Effects of prepartum roughage neutral detergent fiber levels on periparturient dry matter intake, metabolism, and lactation in heat-stressed dairy cows

Heat stress of lactating cattle results in dramatic reductions in dry matter intake (DMI). As a result, energy input cannot satisfy energy needs and thus accelerates body fat mobilization. Decreasing the level of roughage neutral detergent fiber (NDF) in prepartum diets, and thereby increasing the amount of nonfiber carbohydrates, may provide an adequate supply of energy and glucose precursors to maintain and minimize the decrease in DMI while reducing mobilization of adipose tissue. The effects of 3-wk prepartum diets containing different amounts of roughage NDF on DMI, blood metabolites, and lactation performance of dairy cows were investigated under summer conditions in Thailand. Thirty cross-bred cows (87.5% Holstein × 12.5% Sahiwal) were dried off 60 d before their expected calving date and were assigned immediately to a nonlactating cow diet containing the net energy for lactation recommended by the National Research Council (2001) model. The treatment diets contained 17.4, 19.2, and 21.0% DM as roughage NDF from bana grass (Pennisetum purpureum × Pennisetum glaucum) silage. Levels of concentrate NDF were 39.8, 40.2, and 38.6% of dietary NDF, so the levels of dietary NDF were 28.9, 32.1, and 34.2% of DM. After parturition, all cows received a lactating cow diet containing 12.7% roughage NDF and 23% dietary NDF. During the entire experiment, the minimum and maximum temperature-humidity index averaged 77.7 and 86.8, respectively, indicating conditions appropriate for the induction of extreme heat stress. As parturition approached, DMI decreased steadily, resulting in a 12.9, 25, and 32.8% decrease in DMI from d −21 until calving for nonlactating cows fed prepartum diets containing 17.4, 19.2, and 21% roughage NDF, respectively. During the 3-wk prepartum period, intakes of DM and net energy for lactation and concentrations of plasma glucose and serum insulin were higher for cows fed diets containing less roughage NDF. In cows fed the 3-wk prepartum diets containing less roughage NDF, calf birth weights, milk yield, and 4% fat-corrected milk were higher, whereas periparturient concentrations of serum nonesterified fatty acids and plasma β-hydroxybutyrate were lower. There was a carryover effect of the prepartum diet on serum nonesterified fatty acids and plasma β-hydroxybutyrate during the first 7 d in milk, and therefore on milk production. These results suggest that feeding diets containing decreased amounts of roughage NDF during the 3-wk prepartum period may minimize the decrease in DMI and lipid mobilization as parturition approaches. This strategy may thus minimize the effect of hormonal factors and heat stress on periparturient cows.     

Liver proteomic analysis of postpartum Holstein cows exposed to heat stress or cooling conditions during the dry period

Heat stress negatively affects cow performance, compromises immune function, and increases susceptibility to metabolic disorders, particularly during the dry period and as cows transition from gestation to lactation. Metabolic adaptations of the liver are critical for successful transition, yet it is unclear how heat stress affects metabolic pathways within the liver at the proteomic level. The objective of this study was to investigate the liver proteome of postpartum cows that were cooled or heat stressed during the dry period to gain insight into how protein expression is altered by prior heat stress and may contribute to performance and disease outcomes. During the dry period, cows were either housed in shaded barns with fans and water soakers [cooled group (CL); n = 5] or in shaded barns lacking these cooling devices [heat-stressed group (HT); n = 5]. Liver biopsies were collected at 2 d postpartum, and protein content was analyzed by label-free quantitative shotgun proteomics (nanoscale liquid chromatography coupled to tandem mass spectrometry). In the most comprehensive bovine liver proteomics analysis completed to date, we identified 3,270 proteins, 75 of which were differentially expressed between HT and CL cows (fold change ±1.2). The top pathways differing between HT and CL cows were oxidative phosphorylation, mitochondrial dysfunction, farnesoid X receptor/retinoid X receptor (FXR/RXR) activation, and the methylmalonyl pathway. Cooling cows during the dry period likely improves ATP production, reduces oxidative stress, and prevents excessive accumulation of hepatic triglycerides and cholesterol, which may contribute to greater milk yield and lower susceptibility to transition-related diseases.     

