Impact of heat stress on rumen functions

The livestock sector is evolving in response to rapidly increasing demand for livestock products. Ruminant population is the main driver of the growth of the livestock sector besides pig and poultry. The rise in environment temperature due to climate change alters the basic physiology of rumen which negatively affects production. Dry matter intake begins to decline in an adaptive response to heat stress. Increased environmental temperature reduces the gut motility, rumination, ruminal contractions and depresses appetite in ruminants. Heat stress reduces the total production of volatile fatty acid (VFA) with individual variation and also results in changes in ruminal pH. Passage rate and retention time of digesta is also influenced by rise in ambient temperature and thus affects digestibility. The change in microbiota due to heat stress may change the fermentation pattern in the rumen resulting in variation in digestibility, VFA production and also methane emission.


Introduction
reduces the dry matter intake, decreases ruminal motility and contraction, changes the fermentation The livestock sector globally is highly progressive.
pattern and volatile fatty acid production, affects the In developing countries, it is evolving because of digestibility and nutrient utilization, and thus impairs rapidly increasing demand for livestock products, productivity.This review is aimed to give an insight however in developed countries; demand for livestock into alterations in rumen function in response to products is stagnating [1].Currently, livestock is one of increase in environmental temperature.the fastest growing sector among agriculture in developing countries [1].Its share to agricultural GDP Effect of heat stress on feed intake is 33% and is quickly increasing [1].This growth is Feed intake in lactating cows begins to decline at driven by the rapidly increasing demand for livestock ambient temperatures of 25-26°C and reduces more products and this demand is being driven by population rapidly above 30°C [8].At 40°C, the dietary intake may explosion, urbanization and increasing per capita decline by as much as 40% [7].Increased heat load income in developing countries [2].India possesses the decreases nutrient uptake in almost all species and in largest livestock ruminant population in the world case of cattle, the nutrient uptake decreases upto about (520.6 million), and accounts for the largest number of 30% of dry matter intake [8,9].Heat stress leads to the cattle (world share 16.1%), buffaloes (57.9%), second rostral cooling center of the hypothalamus to stimulate largest number of goats (16.7%) and third highest the medial satiety center which inhibits the lateral number of sheep (5.7%) in the world [3].Climate appetite center, and thus reduced dietary intake and change projections for India suggest that temperature is consequently lower milk production [10].Increasing expected to increase between 2.3 and 4.8°C because of environmental temperature and rising rectal temperadoubling of carbon dioxide concentration in the ture above critical thresholds are related to decrease in atmosphere [4] and further this increase in temperature the dry matter intake (DMI) [10].A decrease in the dry is expected for all the months of the year.Rise in the matter intake in dairy cattle was found on exposure to environmental temperature may impair production heat stress [11].High environmental temperature through reduced growth, meat, milk and egg, impaired reported to reduce feed intake in Friesian heifers [12].reproductive performance, imbalanced biochemical At 28°C, dry matter intake was similar to that at 20°C; and physiological process of metabolism and immune whereas, at 33°C dry matter intake was lower by about response [5].The rise in environment temperature 9% as compared to DM intake at 20°C [13] in Holstein alters the basic physiology of rumen which negatively heifers.Food intake by the Alentejana and Limousine affects the nutrient energy balance [6].Heat stress breeds decreased by 10% and 9.6% respectively, at 36.0±1°C and 45±1% humidity; whereas, in Mertolenga and Frisian animals feed intake did not change [14] at similar environmental conditions which indicates variation in feed intake in response to heat stress also depends upon breed.A decrease in DM ture decreased total VFA concentration as compared intake in crossbred cattle was reported at 35 and 40°C with the normal ruminal temperature however; ruminal thermal exposure as compared to thermal exposure at temperature did not affect molar proportion of VFA 25 and 30°C [15].A reduction in DMI in Jersey cows [32,33].Decrease in molar concentration of volatile was also reported when minimum temperature fatty acid during heat stress was mainly attributed to humidity index (THI) exceeded 56 and continued until decrease in roughage intake [30] and variation in THI reached 72 [16].During heat stress, DMI was fermentation pattern due to changes in microbial reduced in the lactating goats because of metabolic rate population [34].High ruminal temperature increased and more heat production [17].Daily DMI of cooled culture pH from 5.73 to 5.82 on averages; whereas, an animals was higher while water intakes were lower increase in ruminal pH form 5.82 to 6.03 was reported than those of noncooled animals where the maximum during heat stress in lactating dairy cattle [16].ambient temperature and relative humidity for the non-Effect of heat stress on nutrient digestibility cooled group were 33°C and 61%, with the corresponding values for the cooled barn being 28°C and 84%, Some authors reported an increase in diet respectively [18].The concentrate intake during cool digestibility in cattle exposed to hot environments [13, comfort, hot-dry and hot-humid exposures did not [35][36][37].In contrast, negative or no relationships change in buffalo calves; whereas, wheat straw intake between high ambient temperatures and diet digestibility decreased significantly by 29.65% and 30.09% during have been reported in dairy cattle [35,38,39] and small hot-dry and hot-humid exposures, respectively [19].
