Studies on the relationship between thyroid function, cold acclimation and retention time of digesta in sheep

Invited Review: Glucosinolates Might Result in Low Methane Emissions From Ruminants Fed Brassica Forages

Methane is formed from the microbial degradation of feeds in the digestive tract in ruminants. Methane emissions from ruminants not only result in a loss of feed energy but also contribute to global warming. Previous studies showed that brassica forages, such as forage rape, lead to less methane emitted per unit of dry matter intake than grass-based forages. Differences in rumen pH are proposed to partly explain these low emissions. Rumen microbial community differences are also observed, but the causes of these are unknown, although altered digesta flow has been proposed. This paper proposes a new mechanism underlying the lower methane emissions from sheep fed brassica forages. It is reported that feeding brassica forages to sheep can increase the concentration of free triiodothyronine (FT3) in serum, while the intramuscular injection of FT3 into sheep can reduce the mean retention time of digesta in the rumen. The short retention time of digesta is associated with low methane production. Glucosinolates (GSLs) are chemical components widely present in plants of the genus Brassica. After ruminants consume brassica forages, GSLs are broken down in the rumen. We hypothesize that GSLs or their breakdown products are absorbed into the blood and then may stimulate the secretion of thyroid hormone FT3 in ruminants, and the altered thyroid hormone concentration may change rumen physiology. As a consequence, the mean retention time of digesta in the rumen would be altered, resulting in a decrease in methane emissions. This hypothesis on mitigation mechanism is based on the manipulation of animal physiological parameters, which, if proven, will then support the expansion of this research area.

In situ rumen degradation of dry matter and crude protein in ewes and dairy cows

Comparative studies were performed on the in-situ rumen degradation rate (kd) and the in-situ rumen undegradable residue (U) of dry matter (DM) and crude protein (CP) in sheep and dairy cows. The effect of different treatmentsfor sheep were investigated together with the validity of using cow-based reference protein values of foods for sheep. The effect of different breeds (Texel ewes and crossbred ewes), dietary roughage: concentrate ratio and shearing (mild cold exposure) were investigated for sheep.

The in-situ rumen incubations were performed in accordance with standard operational procedures which were comparable for both species. The foods under consideration were hay and concentrates. Cows displayed lower (DM 0·53 , CP 0·86) Kds than sheep for concentrates.

For hay a clear trend between cows and sheep was not observed but cows displayed 0·40 lower Kds than Texel ewes and 0·30 higher Kds than crossbred ewes. For concentrates U levels did not differ between species but for hay cows had 0·45 higher U values than Texel ewes. Texel ewes showed 0·45 higher Kd and 0·35 lower U for DM and CP than crossbred ewes. Dietary roughage: concentrate ratio in sheep diets did not influence the Kds and Us of the foods under consideration. Shearing of crossbred ewes increased Kd of CP by 0·25 and reduced U for DM and CP by 0·20 and 0.24 .

In conclusion different in situ rumen degradation rates were found between dairy cows and sheep. This probably makes using reference protein values of foods invalid for sheep since they are based on data for cows. Differences (Kd andU) were also found between sheep breeds and between unshorn and shorn crossbred ewes (mild cold exposure). No effect on rumen degradation kinetics could be attributed to dietary roughage: concentrate ratio.

Effects of ruminal digesta retention time on methane emissions: a modelling approach

The reasons for among-animal variations in methane (CH4) emissions are not fully understood. There is experimental evidence that ruminal digesta mean retention time (MRT) can affect CH4 emissions. The objective of the present study was to evaluate the contribution of among-animal variations in MRT on CH4 emissions and nutrient supply for dairy cow (default MRT = 34 h) and sheep (default MRT = 41 h), using the mechanistic Nordic dairy cow model Karoline. The simulations (n = 100) were made for a cow (bodyweight 600 kg) and for a sheep (bodyweight 60 kg) eating 20 kg and 1.0 kg DM/day, respectively. The diet for the dairy cow consisted of grass silage, barley and rapeseed meal (60 : 30 : 10 on a DM basis; crude protein 156 g/kg DM, neutral detergent fibre 450 g/kg DM) and the sheep diet was grass alone. Normal distribution of MRT values was assumed. Variability (coefficient of variation (CV) = 0.086) on default MRT was introduced by random-number generator of Excel. Intake, diet composition and digestion kinetic parameters were constant in all simulations, only ruminal MRT variables were changed in each simulation. Predicted CH4 emission increased with increased MRT for dairy cow (range from 407 to 488 g/day) and sheep (from 25.0 to 29.2 g/day). Increases in predicted CH4 emissions were partly associated with enhanced organic matter (OM) digestibility in dairy cow (from 0.715 to 0.758) and sheep (from 0.731 to 0.773). Greater CH4 emissions per kilogram digested OM with increased MRT were mainly related to reduced efficiency of microbial cell synthesis in the rumen both for dairy cows (22.8 ± 0.91 g N/kg OM truly digested; CV = 0.040) and for sheep (20.7 ± 0.92 g N/kg OM truly digested; CV = 0.044). Predicted CH4 yield was 20% and 17% greater in dairy cow and sheep, respectively, with the short (n = 10) compared with the long (n = 10) ruminal digesta MRT. Linear regression indicated that CH4 emissions increased by 0.37 (dairy cow) and 0.33 (sheep) g/kg DM intake per 1 h increase in ruminal digesta MRT. It is concluded that among-animal variation in MRT can markedly contribute to among-animal variation in CH4 emissions from ruminants.