Transportation of Cattle in the Dairy Industry: Current Research and Future Directions

Traditionally, a heifer lived on one farm from birth to death. Recently, transportation has become a routine management practice. The needs of cattle are different depending on age and stage of reproduction and lactation. Ramps are not an obstacle for adult cattle; however, for neonatal calves ramp inclines and deck height can create well-being issues for loading and unloading calves. Mixing of dairy cattle is relatively unexplored. Stationary confinement of cows is less stressful than the motion of transport. During transport, adult cattle stand more, but lie more during the recovery period. Room to orient themselves in a particular direction is important. Footing is affected by driver, driving conditions, and stocking density, but flooring does not contribute to falls. Environmental studies indicate that upper critical temperature for adult cattle is 30°C, but neonatal calves are most affected by low temperatures. Therefore, ventilation requirements are based on specific circumstances. Young calves exhibit less physiological stress with transport, but succumb to postsecondary mortality, which is correlated with age at transport. Characteristics of stressed cattle during and following transport include increased heart rate and cortisol concentrations, enzymatic changes, impaired LH surge, and immunological effects. The duration of the journey has a greater impact than the distance and after long transport, most animals drink and then lie down. Therapies during and following transport show that water or electrolytes are important. Studies showed that calves habituate to transport, unlike cows. These data point to the need for research of better methods of transport to reduce stress.     

Symposium review: The influences of heat stress on bovine mammary gland function

Heat stress reduces cow milk yield and results in a significant economic loss for the dairy industry. During lactation, heat stress lowers milk production by 25 to 40% with half of the decrease in milk synthesis resulting from the reduced feed intake. In vitro studies indicate that primary bovine mammary epithelial cells display greater rates of programmed cell death when exposed to high ambient temperatures, which may lead to a decrease in the total number of mammary epithelial cells in the mammary gland, partially explaining the lower milk production of lactating cows under heat stress. The function of mammary cells is also altered by heat stress. In response to heat stress, mammary cells display higher gene expression of heat shock proteins, indicating a need for cytoprotection from protein aggregation and degradation. Further, heat stress results in increased gene expression without altering protein expression of mammary epithelial cell junction proteins, and does not substantially influence the integrity of mammary epithelium. These data suggest that the mammary gland strives to maintain cell-to-cell junction integrity by synthesizing more proteins to compensate for protein losses induced by heat stress. During the dry period, heat stress negatively affects mammary gland development by reducing mammary cell proliferation before parturition, resulting in a dramatic decrease in milk production in the subsequent lactation. In addition to mammary growth, the mammary gland of the heat-stressed dry cow has reduced protein expression of autophagic proteins in the early dry period, suggesting heat stress influences mammary involution. Emerging evidence also indicates that heifers born to cows that experience late-gestation heat stress have lower milk yield during their first lactation, implying that the maternal environment may alter mammary gland development of the offspring. It is not clear if this is due to a direct epigenetic modification of prenatal mammary gland development by maternal heat stress. More research is needed to elucidate the effect of heat stress on mammary gland development and function.     