ruminants [25].The positive effects of high environmental temperature on the digestibility of feed is

Effect of heat stress on rumination and rumen motility
attributed to either the reduction in the passage rate of digesta [21,22,35,40], the changes in feed composition Alteration of the dynamic characteristics of digestion [12,22,35] or the reduction in DMI [12,35].The DM is recognized as a possible mechanism through which digestibility in cattle was reported to increase in a hot heat stress can affect the nutrition of animals [20].
environment when given forage-based diets [35].The Increased environmental temperature reduces the DM digestibility in Ayrshire cattle was significantly rumination time [21] and depresses appetite [22] by higher at 33°C than at 20°C with moderate quality diet, having a direct negative effect on appetite center of the but was similar at 33°C and at 20°C with a high-quality hypothalamus [23].Available data suggest that diet [38].rumination is depressed during dehydration and heat The digestibility patterns of different feed stress [21,24].Also, blood flow to rumen epithelium is components during hot dry and hot humid thermal depressed during heat stress and reticular motility and exposures were studied [41] and found that the digestirumination is decreased whereas, the volume of digesta bility of CP, OM, NDF, ADF and NFCD increased during in the rumen of beef cows, Bedouin goats [25], riverine both type of exposures as compared to cool comfort buffalo [19] and Egyptian buffalo [26] increased.
whereas the digestibility of NDF, ADF and NFCD was Higher concentrations of lactic acid and lower ruminal lower during hot-humid exposure compared to hot-dry.pH was observed in heat-stressed cattle, which may High ruminal temperature did not affect DM and NDF imply that a higher lactic acid concentration and lower digestibility whereas it tended to decrease OM digestiruminal pH might be involved in inhibiting rumen bility as compared with the normal ruminal temperature motility during heat stress [27].There is no clarity on (66.6 vs. 67.4%)[33].The digestibility patterns at the involvement of gastrointestinal hormones and different thermal exposures were investigated [15] and peptidergic neurons in mediating the effect of reported that digestibility at 25 and 30°C did not temperature on gastrointestinal motility.Some of the change whereas digestibility increased at 35°C and gastrointestinal hormones that influence motility also then decreased at 40°C thermal exposure.The decrease affect feed intake in ruminants [28].The large number 0 in nutrient digestibility at 40 C could be attributed to of biologically active compounds produced in the gut change in rumen environment (pH, rumen temperature, that may influence motility and passage makes this rumen motility, rumen flora and fauna) due to higher area very complex [29].

