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Heat stress is detrimental to dairy production and affects numerous variables including feed intake and milk production. It is unclear, however, whether decreased milk yield is primarily due to the associated reduction in feed intake or the cumulative effects of heat stress on feed intake, metabolism, and physiology of dairy cattle. To distinguish between direct (not mediated by feed intake) and indirect (mediated by feed intake) effects of heat stress on physiological and metabolic indices, Holstein cows (n = 6) housed in thermal neutral conditions were pair-fed (PF) to match the nutrient intake of heat-stressed cows (HS; n = 6). All cows were subjected to 2 experimental periods: 1) thermal neutral and ad libitum intake for 9 d (P1) and 2) HS or PF for 9 d (P2). Heat-stress conditions were cyclical with daily temperatures ranging from 29.7 to 39.2°C. During P1 and P2 all cows received i.v. challenges of epinephrine (d 6 of each period), and growth hormone releasing factor (GRF; d 7 of each period), and had circulating somatotropin (ST) profiles characterized (every 15 min for 6 h on d 8 of each period). During P2, HS cows were hyperthermic for the entire day and peak differences in rectal temperatures and respiration rates occurred in the afternoon (38.7 to 40.2°C and 46 to 82 breaths/min, respectively). Heat stress decreased dry matter intake by greater than 35% and, by design, PF cows had similar reduced intakes. Heat stress and PF decreased milk yield, although the pattern and magnitude (40 and 21%, respectively) differed between treatments. The reduction in dry matter intake caused by HS accounted for only approximately 35% of the decrease in milk production. Both HS and PF cows entered into negative energy balance, but only PF cows had increased (approximately 120%) basal nonesterified fatty acid (NEFA) concentrations. Both PF and HS cows had decreased (7%) plasma glucose levels. The NEFA response to epinephrine did not differ between treatments but was increased (greater than 50%) in all cows during P2. During P2, HS (but not PF) cows had a modest reduction (16%) in plasma insulin-like growth factor-I. Neither treatment nor period had an effect on the ST response to GRF and there was little or no treatment effect on mean ST levels or pulsatility characteristics, but both HS and PF cows had reduced mean ST concentrations during P2. In summary, reduced nutrient intake accounted for just 35% of the HS-induced decrease in milk yield, and modest changes in the somatotropic axis may have contributed to a portion of the remainder. Differences in basal NEFA between PF and HS cows suggest a shift in postabsorptive metabolism and nutrient partitioning that may explain the additional reduction in milk yield in cows experiencing a thermal load.

It has long been known that season of the year has major impacts on dairy animal performance measures including growth, reproduction, and lactation. Additionally, as average production per cow has doubled, the metabolic heat output per animal has increased substantially rendering animals more susceptible to heat stress. This, in turn, has altered cooling and housing requirements for cattle. Substantial progress has been made in the last quarter-century in delineating the mechanisms by which thermal stress and photoperiod influence performance of dairy animals. Acclimation to thermal stress is now identified as a homeorhetic process under endocrine control. The process of acclimation occurs in 2 phases (acute and chronic) and involves changes in secretion rate of hormones as well as receptor populations in target tissues. The time required to complete both phases is weeks rather than days. The opportunity may exist to modify endocrine status of animals and improve their resistance to heat and cold stress. New estimates of genotype×environment interactions support use of recently available molecular and genomics tools to identify the genetic basis of heat-stress sensitivity and tolerance. Improved understanding of environmental effects on nutrient requirements has resulted in diets for dairy animals during different weather conditions. Demonstration that estrus behavior is adversely affected by heat stress has led to increased use of timed insemination schemes during the warm summer months to improve conception rates by discarding the need to detect estrus. Studies evaluating the effects of heat stress on embryonic survival support use of cooling during the immediate postbreeding period and use of embryo transfer to improve pregnancy rates. Successful cooling strategies for lactating dairy cows are based on maximizing available routes of heat exchange, convection, conduction, radiation, and evaporation. Areas in dairy operations in which cooling systems have been used to enhance cow comfort, improve milk production, reproductive efficiency, and profit include both housing and milking facilities. Currently, air movement (fans), wetting (soaking) the cow's body surface, high pressure mist (evaporation) to cool the air in the cows’ environment, and facilities designed to minimize the transfer of solar radiation are used for heat abatement. Finally, improved understanding of photoperiod effects on cattle has allowed producers to maximize beneficial effects of photoperiod length while minimizing negative effects.

Environmental stress is one of the most significant factors affecting livestock performance and health, and it is only expected to increase with effects of global warming.Environmental Physiology of Livestock brings together the latest research on environmental physiology, summarizing progress in the field and providing directions for future.