Effect of heat stress during early,late, and entire dry period on dairy cattle

Cooling during the entire dry period abates the negative effects of heat stress postpartum, yet the temporal relationship of cooling (i.e., early or late dry period) to performance is unknown. We evaluated the effect of heat stress early, late, and for the entire dry period on subsequent performance. Cows were selected based on mature-equivalent milk yield and dried off 45 d before expected calving. Cows were blocked by parity, previous 305-d mature equivalent milk yield, and body weight (BW) and randomly assigned to cooling (shade, fans, and soakers; CL) or heat stress (shade; HT). Treatments included CL (n = 20) or HT (n = 18) during the entire dry period, HT during the first 3 wk dry and then CL until calving (HTCL, n = 21), or CL during the first 3 wk dry period and then HT until calving (CLHT, n = 19). Heat stress increased rectal temperature (RT; CL, 38.8; HT, 39.1 ± 0.04°C) and respiration rate (RR; CL, 52.9; HT, 70.5 ± 1.9 breaths/min) during the early dry period. In the late dry period, HT increased RT and RR relative to CL cows (RT = CL, 38.7; HT, 39.1; CLHT, 39.1; HTCL, 38.9 ± 0.05°C; RR = CL, 47; HT, 64; CLHT, 66; HTCL, 53 ± 2.1 breaths/min). During the early dry period, HT decreased dry matter intake (CL, 11.8; HT, 10.5 ± 0.35 kg/d) but dry matter intake did not differ among treatments during late dry period (HT, 10.7; HTCL, 11.1; CL, 11.2; CLHT, 10.1 ± 0.55 kg/d). Cows exposed to prepartum cooling during the entire dry period had increased dry matter intake compared with cows exposed to heat stress during the late dry period (CL vs. CLHT, 11.2 ± 0.55 and 10.1 ± 0.55 kg/d, respectively). Heat stress at any time reduced gestation length compared with cows under prepartum cooling during the entire dry period (CL, 277 vs. HT, 274; CLHT, 273; and HTCL, 274 ± 1.17 d). Dry period length decreased by approximately 4 d if cows were exposed to HT at any time. During the early dry period, HT decreased BW, whereas CL increased BW relative to that at dry-off (CL, 6.9; HT, −9.4 ± 3.7 kg). In the late dry period, we detected no differences in BW gain among treatments, but cows exposed to prepartum cooling for the entire dry period tended to have increased BW gain compared with HT and HTCL. Prepartum cooling during the early or late dry period alone partially rescued milk yield only in the first 3 wk of lactation (CL, 32.9; HT, 26.6; CLHT, 29.7; HTCL, 30.7 ± 1.37 kg/d). Cooling for the entire dry period increased milk yield up to 30 wk into lactation compared with all other treatments. Thus, HT at any time during the dry period compromises performance of cows after calving.     

Yield gap analysis in dairy production systems using the mechanistic model LiGAPS-Dairy

The difference between the theoretical maximum (potential) production and the actual production realized by farmers is referred to as the yield gap. The objectives of this study are to develop a mechanistic model for dairy cows that allows yield gap analysis in dairy production systems and to evaluate model performance. We extended and adapted an existing model for beef cattle to dairy cattle, and the new model was named Livestock simulator for Generic analysis of Animal Production Systems—Dairy cattle (LiGAPS-Dairy). Milk production and growth of an individual cow over its entire lifespan were described as a function of the animal's genotype, the ambient climate, feed quality, and available feed quantity. The model was parameterized for Holstein-Friesian cows. After calibration, we evaluated model performance by comparing simulated results and measured results from experimental farms in the Netherlands, which were not used for model calibration. Cows were permanently housed in stables, where the diet consisted of predetermined amounts of concentrates and ad libitum high-quality roughage. The mean absolute error (MAE) for simulated milk production per lactation was 12% of the measured milk production, whereas the MAE for simulated daily milk yields was 19%. The MAE for simulated feed intake per lactation was 10% of the measured feed intake, whereas the MAE for simulated daily feed intake was 19%. The average yield gap for dairy cows was 11% of the potential milk production (Y_P). Yield gap analysis indicated that for experimental farms in the Netherlands, the difference between Y_P and feed quality limited milk production (Y_L) of 1,009 kg fat- and protein-corrected milk was mainly explained by feed intake capacity (33%), protein deficiency (25%), cow weight at the start of experiments (23%), and heat stress (19%). The LiGAPS-Dairy model also indicated the periods during lactation in which these factors affected milk production. In our opinion, the overall model performance is acceptable for permanently housed cows under Dutch conditions. The model needs to be evaluated further for other production systems, countries and breeds. Thereafter, LiGAPS-Dairy can be used for yield gap analysis and exploration of options to increase resource use efficiency in dairy production.     

