .12) to graze less total time than HSC bulls (7.8 vs 9.0 h/d, respectively). However, both groups of bulls exhibited similar diurnal grazing patterns with two major daily grazing periods; the first (0400 to 1300) peaked early in the morning (0600) and the second (1700 to 2200) occurred in late afternoon and evening. During the S cow treatment, LSC and HSC bulls did not differ (P = .60) in grazing time or pattern, but similar mating activity was exhibited by both SC groups while exposed to S cows. Bulls grazed little during the hottest part of the day (1300 to 1700), and LSC and HSC bulls spent only 2.2 and 2.4 h, respectively, grazing during dark, cooler periods near midnight. Body temperature did not differ between SC groups and averaged 39.3 degrees C during N cow treatment and 39.4 degrees C during S cow treatment. Body temperature had a distinct, monophasic diurnal pattern in bulls exposed to an average mating load.(ABSTRACT TRUNCATED AT 250 WORDS)

Stressor-associated alterations in porcine plasma prolactin.

Experiments were conducted to determine effects of restraint and thermal stressors on plasma prolactin (PRL) in castrated male pigs. A single 20-min restraining period in a restraining cage which prevented both movement and injury increased (P less than 0.05) plasma PRL when applied at either 0800 or 1600 hr. Exposure to 32 degrees C at 0800-1000 hr or at 1600-1800 hr produced more moderate increases (P less than 0.05). A combination of 20 min restraint and 2 hr at 32 degrees C produced a response similar to restraint alone. Twenty minutes after stressor application plasma PRL concentrations in pigs exposed to restraint or restraint +32 degrees C at 1600 h were greater (P less than 0.05) than concentrations measured in all other treatment groups at that time interval. However, there were no statistically significant differences in additional quantitative indices of the plasma PRL responses (maximal level, maximal change, or integrated response above basal levels) among restraint, 32 degrees C, or restraint +32 degrees C, nor between morning and afternoon applications of treatment. Such data do not provide, therefore, any strong evidence for stressor-dependent or circadian differences in plasma PRL response. A second study subjected castrated male pigs to 20 degrees C (controls), 20 +/- 12 degrees C (cyclic temperature, sine wave variation), 5 degrees C constant, and 5 +/- 12 degrees C cyclic for 20 days. After 6 days exposure to 5 degrees C constant or 5 +/- 12 degrees C cyclic there were decreases (P less than 0.05) of 59 and 67% respectively in plasma PRL when compared either with pretreatment levels or with levels in pigs at 20 or 20 +/- 12 degrees C. There were no differences in PRL responses between cyclic vs constant temperatures. These results are the first to indicate that plasma PRL in pigs is affected by acute restraint and thermal stressors.

Effect of transportation on cortisol concentrations and on the circadian rhythm of cortisol in gilts.

A study was conducted to determine whether the circadian rhythm of cortisol in gilts is disrupted or altered by transport. Sixteen ovariectomized gilts with indwelling jugular catheters were individually penned in an enclosed building (location 1). Blood samples were collected at 0700 and 1900 hours for 6 days. On day 7, gilts in groups of 4 were transported 5.6 km to environmentally controlled chambers (25 C) and were individually penned (location 2). On the day of transport, samples were collected at 0700 hours at location 1, immediately before and after transport in a trailer, after unloading at location 2, and at 1900 hours at location 2. For the first 6 days at location 2, blood samples were collected daily at 0700 and 1900 hours. For the 6 days at location 1, circadian rhythm was evidenced by higher cortisol concentrations in the AM hours than in the PM hours. During transport, serum cortisol concentrations increased (P less than 0.01). Highest concentrations developed at 0.5 hour after unloading; concentrations declined thereafter. During the first 6 days at location 2, circadian rhythm was evidenced by higher serum cortisol concentrations in the AM hours than in the PM hours. Therefore, the transportation of gilts 5.6 km to new pens was a transient stress causing a temporary increase in serum cortisol concentrations, but did not cause a disruption in the endogenous rhythm of cortisol.

Peripheral concentrations of cortisol as an indicator of stress in the pig.

Twenty-four crossbred gilts (7 to 9 months old) were exposed to 3 different stressors to evaluate changes in peripheral concentrations of the adrenal hormone cortisol. Gilts were confined in a box for 1 hour, electrically stimulated for 6 minutes, or heat-stressed for 6 hours; these stressors were imposed for 3 consecutive days. Blood samples were collected repeatedly, and serum cortisol concentrations were determined by radioimmunoassay. Peak cortisol concentrations for pigs in the box, electrically stimulated, and heat-stressed were 67.8 +/- 9.9 ng/ml (mean +/- SEM), 43.2 +/- 5.8 ng/ml, and 25.2 +/- 3.0 ng/ml greater than the control concentrations. The times at which these peaks occurred varied with each stressor. Overall serum cortisol concentrations decreased from day 1 to day 3, but the response to a given stressor was not diminished. Our data substantiate the use of cortisol as an indicator of stress in swine and emphasize consideration for the interval between exposures to stressors.

Cortisol response of gilts in tether stalls.

Several experiments were conducted to evaluate serum cortisol concentrations and the circadian rhythm of this hormone in gilts tethered in stalls. Control animals were penned individually. In the initial experiment, 18 nongravid gilts were placed in tether stalls after being in either tether stalls or individual pens for 2 wk. No significant differences were found in serum cortisol concentrations. In a second experiment, 16 ovariectomized gilts were placed in tether stalls or individual pens for up to 5 wk. Estrus was induced during wk 3 and 4. During the first day in tether stalls, serum cortisol concentrations increased (P less than .05) and the circadian rhythm of cortisol was disrupted for 4 d. During estrus, the circadian rhythm of cortisol was interrupted for several days in the gilts, regardless of housing. After 4 wk, morning concentrations of cortisol were higher for gilts in tether stalls. The results indicate that: 1) the initial response to tethering varies according to previous penning and handling experience, 2) although the circadian rhythm of cortisol was either altered or disrupted during estrus, such disruptions were not influenced by type of penning and 3) tether stalls may chronically increase cortisol concentrations in gilts.

Twenty-four-hour profiles of prolactin and testosterone in ram lambs exposed to skeleton photoperiods consisting of various light pulses.

Individual groups of 6 ram lambs were housed within a controlled environment and exposed to one of 6 photoperiod schedules. Groups I and II received 8 (short day) or 16 (long day) h of continuous light, respectively; Groups III, IV and V were exposed to asymmetrical skeleton photoperiods consisting of a main light period of 7 h followed 9 h later by a light pulse of 1 h, 15 min or 1 min duration, respectively, and Group VI was exposed to a symmetrical skeleton photoperiod consisting of two 1-h light pulses positioned 16 h apart. After 4 weeks of treatment serum concentrations of prolactin and testosterone were measured over 24 h. Long-day responses characteristic of the 16L:8D photoperiod (i.e. elevated prolactin and reduced testosterone) were obtained in each of the asymmetric light-pulse treatment groups, but whereas prolactin was elevated over the full 24 h in lambs exposed to 16L:8D, two prominent nocturnal prolactin releases were largely responsible for the high 24-h mean prolactin values in Groups III, IV and V. Reduced serum testosterone in these same groups could not be attributed to a diurnal pattern of secretion but was associated with an overall decrease in testosterone pulse frequency. Prolactin and testosterone levels in Group IV were intermediate between those observed in lambs exposed to 8 or 16 h of light. In summary, light pulses of short duration (1 min) positioned at 17 h after dawn can produce endocrine changes in lambs similar to those observed in lambs exposed to 16 h of continuous light.