Dairy products are the main source of conjugated linoleic acid (CLA), a functional food component with health benefits. The major source of cis-9, trans-11 CLA in milk fat is endogenous synthesis via delta9-desaturase from trans-11 18:1, with the remainder from incomplete rumen biohydrogenation of linoleic acid. Diet has a major influence on milk fat CLA; however, effects of physiological factors have received little attention. Our objectives were to examine milk fat content of CLA and the CLA-desaturase index with regard to: 1) effect of breed, parity, and stage of lactation, and 2) variation among individuals and the relationship to milk and milk fat. Holstein (n = 113) and Brown Swiss (n = 106) cows were fed a single diet and milk sampled on the same day to avoid confounding effects of diet and season. Frequency distributions demonstrated that milk fat content of CLA and CLA-desaturase index varied over threefold among individuals, and this needs to be considered in the design of experiments. Holsteins had a higher milk fat content of CLA and CLA-desaturase index, but breed differences were minor. Parity and days in milk also had little or no relationship to the individual variation for these two CLA variables. Breed, parity, and days in milk accounted for < 0.1, < 0.3, and < 2.0% of total variation in CLA concentration in milk fat, respectively. Milk fat content of CLA and CLA-desaturase index were essentially independent of milk yield, milk fat percent, and milk fat yield. We speculate that the basis for the genetic variation among individuals is related to rumen output of trans-11 18:1 and to a lesser extent cis-9, trans-11 CLA, and to the tissue amount and activity of delta9-desaturase.

The cellular heat stress (HS) response is one component of the acute systemic response to HS. Gene networks within and across cells and tissues respond to environmental heat loads above the thermoneutral zone with both intra- and extracellular signals that coordinate cellular and whole-animal metabolism. Activation of these systems appears to be initiated at skin surface temperatures exceeding 35°C as animals begin to store heat and rapidly increase evaporative heat loss (EVHL) mechanisms. Gene expression changes include 1) activation of heat shock transcription factor 1 (HSF1); 2) increased expression of heat shock proteins (HSP) and decreased expression and synthesis of other proteins; 3) increased glucose and amino acid oxidation and reduced fatty acid metabolism; 4) endocrine system activation of the stress response; and 5) immune system activation via extracellular secretion of HSP. If the stress persists, these gene expression changes lead to an altered physiological state referred to as \"acclimation,\" a process largely controlled by the endocrine system. In the acclimated state, metabolism is adjusted to minimize detrimental effects of increased thermal heat load. The role of secreted HSP in feedback regulation of the immune and endocrine system has not yet been investigated. The variation in EVHL among animals and the central role that HSF1 has in coordinating thermal tolerance suggest that there is opportunity to improve thermal tolerance via gene manipulation. Determining the basis for altered energy metabolism during thermal stress will lead to opportunities for improved animal performance via altered nutritional management.

A major goal in understanding the pathogenesis of the anthrax bacillus is to determine how the protective antigen (PA) pore mediates translocation of the enzymatic components of anthrax toxin across membranes. To obtain structural insights into this mechanism, we constructed PA-pore membrane complexes and visualized them by using negative-stain electron microscopy. Two populations of PA pores were visualized in membranes, vesicle-inserted and nanodisc-inserted, allowing us to reconstruct two virtually identical PA-pore structures at 22-Å resolution. Reconstruction of a domain 4-truncated PA pore inserted into nanodiscs showed that this domain does not significantly influence pore structure. Normal mode flexible fitting of the x-ray crystallographic coordinates of the PA prepore indicated that a prominent flange observed within the pore lumen is formed by the convergence of mobile loops carrying Phe427, a residue known to catalyze protein translocation. Our results have identified the location of a crucial functional element of the PA pore and documented the value of combining nanodisc technology with electron microscopy to examine the structures of membrane-interactive proteins.