Influence of prepartum dietary crude protein on growth hormone, insulin, reproduction, and lactation of dairy cows

Effects of prepartum dietary crude protein on growth hormone, insulin in blood plasma, and subsequent reproduction and lactation were investigated in 30 Holsteins. Cows were fed either 80 or 100% of the crude protein recommended by National Research Council starting about 60 days precalving. Postcalving all animals were fed according to recommendations. Blood was sampled on days 220 (start of experiment), 250, 270, and 280 of gestation and on days 10 and 30 postpartum. Cows fed 80% crude protein prepartum showed decreased intake of dry matter and urea nitrogen in blood plasma prepartum, and yields of milk and total solids decreased by about 15%. Body weight changes through 30 days postpartum were not significantly different between treatments. There were no significant differences for growth hormone and insulin except growth hormone tended to be higher on day 270 of gestation for cows fed 80% crude protein. Insulin was negatively correlated with lactation number -5.1 to -5.6. Reproductive efficiency was similar between treatment groups. Lower crude protein intake during the dry period decreased dry matter intake and yields of milk and total solids.     

Effects of late gestation heat stress on postpartum milk production and reproduction in dairy cattle.

Carry-over effects of late gestation heat stress on postpartum productive and reproductive traits were estimated from DHI records using 341 lactations from six sites in Mississippi. Climatological data were gathered from records of weather stations near the sites. Using multiple linear regression analyses, predictor variables for lactations were age at calving, lactation number, maximum degree-days (above 32.2 degrees C) during the periods 30 and 60 d prepartum, and precipitation 30 and 60 d prepartum. Months and sites were indicator variables. Dependent variables included milk and fat production during early, mid, and late lactation; days to peak lactation; days open; services per conception; and body weight. Age at calving affected milk and fat production in mid and late lactation and services per conception. Degree-days for 60 d prepartum had the greatest negative influence on production variables; its statistical significance was shown in predictions of milk and fat production in early and midlactation. Days open were higher for July than for cows calving in August or September. Sites had effects on many milk and fat measurements and some reproductive traits. These results indicate that heat stress in the last 60 d of gestation has negative effects on some production variables.     

Heat stress reduces the accumulation of rosmarinic acid and the total antioxidant capacity in spearmint (Mentha spicata L)

Selected high‐phenolic lines of spearmint were subjected to a constant 30 °C heat regimen for a period of 4 weeks to determine the effects of heat stress on soluble phenolics, phenols and rosmarinic acid biosynthesis and antioxidant capacity. Heat stress significantly reduced levels of total phenolic acids (71–87%) and soluble phenols (75–87%). This loss was concomitant with a loss of total antioxidant capacity of 21–60% after week 1 and up to 95% by week 4. High‐performance liquid chromatography profiling of heat‐stressed plants at 270 and 320 nm detected nearly a complete loss of rosmarinic acid in all seven chemotypes. High‐temperature drying of non‐heat‐stressed plants at 80 °C resulted in a similar loss of total antioxidant capacity and rosmarinic acid content an effect not observed in material that was subjected to low‐temperature drying first, followed by exposure to high temperature. This suggests that heat stress negatively regulates rosmarinic acid biosynthesis and causes a potential rapid biological breakdown of rosmarinic acid in tissues. 2,2‐Diphenyl‐1‐picrylhydrazyl radical assays of heat‐stressed and non‐stressed plants clearly show that rosmarinic acid is the major contributor to the antioxidant capacity in spearmint. Copyright © 2005 Society of Chemical Industry     