Effect of heat stress on volatile fatty acid (VFA)
of lactic acid and lower ruminal pH were observed in production and ruminal pH heat-stressed cattle, which may imply that a high lactic Heat stress reduced the total production of acid concentration and lower ruminal pH might be volatile fatty acid [13,30,31].The ratio of acetate to involved in inhibiting rumen motility during heat stress propionate decreased during heat stress and more [27].specifically, the molar concentration of acetate decrea-Variation in the rate at which feed passes through sed whereas propionate and butyrate concentration the digestive tract is a major factor in the positive increased nonsignificantly (p>0.05)[13].The effects relationship between environmental temperature and of ruminal temperature on a dual-flow, continuousdigestibility [40].The increase in diet digestibility in culture system on in vitro fermentation characteristics heat-stressed ruminants was explained by increased was investigated and found that high ruminal temperamean retention time in the whole gastrointestinal tract [42].Slower passage rate and longer mean retention in the kingdom of Euryarchaeota and are found in a time of digesta have been described in dairy cows [43], wide range of other anaerobic environments [48].buffalo [19] and heifers [13] maintained under hot Through a series of biochemical reduction of carbon environments.A significant increase in DM digestidioxide (CO ) with hydrogen (H ), and some methano- to hot conditions was reported; however, it was found slow-growing methanogens (regeneration time about to be reduced when the exposure to hot conditions was 130 h) that produces CH from acetate (Methanosarcina) 4 prolonged [44].In the same study, changes in diet and fast growing methanogens (generation time 4-12 digestibility observed under hot environments were not h) that reduce CO with H .In the rumen, methano- related to DMI and passage rate of digesta.Although a genesis occurs mostly by the fast-growing methanogens positive relationship between digestibility, especially as ruminal retention time is too short to permit of fiber components, and ambient temperatures has establishment of the slow growing species.Methane been reported [45], high temperatures had little effect emissions in ruminants also account for a 2% to 12% of on diet digestibility in dairy cows [46] and goats [45].
gross energy loss of feeds depending upon the type of Digestibility coefficients of dry matter, organic matter, diets [49].Methane is one of the by-products formed neutral detergent fiber and acid detergent fiber in sheep from the degradation of carbohydrates during enteric were not affected by short exposure (10 days) to a THI fermentation in feed and anaerobic digestion in of 82, but were lower after prolonged exposure to heat manure.The rumen is the most important part of [39].Dilution of rumen content due to higher water methane production in ruminants like cattle, while intake, reduction in rumen bacteria activity, decline in methane is mainly produced in the large intestines for rumen motility and reduction of saliva production may be responsible for digestibility changes when animals monogastric animals like pigs.Enteric fermentation are chronically exposed to extreme temperature humidity accounts for about 80% of methane in dairy cow [50].index [47].In the later study [47] it was suggested that Methane production from enteric fermentation is a ewes chronically exposed to heat showed lower diet function of the rate of organic matter fermentation, the digestibility and had lower pH and cellulolytic and type of volatile fatty acid produced and the efficiency amylolytic bacteria concentrations, slower digesta of microbial biosynthesis [51,52].The major factors passage rate and lower osmolarity of rumen content, that affect methane production in the ruminants are pH, indicating a possible impairment of bacterial activity volatile fatty acids, diet, feeding strategy, animal and high dilution of rumen fluid.The negative effect of species and environmental stresses.The optimum pH such a depression of rumen bacteria activity on diet for methane production is 7.0-7.2,but the gas production digestibility might have overcome the positive effects can occur in the pH range of 6.6-7.6.However, beyond caused by the decline in DMI and digesta outflow rate, this range, the activity of fiber degraders reduces resulting in a net reduction of diet digestibility in chro- [53,54].nically heat-stressed ewes.The microbiota composition The emission rates of CH showed significant 4 was significantly different at elevated environmental diurnal variations with two peaks which were probably temperatures and humidity [31].In a recent study, four related to the feeding routine [55].Daily CH emissions 4 Holstein heifers were exposed at three temperatures increased significantly with the activity of the cows (r (20 °C, 28 °C and 33 °C) in a climatic chamber for two = 0.61) while daily CH emissions were negatively 4 weeks and found that the relative populations of the correlated to the indoor air temperature (r = -0.84).

