Critical THI thresholds based on the physiological parameters of lactating dairy cows

The severity of heat stress conditions in high-yielding dairy cows is currently underestimated. The present study aimed to determine the heat load threshold of the temperature-humidity index (THI) on physiological parameters of lactating Holstein-Friesian cows under a continental climatic zone in Germany. Physiological parameter measurements, such as respiration rate (RR), measured hourly, and heart rate (HR) and rectal temperature (RT), both measured twice daily, were performed in a total of 139 multiparous cows on three randomly chosen measurement days per week. In addition, the ambient temperature and relative humidity of the barn were recorded every 5 min to calculate the current THI. The physiological parameter data were linked to the THI, and the heat load thresholds were determined using the broken-stick model. The heat load duration effect of each physiological parameter was obtained by regression analysis. Considering the increases in the physiological parameters, our study provided reliable data to determine heat load thresholds for lactating high-yielding dairy cows in a moderate climatic zone. The heat load threshold could be determined for RR in standing cows (THI = 70) and lying cows (THI = 65) and for HR (THI = 72) and RT (THI = 70) in standing cows. The heat load duration also demonstrated a significant effect on the increases in physiological parameters among dairy cows. In particular, the present study enabled a strategy to be devised to initiate heat mitigation in high-yielding dairy cows when they are exposed to THIs above 65.     

Evaporative cooling for Holstein dairy cows under grazing conditions

Twenty-four grazing Holstein cows in mid and late lactation were randomly assigned to two treatment groups: control and cooled. The trial was performed at the Experimental Dairy Unit, Rafaela Agricultural Experimental Station (INTA), Argentina. The objective was to evaluate the effects of sprinkler and fan cooling before milkings on milk production and composition. The effects of the cooling system on rectal temperature and respiration rate were also evaluated. Cooled cows showed higher milk production (1.04 l cow^–1 day^–1). The concentration and yield of milk fat and protein increased in response to cooling treatment. The cooling system also reduced rectal temperature and respiration rate. No effects were observed on body condition. It was concluded that evaporative cooling, which is efficient for housed animals, is also appropriate to improve yields and animal well-being under grazing systems. These results are impressive since the cooling system was utilized only before milkings, in a system where environmental control is very difficult to achieve. This trial was performed during a mild summer. The results would probably be magnified during hotter weather.     

Metabolic conditions of lactating Friesian cows during the hot season in the Po valley. 1. Blood indicators of heat stress

In two consecutive summers, 21 and 18 cows, respectively, were monitored for blood parameters and milk yield, to assess their variation according to the level of heat stress at different stages of lactation. During both years, the cows were monitored according to their lactation phase (early, mid-, and late) at the beginning of the summer. Climatic conditions were described through the temperature humidity index. Cows were monitored for breathing rate, rectal temperature, blood metabolites and enzymes, and milk yield. In the first year, two hotter periods were identified, with more severe conditions in the second one, when cows had rectal temperatures higher than 40°C. In the second year, only one hotter period was identified, with a heat stress comparable to that of the first period of the first year. Milk yield declined during the hotter period; in both years, a higher reduction in milk yield was recorded for cows in the mid-stage of lactation. The decrease in plasma glucose during the hotter period was evident in both years. Plasma cholesterol also decreased in those periods. The reduction in plasma ALP activity in the hotter period of both years confirmed the role of this enzyme as a quick and reliable heat-stress blood marker.     

Effects of feeding supplemental palmitic acid (C16:0) on performance and milk fatty acid profile of lactating dairy cows under summer heat