Heat stress (HS) is a multibillion-dollar problem for the global dairy industry, and reduced milk yield is the primary contributor to this annual economic loss. Feed intake declines precipitously during HS but accounts for only about 35% of the decreased milk synthesis, indicating that the physiological mechanisms responsible for decreased milk production during HS are only partly understood. Thus, our experimental objectives were to characterize the direct effects of HS on the somatotropic axis, a primary regulator of metabolism and milk yield. We recently reported no differences in mean growth hormone (GH) concentrations, GH pulsatility characteristics, or GH response to growth hormone releasing factor in HS versus pair-fed (PF) thermoneutral controls. Despite similarities in circulating GH characteristics, plasma insulin-like growth factor (IGF)-I concentrations were reduced during heat stress conditions but not in PF animals, suggesting that uncoupling of the hepatic GH-IGF axis may occur during HS. We investigated this possibility by measuring proximal indicators of hepatic GH signaling following a GH bolus. Heat stress but not PF decreased abundance of the GH receptor and GH-dependent signal transducer and activator of transcription (STAT)-5 phosphorylation. Consistent with reduced GH signaling through STAT-5, basal hepatic IGF-I mRNA abundance was lower in HS cows. Thus, the reduced hepatic GH responsiveness (in terms of IGF-I gene expression) observed during HS appears to involve mechanisms at least partially independent of reduced nutrient intake. The physiological significance of reduced hepatic GH receptor abundance during HS is unclear at this time. Aside from reducing IGF-I production, it may reduce other GH-sensitive bioenergetic processes such as gluconeogenesis.

The dry period is required to facilitate cell turnover in the bovine mammary gland in order to optimize milk yield in the next lactation. Traditionally, an 8-week dry period has been a standard management practice for dairy cows based on retrospective analyses of milk yields following various dry period lengths. However, as milk production per cow has increased, transitioning cows from the nonlactating state to peak milk yield has grown more problematic. This has prompted new studies on dry period requirements for dairy cows. These studies indicate a clear parity effect on dry period requirement. First parity animals require a 60-day dry period, whereas lactations following later parities demonstrate no negative impact with 30-day dry period or even eliminating the dry period when somatotropin (ST) is also used to maintain milk yields. Shortened dry periods in first parity animals were associated with reduced mammary cell turnover during the dry period and early lactation and increased numbers of senescent cells and reduced functionality of lactating alveolar mammary cells postpartum. Use of ST and increased milking frequency postpartum reduced the impact of shortened dry periods. The majority of new intramammary infections occur during the dry period and persist into the following lactation. There is therefore the possibility of altering mastitis incidence by modifying or eliminating the dry period in older parity animals. As the composition of mammary secretions including immunoglobulins may be reduced when the dry period is reduced or eliminated, there is the possibility that the immune status of cows during the peripartum period is influenced by the length of the dry period.

To evaluate effects of different dry period lengths on milk yield, milk composition, and energy balance of dairy cows, 122 multiparous and primiparous Holstein dairy cows were used in a completely randomized experimental design with 56-, 42-, and 35-d dry period lengths. Actual dry period lengths for respective treatments (TRT) were 56±5.1 d, 42±2.1 d, and 35±2.7 d. Overall, cows in the 42- and 56-d TRT gained more body condition than those in 35-d TRT during the dry period; however, postpartum body condition score did not change substantially among the TRT. Although from 3 to 210 DIM, differences were not detected in the milk yield of multiparous cows between the 35- and 56-d TRT, primiparous cows in the 35-d TRT produced less milk than those in 56-d TRT. In primiparous cows, the milk production at wk 9, 10, and 11 of lactation was lower in the 35-d compared with the 56-d TRT. Primiparous cows in the 35-d compared with the 56-d TRT produced less milk protein. In the 35-d TRT, serum triglyceride concentration was greater in primiparous cows than in multiparous cows during the peripartum period. Among primiparous cows, those in the 56-d TRT had greater concentrations of nonesterified fatty acids than those in the 35-d TRT during the peripartum period. No significant differences were observed in concentrations of serum glucose, insulin, and insulin-like growth factor-I during early lactation among TRT. There was also no difference among TRT for incidence of metabolic disorders. Thus, this study indicates that shortening the dry period to 35 d may be beneficial in multiparous and overconditioned cows, but not in primiparous cows.

Selecting domestic animals for tolerance to thermal stress (TS) has been counterproductive, because acclimation involves reducing or diverting metabolizable energy from production to balance heat gain and loss. Ideally, simultaneous selection for increased production and improved thermotolerance is desirable, but to accomplish this at the genomic level the genes associated with acclimation, adaptation, and thermo-tolerance need to be identified. We evaluated the effects of TS on mammary development and gene expression in vitro using a bovine mammary epithelial cell collagen gel culture system. Acute TS was characterized by inhibition and regression of the ductal branches. Gene expression profiling revealed an overall upregulation of genes associated with the stress response and protein repair. In contrast, genes associated with cellular and mammary epithelial cell-specific biosynthesis, metabolism, and morphogenesis were generally downregulated by TS. Future studies will examine the impact of acclimation and adaptation on gene expression to identify those genes associated with acquisition of thermal tolerance.