Clostridium coccoides-Eubacterium rectal group and
This suggests that increased daily indoor air the genus Streptococcus increased and that of the genus temperatures due to seasonal changes may bring about Fibrobacter decreased in response to increasing decreased animal activity which may decrease the temperature [34].The change in microbiota due to release of CH from dairy cows [55].The methane 4 thermal exposure may change the fermentation pattern production was found to be correlated with DMI [56] in the rumen resulting in variation in digestibility of whereas the forage portion of the diet has also been different feed components and also composition of used to predict the methane production [57].The effect fermentation products.Other than alteration of bacterial of different thermal exposure on methane emission activity, the different responses in digestibility when were studied [58] and found that methane emission per ewes were exposed to thermal exposure for different kg DMI was reduced during 35°C and as compared to times might be related to the changes of ruminal and exposure at 25 and 30°C, then increased at 40°C.The intestinal absorption of nutrients that can occur in lowest methane emission at 35°C might be due to animals chronically exposed to high ambient temperahigher digestibility whereas methane production at 25 ture [35].especially the methane producing bacteria.In order to Wageningen Academic Publ., Wageningen, the Netherlands mitigate and formulate successful strategies to combat pp.204.17.Hamzaoui, S., Salama, A.A.K., Albanell, E., Such, X. and the negative effects of increased environmental

0 and 30 C
was higher as digestibility was lower at Effect of heat stress on methane production in respective temperatures than at 35°C temperature rumen exposure and therefore, more organic matter was Methanogens belong to a separate domain archaea available for microbes to convert them into methane energetic metabolism in lactating Holstein cows.J. Dairy and so methane production was comparatively higher 0 Sci.93: 644-655.at 25 and 30 C temperature.The highest methane emission 10.Albright, J.L. and Alliston, C.W. (1972) Effects of varying per kg DMI at 40°C might be attributed to lower organic the environment upon performance of dairy cattle.J. Anim.matter digestibility and shift in methane producing Sci.32: 566-577.11.Rhoads, M.L., Rhoads, R.P., VanBaale, M.J., Collier, R.J., microbes and other microbial fermentation, due to Sanders, S.R., Weber, W.J., Crooker, B.A. and Baumgard, change in rumen environment because of higherL.H. (2009) Effects of heat stress and plane of nutrition on environmental temperature [58].An increase in methane lactating Holstein cows: I. Production, metabolism, and emission was observed during higher ruminal temperature aspects of circulating somatotropin.J. Dairy Sci.9(5):986-[33].Methane production from enteric fermentation 997.12. Smith, D.L., Smith, T., Rude, B.J. and Ward, S.H. (2013) was a function of the rate of organic matter fermentation, Comparison of the effects of heat stress on milk and the type of volatile fatty acid produced and the component yields and somatic cell score in Holstein and efficiency of microbial biosynthesis [51,52].
Caja, G. (2013) Physiological responses and lactational temperature on rumen function, it is highly pertinent to performances of late-lactation dairy goats under heat stress carry out studies to ascertain the physiological and conditions.J. Dairy Sci.96(10): 6355-6365.microbiological basis of alteration in rumen function 18. Chaiyabutr, N., Chanpongsang, S. and Suadsong, S. (2008) during heat stress.Effects of evaporative cooling on the regulation of body water and milk production in crossbred Holstein cattle in a flora and fauna in the rumen.There are only few 15.Yadav, B., Singh, G., Wankar, A., Dutta, N., Verma, A.K. and studies related to the change in the ruminal microbial Chaturvedi, V.B. (2012a) Effect of heat stress on population in response to heat stress.Methane is one of th digestibility in crossbred cattle.VIII Biennial Conference the main products of ruminal fermentation which of ANAC and symposium on Animal Nutrition Research might also be affected by heat stress.In the scenario of Strategies for Food Security.pp 138.climate change, methane emission from livestock 16.Hall, M. B. (2009) Heat Stress Alters Ruminal Fermentation and Digesta Characteristics, and Behavior in Lactating Dairy animals is of paramount importance.Therefore, it of Cattle.In Proceeding of 11th International Symoosium on necessitates a comprehensive study relating to the Ruminant Physiology, Y. Chilliard, F. Glasser, Y.microbial population being affected by heat stress Faulconnier, F. Bocquier, I. Veissier, and M. Doreau, ed.