The objective was to determine performance and milk fatty acid changes of high producing dairy cows in early lactation, under summer heat, by adding a supplemental rumen inert fat in the form of a saturated free fatty acid (856 g/kg C16:0/kg of total fatty acids) to the total mixed ration (TMR). Early lactation multiparous Holstein cows in two similar pens of 99 and 115 cows were used in a 2 × 2 Latin Square design experiment with 35 d periods during a period when daily high and low temperatures averaged 34.3 and 15.9 °C, the relative humidity averaged 51% and there were no rain events. The TMR was the same for both groups, consisting of approximately 435 g/kg forage and 565 g/kg concentrate, except that the vitamin/mineral premix had no added fat (control, C) or added fat (C16:0) at a level designed to deliver approximately 450 g/cow/d of supplemental fat if cows consumed 26.5 kg/d of dry matter (DM). The two TMR averaged 905 g/kg organic matter (OM), 318 g/kg neutral detergent fiber (aNDF), and 186 g/kg crude protein (CP). The ‘C’ TMR had 58 g/kg total fatty acids with an estimated net energy for lactation (NE_l) of 7.3 MJ/kg (DM), while the C16:0 TMR had 72 g/kg total fatty acids and 7.5 MJ/kg NE_l (DM). Whole tract digestibility of DM, OM, aNDF and CP tended (P<0.10) to increase, and that of fatty acids increased substantially (P<0.01), with C16:0 feeding, whereas, DM intake was not affected. Milk fat content decreased (P<0.01) with C16:0 feeding (37.5 versus 36.0 g/kg), whereas, true protein content tended (P=0.09) to increase. There was a tendency (P=0.07) for increased milk yield (36.69 versus 38.04 kg/d), while milk protein yield increased (P=0.03) with C16:0 supplementation (1.08 versus 1.13 kg/d). Milk fat yield was unaffected by treatment. Concentrations of short and medium chain milk fatty acids (C6:0–C15:0), decreased, or tended to decrease, with C16:0 addition (C13:0 and C15:0, P<0.10; all others, P≤0.05). The concentration of C16:0 increased (P<0.001) in milk triglycerides from cows fed C16:0 (27.10 versus 31.57 g/kg), the longer chain saturated fatty acids C17:0 and C18:0 decreased (P≤0.05) and other long chain unsaturated fatty acids were unaffected. Benefits of C16:0 feeding on cow productivity must be balanced against negative effects on the nutritive value of the milk (i.e., increased C16:0 in milk fatty acids) produced for human consumption. However, relatively low amounts of supplemental C16:0 (27.10 versus 31.57 g/kg in milk triglycerides for C and C16:0 supplemented cows, respectively) were actually secreted in milk, in spite of them being essentially fully digested in the digestive tract. Strategies to divide cows into production groups based on milk yield and/or milk fat proportions could further limit C16:0 secretion in milk. Supplemental dietary C16:0 may have positive effects on milk production that outweigh the negative health effects of the increased C16:0 content in the milk fat.     

Adaptive responses to fescue toxicosis under thermoneutral and heat stress conditions

This study determined the potential for short-term adaptation to fescue toxicosis and heat stress in rats. Male CD outbred rats (n=24) were implanted with temperature transmitters (Respironics, Bend, OR) to measure core temperature (Tc) and general activity. All rats were initially fed diets with ground, uninfected tall fescue seed (E−) and exposed to 21 °C (thermoneutral, TN) to establish baseline values. In Period 1, all groups were maintained at TN for 7 days, with one group fed a diet containing ground, endophyte-infected tall fescue seed (E+, approximately 165 μg ergovaline/kg BW/d) and two groups fed E− diet. Ergovaline is thought to be the primary toxin responsible for many symptoms associated with fescue toxicosis. Period 1 was followed by 7 days at 31 °C (heat stress, HS, Period 2) on the same diets. All animals were fed E− diet during the second 7 day of HS (Period 3). In the final 7 day (Period 4), E+ diet was returned to the original group and fed to one of the previously E− groups, with the third group remaining on E− diet. A 40% decrease in FI occurred with E+ treatment at TN (P<0.05), with a comparable BW reduction (P<0.05) after 4 day. Both responses worsened during HS. Treatment with E+ in Period 4 indicated that FI and BW had not adapted to fescue toxicosis. A reduction in daily Tc occurred with E+ treatment at TN (P<0.05) followed by hyperthermia during the initial stage of HS (P<0.05). Although feed intake and growth rate showed no change over time, there was a reduction in fescue toxicosis-induced hyperthermia in the heat with repeat treatment. Conditioning animals to fescue toxicosis and heat stress prior to exposure may be beneficial in reducing impacts on thermal status of the animal.     