Plasma concentrations of gut peptides in dairy cattle increase after calving

Effects of transition from late gestation to early lactation on plasma concentrations of glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide 1-(7-36) amide (GLP-1), and cholecystokinin (CCK) have not been reported in cattle. The objective of the present study was to measure plasma concentrations of GLP-1, GIP, CCK, insulin, glucose, and nonesterified fatty acids in blood plasma obtained from the coccygeal vein of 32 Holstein cows at an average of 11 d before, and 5, 12, and 19 d after calving. Feed dry matter intake (DMI) averaged 14.4, 17.7, and 19.9 kg/d on d 5, 12, and 19 of lactation, respectively, as milk yield increased (30.6, 36.6, and 39.7 kg/d, respectively). Plasma concentrations of insulin and glucose were lower postpartum than prepartum, but did not differ among samples collected after calving. In contrast, plasma concentration of gut peptides increased linearly after calving, perhaps as a consequence of increased feed intake and nutrient absorption; however, the increases in plasma concentrations of GIP and GLP-1 as lactation progressed were not associated with increased DMI per se, and likely reflect the endocrine and metabolic adaptations of lacto-genesis. In contrast, increased concentration of CCK was related both to increasing days in milk and DMI. By 19 d postpartum, concentrations of GLP-1, GIP, and CCK increased by 2.3-, 1.8-, and 2.8-fold, respectively, compared with values at 11 d before calving. Although these peptides have direct and indirect effects that reduce appetite and DMI in other species (including increased insulin secretion), these may be glucose- or insulin-dependent functions, and insulin and glucose concentrations were reduced in early lactation.     

Evaluation of whisky distillery by-products IV.—Evaporated spent wash as a supplementary food with low-quality roughages for cattle

Evaporated spent wash, a whisky distillery by-product, has been evaluated as a supplementary food with low-quality roughage for beef cattle. Three levels of the material were used, the two lower levels with small amounts of barley. None of the supplements produced a marked increase in intake of roughage. Digestibility of the diet was substantially increased when spent wash or spent wash and barley was included and an intake of metabolisable energy of approximately 80% of the animal's maintenance requirement was obtained. The spent wash was found to be rich in nitrogenous compounds, phosphorus and copper but very low in calcium and sodium.     

Influence of dietary sodium and potassium bicarbonate and total potassium on heat-stressed lactating dairy cows

Objectives were to study effects of heat stress, 0 or .85% sodium bicarbonate, 0 or 1.0% potassium bicarbonate, and 1.0 or 1.5% total dietary potassium on production and physiological responses of dairy cows. Eighteen lactating Holsteins were assigned to shade (control) or no shade (heat stress) lots continuously for three consecutive 35-day periods and to different dietary treatments each period. Basal diet was 25% cottonseed hulls and 75% concentrate. Daytime and nighttime feed intake and production were measured the last 2 wk of each period, and milk and blood were sampled the final day of each period. Black globe temperature, rectal temperature, respiration rate, and blood pH were higher in no shade. Daytime intake was 132% greater in shade, nighttime intake was not different between environments. Milk production was about 19% greater for evening and morning milkings in shade. Daytime intake, daytime and nighttime milk production were higher with sodium bicarbonate. Potassium bicarbonate reduced intake and production. Higher total dietary potassium increased total daily milk production. Lactating cows appear adept at withstanding environmental and dietary challenges to acid-base homeostasis. Supplementation of sodium bicarbonate and 1.5% dietary potassium, but not potassium bicarbonate, were beneficial to lactating dairy cows.     

ADSA Foundation Scholar Award: Early-life exposure to hyperthermia: Productive and physiological outcomes,costs, and opportunities

Global rising temperature is a considerable threat to livestock production and an impediment to animal welfare. In fact, the 5 warmest years on record have occurred since 2016. Although the effect of heat stress on lactating cattle is well recognized and extensively studied, it is increasingly evident that rising temperatures will affect dairy cattle of all ages and lactation states. However, the extent and consequences of this effect are less understood and often overlooked in the literature and dairy industry. Early-life experiences, such as exposure to hyperthermia, can have life-long implications for health and productivity. This review highlights the body of work surrounding the effects of heat-stress exposure in young dairy cattle, including the prenatal fetus (in utero), postnatal calves (preweaning), and growing heifers, which are all categories that are typically not considered for heat-stress abatement on farm. Insights into the physiological and molecular mechanisms that might explain the adverse phenotypic outcomes of heat-stress exposure at different stages of development are also discussed. The estimated economic loss of in utero hyperthermia is addressed, and the ties between biological findings and opportunities for the application of cooling management interventions on farm are also presented. Our research highlights the importance of heat-stress abatement strategies for dry-pregnant cows to ensure optimal multigenerational productivity and showcases the benefits of cooling neonatal calves and growing heifers. Understanding the implications of heat stress at all life stages from a physiological, molecular, economic, and welfare perspective will lead to the development of novel and refined practices and interventions to help overcome the long-lasting effects of climate change in the dairy industry.     

