Guide for the Care and Use of Laboratory Animals

Interrelationships between energy balance and postpartum reproductive function in dairy cattle.

Analysis of repeated measurements of animals.

Environmentally induced periods of heat stress decrease productivity with devastating economic consequences to global animal agriculture. Heat stress can be defined as a physiological condition when the core body temperature of a given species exceeds its range specified for normal activity, which results from a total heat load (internal production and environment) exceeding the capacity for heat dissipation and this prompts physiological and behavioral responses to reduce the strain. The ability of ruminants to regulate body temperature is species- and breed-dependent. Dairy breeds are typically more sensitive to heat stress than meat breeds, and higher-producing animals are more susceptible to heat stress because they generate more metabolic heat. During heat stress, ruminants, like other homeothermic animals, increase avenues of heat loss and reduce heat production in an attempt to maintain euthermia. The immediate responses to heat load are increased respiration rates, decreased feed intake and increased water intake. Acclimatization is a process by which animals adapt to environmental conditions and engage behavioral, hormonal and metabolic changes that are characteristics of either acclimatory homeostasis or homeorhetic mechanisms used by the animals to survive in a new ‘physiological state’. For example, alterations in the hormonal profile are mainly characterized by a decline and increase in anabolic and catabolic hormones, respectively. The response to heat load and the heat-induced change in homeorhetic modifiers alters post-absorptive energy, lipid and protein metabolism, impairs liver function, causes oxidative stress, jeopardizes the immune response and decreases reproductive performance. These physiological modifications alter nutrient partitioning and may prevent heat-stressed lactating cows from recruiting glucose-sparing mechanisms (despite the reduced nutrient intake). This might explain, in large part, why decreased feed intake only accounts for a minor portion of the reduced milk yield from environmentally induced hyperthermic cows. How these metabolic changes are initiated and regulated is not known. It also remains unclear how these changes differ between short-term v. long-term heat acclimation to impact animal productivity and well-being. A better understanding of the adaptations enlisted by ruminants during heat stress is necessary to enhance the likelihood of developing strategies to simultaneously improve heat tolerance and increase productivity.

https://dspace.unitus.it/bitstream/2067/1536/1/FINAL%20ANIMAL%20repo_A77rkGLY.pdf

Metabolic and hormonal acclimation to heat stress in domesticated ruminants.

Exploration of the hypothalamic–pituitary–adrenal function as a tool to evaluate animal welfare

Physiological control of mammary growth, lactogenesis, and lactation.

Serum prolactin was measured in 55 cows 2–4 hr before, immediately after and 1 hr after milking for the duration of lactation or week 44, whichever occurred first. Serum prolactin increased in response to stimuli associated with milking, but this response was largest at 8 weeks; of lactation (77 ng/ml) and then gradually decreased as lactation advanced until at 32 weeks prolactin was no longer released. As prolactin concentrations increased in the pre—milking serum samples the amount subsequently released by the milking stimulus was reduced (p < 0.01). The correlation between serum prolactin measured 2 to 4 hr before milking and milk yield was very low (r = -0.03), although similar calculations for samples collected immediately after and 1 hour after milking (r = 0.36 and 0.18, respectively) were significant (p < 0.01). Within—stageof— lactation correlations between serum prolactin and milk yield were very low and sometimes negative during early lactation, but after 12 to 24 weeks of lactation the correla...

Serum Prolactin During Various Physiological States and Its Relationship to Milk Production in the Bovine

Mammary development and pituitary prolactin level of heifers from birth through puberty and during the estrous cycle.

Serum PRL, GH, and progesterone were measured at 1300 h twice weekly in heifers exposed to natural length or 16-h light (16L)-8-h dark (8D) during Nov. 11 to March 9 (n = 20) and April 30 to Aug. 13 (n = 18). Heifers exposed to 16L–8D received fluorescent light between 0600–2200 h daily. Serum PRL increased 4-fold in response to 16 h of artificial illumination between Nov. 11 and March 9 (P < 0.01), and increased 1.6-fold between April 30 and Aug. 13 (P < 0.01). Serum PRL fluctuations in natural and 16L–8D photoperiods were synchronized and positively correlated (r = 0.36–0.53) with changes in ambient temperature. Overall, serum PRL concentrations increased (P < 0.01) within 16L–8D and natural length photoperiods as ambient temperatures increased from -7 to +29 C. When ambient temperatures were below 0 C, serum PRL concentrations were similar in heifers exposed to natural and 16L–8D photoperiods. As ambient temperatures increased, serum PRL concentrations increased synergistically in animals exposed to 16...

Prolactin and Growth Hormone Responses to Photoperiod in Heifers

Serum GH and insulin were measured in 26 cows and serum corticoids were measured in 12 cows. Blood samples were collected 2–4 hr before, immediately after and 1 hr after milking for the duration of a lactation (44 weeks). Milking was associated with the release of corticoids and insulin but not GH. Serum GH was negatively associated with milk yield, especially after 24 weeks of lactation. This negative, relationship may be more directly caused by changes associated with concurrent pregnancy rather than with lactation per se, because serum GH concentrations decreased as lactation advanced, but increased as gestation progressed. Serum GH was greater (p < 0.05) during estrus than during the luteal phase of the estrous cycle of lactating cows. Serum insulin concentrations were inversely related to GH levels (increasing as lactation advanced, but decreasing as pregnancy progressed), and insulin was consistently negatively related to milk yields. Serum corticoids were variable, but on the average they changed v...

H. Allen Tucker

Papers

Physiological control of mammary growth, lactogenesis, and lactation.

Serum Prolactin During Various Physiological States and Its Relationship to Milk Production in the Bovine

Mammary development and pituitary prolactin level of heifers from birth through puberty and during the estrous cycle.

Prolactin and Growth Hormone Responses to Photoperiod in Heifers

Bovine Serum Growth Hormone, Corticoids and Insulin During Lactation