Pregnancy rates following timed embryo transfer with fresh or vitrified in vitro produced embryos in lactating dairy cows under heat stress conditions

Timed embryo transfer (TET) using in vitro produced (IVP) embryos without estrus detection can be used to reduce adverse effects of heat stress on fertility. One limitation is the poor survival of IVP embryos after cryopreservation. Objectives of this study were to confirm beneficial effects of TET on pregnancy rate during heat stress as compared to timed artificial insemination (TAI), and to determine if cryopreservation by vitrification could improve survival of IVP embryos transferred to dairy cattle under heat stress conditions. For vitrified embryos (TET-V), a three-step pre-equilibration procedure was used to vitrify excellent and good quality Day 7 IVP Holstein blastocysts. For fresh IVP embryos (TET-F), Holstein oocytes were matured and fertilized; resultant embryos were cultured in modified KSOM for 7 days using the same method as for production of vitrified embryos. Excellent and good quality blastocysts on Day 7 were transported to the cooperating dairy in a portable incubator. Nonpregnant, lactating Holsteins (n = 155) were treated with GnRH (100 microg, i.m., Day 0), followed 7 days later by prostaglandin F2alpha (PGF2alpha, 25 mg, i.m.) and GnRH (100 microg) on Day 9. Cows in the TAI treatment (n = 68) were inseminated the next day (Day 10) with semen from a single bull that also was used to produce embryos. Cows in the other treatments (n = 33 for TET-F; n = 54 for TET-V) received an embryo on Day 17 (i.e. Day 7 after anticipated ovulation and Day 8 after second GnRH treatment). The proportion of cows that responded to synchronization based on plasma progesterone concentrations on Day 10 and Day 17 was 67.7%. Pregnancy rate for all cows on Day 45 was higher (P < 0.05) in the TET-F treatment than for the TAI and TET-V treatments (19.0 +/- 5.0,6.2 +/- 3.6, and 6.5 +/- 4.1%). For cows responding to synchronization, pregnancy rate was also higher (P < 0.05) for TET-F than for other treatments (26.7 +/- 6.4, 5.0 +/- 4.3, and 7.4 +/- 4.7%). In the TET-F treatment group, cows producing more milk had lower (P < 0.05) pregnancy rates than cows producing less milk. In conclusion, ET of fresh IVP embryos can improve pregnancy rate under heat stress conditions, but pregnancy rate following transfer of vitrified embryos was no better than that following TAI.     

Microclimate modeling in naturally ventilated dairy barns during the hot season: Checking the accuracy of forecasts

Monitoring and predicting the microclimate in naturally ventilated barns (NVB) is important given the adverse effects of high summer temperatures on dairy cows in the context of global climate change. The aim of the work was to verify the accuracy of the microclimate forecast in a NVB using linear regression (LR). Our working hypothesis suggested that multiple periodic measurements of air temperature and relative humidity outside and inside the barns at the same time will allow us to build LR models for predicting the temperature-humidity index (THI). This was done not only for a specific dairy barn based on this indicator outside, but also in other dairy barns with a similar design, located in similar weather conditions. The results of the research indicate that the use of LR had a high accuracy of forecasting (93–96%) the THI in NVB of various designs during the summer heat. At the same time, differences were found between traits (air temperature, relative humidity as well as resulting THI) provided by meteorological weather stations and these data measured simultaneously next to the dairy barns. The proposed LR models can be used to predict THI in NVBs of different designs.     