The effects of dry period versus continuous lactation on metabolic status and performance in periparturient cows

It has been argued that dairy cows with a high genetic milk production potential can maintain high milk production even with total omission of the dry period. Further, when omitting the dry period, cows are believed to experience fewer metabolic changes during the transition from late gestation to early lactation compared with cows having a traditional dry period. The performance and metabolic response to omission of the dry period for cows with an expected peak milk yield higher than 45 kg/d were studied in 28 Holstein dairy cows. The cows were followed in late gestation and in the subsequent 5 wk of early lactation. Fourteen cows were milked through late gestation (CM) and another 14 dairy cows underwent a 7-wk dry period (DRY). In the early lactation period, the cows had the same dry matter (DM) intake but cows in the CM group had a 22% reduction in milk yield compared with the cows in the DRY group. At calving, the experimental groups had the same average body weight and body condition score and there were no significant differences in body weight and body condition score changes in early lactation. However, the cows in the CM group compared with the cows in the DRY group had a higher plasma concentration of glucose and insulin and a lower plasma concentration of nonesterified fatty acids and β-hydroxybutyrate in the following 5 wk of early lactation. Furthermore, the cows in the CM group had lower liver triacylglycerol concentration and higher liver glycogen concentration in the following early lactation. It is concluded that, even in dairy cows with an expected peak milk yield above 45 kg/d, omission of the dry period results in a relatively high reduction in milk yield in the following early lactation. Furthermore, these cows are in less metabolic imbalance in the following early lactation.     

The Effect of Silage and Concentrate Type on Intake Behavior, Rumen Function, and Milk Production in Dairy Cows in Early and Late Lactation

The objective of this experiment was to evaluate the effect of feeding total mixed rations (TMR) that differ in structural and nonstructural carbohydrates to dairy cows in early and late lactation on short-term feed intake, dry matter intake (DMI), rumen fermentation variables, and milk yield. A 5 × 5 Latin square experiment with 15 dairy cows was repeated during early and late lactation. The 5 treatments were a TMR with (all on dry matter basis) 55% roughage (a 50:50 mixture of corn silage and grass silage) and 45% concentrate (a 50:50 mixture of concentrate rich in structural carbohydrates and concentrate rich in nonstructural carbohydrates; treatment CON), a TMR with the concentrate mixture and 55% grass silage (RGS) or 55% corn silage (RCS), and a TMR with the roughage mixture and 45% of the concentrate rich in structural carbohydrates (CSC) or the concentrate rich in nonstructural carbohydrates (CNS). Meal criteria, determined using the Gaussian-Gaussian-Weibull method per animal per treatment, showed an interaction between lactation stage and treatment. Feed intake behavior variables were therefore calculated with meal criteria per treatment-lactation stage combination. Differences in feed intake behavior were more pronounced between treatments differing in roughage composition than between treatments differing in concentrate composition, probably related to larger differences in chemical composition and particle size between corn silage and grass silage than between the 2 concentrates. The number of meals was similar between treatments, but eating time was greater in RGS (227 min/d) and lesser in RCS (177 min/d) than the other treatments. Intake rate increased when the amount of grass silage decreased, whereas meal duration decreased simultaneously. These effects were in line with a decreased DMI of the RGS diet vs. the other treatments, probably related to the high neutral detergent fiber (NDF) content. However, this effect was not found in CSC, although NDF content of the TMR, fractional clearance rate of NDF, and fractional degradation rate of NDF was similar between CSC and RGS. Rumen fluid pH was lesser, and molar proportions of acetic acid and of propionic acid were lesser and greater, respectively, in RCS compared with all other diets. Milk production did not differ between treatments. There was no effect of type of concentrate on milk composition, but diet RCS resulted in a lesser milk fat content and greater milk protein content than diet RGS. Lactation stage did affect short-term feed intake behavior and DMI, although different grass silages were fed during early and late lactation. The results indicate that short-term feed intake behavior is related to DMI and therefore may be a helpful tool in optimizing DMI and milk production in high-production dairy cows.     