Effect of feeding Saccharomyces Cerevisiae on performance of dairy cows during summer heat stress

Effects of feeding a culture of Saccharomyces cerevisiae to lactating cows on their lactational performance during heat stress were determined. Multiparous Holstein cows (n = 723) calving during the summer months from two dairy farms were randomly assigned to a diet containing no yeast culture (control; n = 361) or 30 g/d of a S. cerevisiae yeast culture (YC; n = 362) fed from 20 to 140 d in milk (DIM). Cows were milked twice daily and the production of milk and milk components was measured every 2 weeks. Dry matter (DM) intakes from 6 pens were measured daily and pen temperature and humidity were evaluated hourly from June to November. Rectal temperature was measured in 88 cows (22/treatment/farm), once weekly, and blood was sampled from a subset of 120 cows at 58 and 100 DIM for measurements of plasma glucose, nonesterified fatty acids, 3-OH-butyrate, insulin, and urea N concentrations. Daily temperature, humidity and the temperature-humidity index in the study pens did not differ between treatments, and rectal temperature of cows in the control and YC treatments differed with days postpartum. Intake of DM was similar between diets, but cows fed YC produced 1.2 kg/d more milk, more milk true protein, solids-not-fat and lactose than that produced by control cows. However, energy-corrected milk yield, and concentrations of true protein, solids-not-fat and lactose did not differ between treatments. Feeding YC did not influence plasma metabolites, insulin, or body condition score of cows, but urea N concentrations were reduced. Feeding a yeast culture of S. cerevisiae improved yields of milk and milk components in heat-stressed multiparous Holstein cows.     

Fertility and hormonal responses to temporary relief of heat stress in lactating dairy cows

The influence of temporary cooling on pregnancy rate and ovarian secretion of cortisol, estradiol and progesterone in Holstein cows was evaluated during the months of June to September. Cows were randomly assigned to one of five treatment groups: 1) evaporative cooling, 8 d; 2) evaporative cooling, 16 d; 3) refrigerated air conditioning, 8 d; 4) refrigerated air conditioning, 16 d and 5) controls maintained outdoors with access to shade. Estrus was synchronized by giving two injections of prostaglandin (25 mg). Cows in the cooled groups were placed in box stalls at the time of the second prostaglandin injection, allowing them to be cooled for approximately 3 d prior to breeding. Blood samples were taken on days 0, 5, 10, 15, 20 and 25 postbreeding and serum was analyzed for estradiol, cortisol, progesterone and a pregnancy-specific protein. Pregnancy rates as determined by palpation per rectum at 40 to 60 d post breeding were not different between cows in cooled treatments, regardless of duration or type of cooling. Pregnancy rate was higher (P<0.05) in cooled as compared to control cows. Serum progesterone concentrations were higher on Day 15 in cooled cows as compared to control cows. No differences in serum estradiol or cortisol concentrations were observed between cooled and control cows. Pregnancy-specific protein determinations indicated that 50% of both cooled and control cows conceived. Results indicate that temporary cooling increased pregnancy rates by increasing embryonic survival rates.     

Genetic dissection of reproductive performance of dairy cows under heat stress

Heat stress negatively impacts the reproductive performance of dairy cows. The main objective of this study was to dissect the genetic basis underlying dairy cow fertility under heat stress conditions. Our first goal was to estimate genetic components of cow conception across lactations considering heat stress. Our second goal was to reveal individual genes and functional gene-sets that explain a cow’s ability to conceive under thermal stress. Data consisted of 74 221 insemination records on 13 704 Holstein cows. Multitrait linear repeatability test-day models with random regressions on a function of temperature–humidity index values were used for the analyses. Heritability estimates for cow conception under heat stress were around 2–3%, whereas genetic correlations between general and thermotolerance additive genetic effects were negative and ranged between −0.35 and −0.82, indicating an unfavorable relationship between cows’ ability to conceive under thermo-neutral vs. thermo-stress conditions. Whole-genome scans identified at least six genomic regions on BTA1, BTA10, BTA11, BTA17, BTA21 and BTA23 associated with conception under thermal stress. These regions harbor candidate genes such as BRWD1, EXD2, ADAM20, EPAS1, TAOK3, and NOS1, which are directly implicated in reproductive functions and cellular response to heat stress. The gene-set enrichment analysis revealed functional terms related to fertilization, developmental biology, heat shock proteins and oxidative stress, among others. Overall, our findings contribute to a better understanding of the genetics underlying the reproductive performance of dairy cattle under heat stress conditions and point out novel genomic strategies for improving thermotolerance and fertility via marker-assisted breeding.     