Effect of ratio of roughage concentrate on glucose-induced heat production in sheep rumen fluid in vitro

The effect of ration on heat of glucose fermentation in sheep rumen fluid was investigated. Heat production was measured in a semiadiabatic calorimeter. In trial 1, the effect of glucose (.4 to 6.4 mg) on fermentative heat production was determined in rumen fluid from sheep fed 25 or 100% roughage diet. Heat of glucose fermentation decreased with increase in glucose dose in both diets. Maximal heat of glucose fermentation in both diets agreed with stoichiometric calculations. However, at 6.4 mg glucose, maximal heat was 18 kcal/mol in the 25% roughage diet and 14 kcal/mol in the 100% roughage diet. Purine N and maximal rate of heat production were not affected by diet type. In trial 2, the effect of glucose (1.6 and 6.4 mg) on fermentative heat production was determined in rumen fluid from sheep fed 25, 50, 75, and 100% roughage. In addition, fermentation pattern was measured in donors of the rumen fluid. Heat of glucose fermentation was positively correlated with organic matter digestibility and negatively correlated with rumen pH and acetate concentration. These observations indicate that in addition to the effect of roughage on the fermentation pattern, supplemental adaptation may occur, as indicated by the reduction in the heat of glucose fermentation.     

Effects of heat stress on energetic metabolism in lactating Holstein cows

Heat stress has an enormous economic impact on the global dairy industry, but the mechanisms by which hyperthermia negatively affect systemic physiology and milk synthesis are not clear. Study objectives were to evaluate production parameters and metabolic variables in lactating dairy cows during short-term heat stress or pair-fed conditions coupled with bST administration. Twenty-two multiparous Holstein cows were subjected to 3 experimental periods: 1) thermoneutral conditions with ad libitum intake for 7 d (P1); 2) heat stress (HS) with ad libitum intake (n = 10) or pair-fed (PF) in thermoneutral conditions (n = 12) for 7 d (P2), and 3) 7 d of HS or PF in conditions as described in P2 with recombinant bovine somatotropin administered on d 1 (P3). All cows received an intravenous glucose tolerance test (GTT) on d 5 of each period. Heat stress conditions were cyclical and temperatures ranged from 29.4 to 38.9°C. Rectal temperatures and respiration rates increased during heat stress (38.6-40.4°C and 44-89 breaths/min, respectively). Heat stress reduced dry matter intake by 30% and by design PF cows had similar intake reductions (28%). During heat stress and pair-feeding, milk yield decreased by 27.6% (9.6 kg) and 13.9% (4.8 kg), respectively, indicating that reduced feed intake accounted for only 50% of the decreased milk production. Milk yield increased with recombinant bovine somatotropin in both HS (9.7%) and PF (16.1%) cows. Cows in both groups were in positive energy balance (3.95 Mcal/d) during P1 but entered negative energy balance during P2 and P3 (−5.65 Mcal/d). Heat stress and pair-feeding treatments decreased (9.3%) basal glucose concentrations. Heat stress conditions had no effect on basal NEFA levels during P2; however, PF cows (despite a similar calculated energy balance) had a 2-fold increase in basal NEFA concentrations. Both groups had increased plasma urea nitrogen levels during P2 and P3 compared with P1. Basal insulin levels increased (37%) during P2 and P3 in HS cows but did not differ between periods in PF cows. During P2 and compared with P1, PF cows had a decreased rate of glucose disposal, whereas HS cows had a similar disposal rate following the GTT. During P2 and compared with P1, PF cows had a reduced insulin response whereas HS cows had a similar insulin response to the GTT. In summary, reduced nutrient intake accounted for only 50% of heat stress-induced decreases in milk yield, and feed intake-independent shifts in postabsorptive glucose and lipid homeostasis may contribute to the additional reduction in milk yield.     