Effects of acute and chronic heat stress on feed sorting behaviour of lactating dairy cows

Nutritional strategies to mitigate the negative effects of heat stress on animal welfare and productivity often involve changes in ration formulation. However, cattle commonly sort their ration in favour of certain components, and it is not clear how feed sorting responds to heat stress. This study investigated the association between heat stress and feed sorting behaviour. Lactating Holstein dairy cows (n = 32; parity = 2.8±1.2; mean±SD) were housed in a free stall barn and milked 3×/day. Cows were fed individually using the Calan Broadbent Feeding System and offered ad libitum access to a total mixed ration (containing on a dry matter basis: 3.3% ryegrass hay, 16.5% ryegrass baleage, 24.7% corn silage, 11.1% brewers grains, 19.7% ground corn, 19.8% concentrate and 4.9% protein/mineral supplement), provided 1×/day. Beginning at 186±60 days in milk, cows were exposed to either: heat stress conditions (HT; n = 15) (average temperature–humidity index: 77.6), or evaporative cooling (CL; n = 17), consisting of misters and fans over the freestall and feed bunks. Data were collected during a 4-day baseline period, and two 4-day experimental periods: starting at 10 days after implementing treatments (defined as acute heat stress for HT cows), and at 62 days after implementing treatments (defined as chronic heat stress for HT cows). Daily feed intake and physiological responses to heat stress (body temperature, respiration rate) were recorded. Samples of fresh and refused feed were collected daily from individual cows for particle size analysis. The particle size separator had three screens (19, 8 and 1.18 mm) and a bottom pan, resulting in 4 fractions (long, medium, short and fine particles). Feed sorting was calculated as the actual intake of each particle size fraction expressed as a percentage of the predicted intake of that fraction. During both heat stress periods, HT cows sorted for long particles more than CL cows (105.0% v. 100.6%; SE = 1.1). During acute heat stress, HT cows sorted to a greater extent than CL cows against medium and short particles, whereas sorting of these fractions did not differ during chronic heat stress. Body temperature and respiration rate were associated across treatments with the extent of sorting for long particles and against short particles during acute heat stress. These results suggest that feed sorting is particularly influenced during acute heat stress, and that sorting for longer particles may increase in heat stress.     

mTOR regulates the nucleoplasmic diffusion of Xrn2 under conditions of heat stress

Stress induces various responses, including translational suppression and tRNA degradation in mammals. Previously, we showed that heat stress induces degradation of initiator tRNA^Met (iMet) through 5′–3′ exoribonuclease Xrn1 and Xrn2, respectively. In addition, we found that rapamycin inhibits the degradation of iMet under heat stress conditions. Here, we report that the mammalian target of rapamycin (mTOR) regulates the diffusion of Xrn2 from the nucleolus to the nucleoplasm, facilitating the degradation of iMet under conditions of heat stress. Our results suggest a mechanism of translational suppression through mTOR-regulated iMet degradation in mammalian cells.     

Oxygen and steroid concentrations in preovulatory follicles of lactating dairy cows exposed to acute heat stress

Maternal heat stress reduces oocyte competence for fertilization and post-fertilization development, but the mechanism is unknown. The present experiment investigated two potential mechanisms: (1) reduced oxygen delivery to the preovulatory follicle (due to increased thermoregulatory vascular perfusion of skin and respiratory tract); (2) reduced follicular steroid synthesis. These hypotheses were tested by measuring the fractional concentration of oxygen and concentrations of estradiol-17beta and progesterone in follicular fluid of the preovulatory follicle of lactating Holstein cows. Estrous cycles were synchronized using GnRH on Day -9 and PGF(2alpha) on Day -2. On Day 0, all cows without a CL and with a large preovulatory follicle were assigned to control or heat stress treatments for 1d (beginning at 1030 h). Between 4 and 6 h after treatment (1430-1630 h), follicular fluid was aspirated by transvaginal puncture, and fractional oxygen concentration in follicular fluid of the dominant follicle was determined with a fluorometric fiber-optic oxygen sensor. There was no significant effect of heat stress on follicular fluid P(O2) or concentrations of estradiol-17beta or progesterone among cows that had follicular fluid steroid concentrations considered typical of a preovulatory follicle. Follicular oxygen concentration was 6.9+/-0.4% for control cows and 7.3+/-0.3% for heat-stressed cows. Oxygen concentration tended to be inversely correlated to follicular diameter (P=0.09). In conclusion, it was unlikely that reduced oocyte competence due to acute heat stress was caused by reductions in follicular concentrations of oxygen, estradiol-17beta, or progesterone.     

Assessing heat tolerance through productive vs physiological indicators. Data from dairy sheep under on-farm conditions

The search for criteria that allow the quantification of the level of thermotolerance of an animal is a major challenge in animal production. Different criteria have been proposed to date, mainly the use of routine milk recording and weather information or the collection of physiological measures related with heat stress. This study aimed at quantifying the association between indicators of heat tolerance derived from productive and physiological traits. For this purpose, two physiological traits, rectal temperature (RT) and respiratory rate (RR), and nine productive traits (milk yield, fat, protein and lactose yields and contents, casein and urea contents) were measured from June to September of 2018 in three flocks of Manchega sheep. A total of 462 lactating ewes participated in the study. Air temperature (Ta), relative humidity (RH) and associated temperature and humidity index (THI) were recorded inside the barn and also obtained from the closest weather station from the national meteorological network, and used to produce several measurements of heat load on animals. Based on the results of fits for quadratic and cubic regressions on the alternative heat load measures, the cubic regression on Ta and THI obtained inside the barn at time of recording yielded the best fit for physiological and productive parameters. The use of weather information taken from the official weather station closest to the farm also produced similar estimates and could be considered as a good alternative when on-farm meteorological data are not available. Two-trait random regression models that involved individual intercept and slope of response to heat load were used to obtain correlations between basal levels and heat tolerance within and across traits. Estimated correlations showed that animals with smaller vs larger basal levels of RT and RR tend to be more vs less heat tolerant (correlations up to 0.46) and that slopes of increase for RR and RT under heat stress were highly correlated (0.82). Estimated correlations between tolerance criteria from production vs physiology were up to ?0.5 (between milk yield and RT), indicating that animals that show less increase in body temperature also tend to show a smaller decrease in production under heat stress. However, because of the non-unity correlation between the two types of indicators of heat tolerance, both sources of information, productive and physiological ought to be taken into account to ensure the long-term sustainability of selection programmes aiming at improving productive levels when heat stress is a concerning issue.     

Carry-over effect of summer thermal stress on characteristics of the preovulatory follicle of lactating cows

Low fertility of cattle during the cool autumn is mostly associated with exposure to thermal stress in the previous summer. This study examined carry-over effects of summer heat stress and induction of enhanced follicular turnover, on the characteristics of preovulatory follicles during autumn. Holstein cows were treated with PGF₂ and GnRH to induce six successive 9-day follicular waves during autumn. Intact cows served as controls. Follicular fluid concentrations of androstenedione were higher in treated cows, but those of IGF-I and progesterone did not differ between groups. The concentration of estradiol at the end of the treatment was higher, and the content per follicle increased with time and was numerically higher in treated cows. The volume of follicular fluid increased during the autumn and was greater in treated cows. A delayed effect of heat stress on preovulatory follicles was noted, and induction of follicular waves by GnRH was able to improve follicular characteristics in autumn.     