Effects of heat stress and plane of nutrition on lactating Holstein cows: I. Production, metabolism, and aspects of circulating somatotropin

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.     

Response of lactating dairy cows to fat supplementation during heat stress.

Effects of supplemental prilled long-chain fatty acids on lactation performance during heat stress were examined using eight multiparous Holstein cows in a replicated 4 x 4 Latin square design with 15-d periods. Cows were ruminally cannulated and were assigned randomly to one of four treatments in a 2 x 2 factorial arrangement of treatments. Factors were 0 or 5% supplemental fat and thermoneutral or heat stress conditions. Cows were housed in environmental chambers with thermoneutral conditions of 20.5 degrees C and 38% relative humidity for 24 h/d or heat stress conditions of 31.8 degrees C and 56% relative humidity for 14 h/d and 25.9 degrees C with 56% relative humidity for 10 h/d. Isonitrogenous diets (17% CP) containing 50% alfalfa silage and 50% concentrate were offered for ad libitum intake. Diets contained 1.64 or 1.83 Mcal NEL/kg DM. No diet by environment interactions were significant. Milk fat percentage (3.46 vs. 3.15%) and 3.5% FCM (31.5 vs. 29.2 kg/d) were higher for cows fed 5 vs. 0% fat. Dry matter intake, milk yield, and milk protein percentage did not differ between diets. Heat stress decreased DMI, milk yield, 3.5% FCM, and milk protein percentage but did not affect milk fat percentage. Results suggest that supplemental fat at 5% of diet DM enhances lactation performance similarly under thermoneutral and heat stress conditions.     

Symposium review: Environmental effects on mammary immunity and health

Environmental effects on pathogen abundance and access are precursors to mastitis. Indeed, high heat and humidity, and unsanitary housing and equipment, are associated with greater pathogen load and exposure. Although less is known about effects of environment on a cow's ability to resist infection, several indicators suggest that it can affect pathogen responses. Mastitis incidence and bulk tank somatic cell count vary with season, typically peaking in summer. Recent controlled studies have revealed that heat stress exposure results in changes in the microbiome of the cow and her environment, which may relate to negative effects on milk quality and cow health. Alternatively, specific pathogen loads may vary based on housing dynamics rather than associations with physical environment. Indeed, housing-related stressors, such as overcrowding and social group challenge, influence secretion of glucocorticoids, thus affecting pathogen resistance in the cow. Two key seasonal variables are photoperiod and temperature, specifically the heat stress consequent to elevated temperature and humidity. Shifts in light duration regulate immune function in other species, but apparently have limited effect on udder health of lactating cows. In contrast, in dry cows, short days increase peripheral blood mononuclear cell number and are associated with lower somatic cell count in the next lactation, compared with long days. With heat stress, elevated body temperature directly affects expression of immune-related genes in mammary tissue. Responses depend on duration of exposure and feature acute upregulation of immune-signaling pathways, followed by enrichment of other immune-related pathways after prolonged exposure. Most responses are transient and recover within 1 wk. Functionally, heat stress impairs some aspects of acquired immunity in dry cows, including antigen responses and lymphocyte proliferation, but apparently not innate immune function. However, heat stress in late gestation reduces neutrophil phagocytosis and killing in vitro, and neutrophils in circulation are reduced in vivo as are responses to pathogen challenge in the subsequent lactation. A holistic understanding of the complex interplay of environment, pathogens, and host is needed to inform advances in this area.