Genotype effects on body temperature in dairy cows under grazing conditions in a hot climate including evidence for heterosis

We compared diurnal patterns of vaginal temperature in lactating cows under grazing conditions to evaluate genotype effects on body temperature regulation. Genotypes evaluated were Holstein, Jersey, Jersey × Holstein and Swedish Red × Holstein. The comparison of Holstein and Jersey versus Jersey × Holstein provided a test of whether heterosis effects body temperature regulation. Cows were fitted with intravaginal temperature recording devices that measured vaginal temperature every 15 min for 7 days. Vaginal temperature was affected by time of day (P < 0.0001) and genotype × time (P < 0.0001) regardless of whether days in milk and milk yield were used as covariates. Additional analyses indicated that the Swedish Red × Holstein had a different pattern of vaginal temperatures than the other three genotypes (Swedish Red × Holstein vs others × time; P < 0.0001) and that Holstein and Jersey had a different pattern than Jersey × Holstein [(Holstein + Jersey vs Jersey × Holstein) × time, P < 0.0001]. However, Holstein had a similar pattern to Jersey [(Holstein vs Jersey) × time, P > 0.10]. These genotype × time interactions reflect two effects. First, Swedish Red × Holstein had higher vaginal temperatures than the other genotypes in the late morning and afternoon but not after the evening milking. Secondly, Jersey × Holstein had lower vaginal temperatures than other genotypes in the late morning and afternoon and again in the late night and early morning. Results point out that there are effects of specific genotypes and evidence for heterosis on regulation of body temperature of lactating cows maintained under grazing conditions and suggest that genetic improvement for thermotolerance through breed choice or genetic selection is possible.     

Modeling conductive cooling for thermally stressed dairy cows

Conductive cooling, which is based on direct contact between a cow lying down and a cooled surface (water mattress, or any other heat exchanger embedded under the bedding), allows heat transfer from the cow to the cooled surface, and thus alleviate heat stress of the cow. Conductive cooling is a novel technology that has the potential to reduce the consumption of energy and water in cooling dairy cows compared to some current practices. A three-dimensional conduction model that simulates cooling thermally-stressed dairy cows was developed. The model used a computational fluid dynamics (CFD) method to characterize the air-flow field surrounding the animal model. The flow field was obtained by solving the continuity and the momentum equations. The heat exchange between the animal and the cooled water mattress as well as between the animal and ambient air was determined by solving the energy equation. The relative humidity was characterized using the species transport equation. The conduction 3-D model was validated against experimental temperature data and the agreement was very good (average error is 4.4% and the range is 1.9–8.3%) for a mesh size of 1117202. Sensitivity analyses were conducted between heat losses (sensible and latent) with respect to air temperature, relative humidity, air velocity, and level of wetness of skin surface to determine which of the parameters affect heat flux more than others. Heat flux was more sensitive to air temperature and level of wetness of the skin surface and less sensitive to relative humidity.     

Genetic improvement of egg laying traits in Fayoumi chickens bred under conditions of heat stress through selection and gene expression studies

Thermal stress has been shown to result in decreased egg production, decreased eggshell quality, and ultimately millions of dollars in losses to the industry. Therefore, there are many factors to consider when implementing genetic selection programs aimed at improving egg production under tropical conditions. So, trial is trying to improve the productivity and eggshell quality traits of the Fayoumi chicken under high ambient temperatures via selection programs and gene expression. In the present study, day-old Fayoumi chicks were raised either under normal temperature (control) or conditions of thermal stress (the heated group). At 35 weeks, male and female chickens from the control group were mated randomly and females selected for higher egg production and eggshell strength were mated to male siblings to obtain the progeny of the first generation (F₁). F₁ birds were further selected and mated to obtain the progeny of the second generation. Our results show that egg production and eggshell strength traits improved over successive generations via selection under conditions of heat stress. Furthermore, the reduction in egg production and eggshell strength as a result of heat stress declined from one generation to the next in birds selected for good heat tolerance, and an inverse relationship was observed between the OC-17 and eggshell strength. Additionally, levels of HSP90 and gene expression increased in the two successive generations, indicating that both productivity and heat tolerance were enhanced due to selection in birds raised under conditions of thermal stress. Moreover, generation exerted an important effect on this trait. Thus, desirable traits such as improved heat tolerance in producing lines were observed in Fayoumi chickens exposed to conditions of thermal stress via selection. Therefore, modern advances in studies of poultry breeding and genetics, such as gene expression studies, should be examined.