Thyroid hormones correlate with field metabolic rate in ponies, Equus ferus caballus

During winter free living herbivores are often exposed to reduced energy supply at the same time that energy needs for thermoregulation increase. Several wild herbivores as well as robust horse breeds reduce their metabolism during times of low ambient temperature and food shortage. Thyroid hormones (TH) affect metabolic intensity and a positive effect of TH on basal metabolic rate (BMR) has been demonstrated in mammals and birds. As BMR and field metabolic rate (FMR) are often assumed to be intrinsically linked, TH may represent a reliable indicator for FMR. To test this hypothesis, ten Shetland pony mares were kept under semi-extensive central European conditions. During the winter month one group was fed 60% and one group 100% of their maintenance energy requirements. We measured FMR, locomotor activity, resting heart rate and TH levels in summer and winter. FMR, locomotor activity, resting heart rate and total T3 concentrations decreased substantially in winter compared to summer, whereas total T4 increased. Feed restriction led to a reduced FMR and resting heart rate, while TH and locomotor activity were not affected. Across both seasons FMR, resting heart and locomotor activity were positively correlated with total T3 but negatively and more weakly related with total T4.

Effect of drinking water temperature on physiological variables of crossbred dairy cattle at high altitude temperate region of Himalayas

AIM

The objective of study was to investigate the effects of drinking water on certain physiological parameters such as heart rate (HR), respiration rate (RR), rectal temperature (RT) and, ruminal motility (RM).

MATERIALS AND METHODS

The experiment was carried out on 18 farm bred lactating crossbred cows. The animals selected for the study were divided into three groups of six animals each on the basis of milk yield and parity and were allotted to three treatment group of six each such as ambient drinking water temperature at 10.25±0.28°C (ambient water, T1), drinking water temperature at 15-20°C (T2) and drinking water temperature at 35-40°C (T3). All the managemental practices were kept similar during experiment except drinking water temperatures physiological variables such as HR, RR, RT, and RM of the individual cow was measured and recorded twice in a day at 800 h and again at 1400 h two consecutive days in a week 15 min after providing drinking water.

RESULT

HR and RR at morning and at evening recorded were within the normal physiological level for all the treatment groups. However, RT at morning was comparable in all the treatments whereas at evening it was significantly (p<0.01) higher for cows consuming in T2 and in T3 than cows consumed (T1). The RM during morning among the treatments were non-significant as compared to the rumen motility at evening was significantly higher for (T1) and (T2) than for cows in (T3).

CONCLUSION

It can be concluded that offering warm drinking water at 35-40°C to crossbred lactating dairy cow is beneficial during winter at high altitude temperate region.

The effect of altering the hormone status of ewes on the outflow rate of protein supplements from the rumen and so on protein degradability

Four experiments were carried out on individually penned Finn Dorset and Suffolk × Finn Dorset ewes receiving complete diets of milled hay and concentrates and rationed at various levels below voluntary intake. In the first experiment, 20 non-pregnant non-lactating ewes were each given daily 1·6 kg food and injected intramuscularly with either isotonic saline (control), progesterone plus oestrogen (20 mg + 80 ng per 24 h) to mimic hormone status in pregnancy, bromocriptine (1 mg per 12 h) to suppress plasma prolactin, or thyrotrophin releasing factor (TRF) to elevate plasma concentrations of prolactin and the iodothyronines. Fractional outflow rates of chromium-mordanted fish meal from the rumen were estimated from the rate of excretion of chromium in faeces and were proportionately 0·25 higher for ewes given TRF than for controls (P < 0·025). None of the other treatments had a significant effect on outflow rate.

In experiment 2, fractional outflow rates from the rumen were determined during July (long daylength) and again during December (short daylength) in 12 ewes that had their seasonal pattern of plasma prolactin modified by removal of their anterior cervical sympathetic ganglia. Another 12 sham-operated ewes were used as controls. Neither the seasonal changes in prolactin nor their modification by cervical ganglionectomy significantly affected outflow rate.

Twelve non-pregnant non-lactating ewes were used in the third experiment to test the effects of mid-winter shearing (mean ambient temperature −4°C) on fractional outflow rates from the rumen and on the plasma concentrations of prolactin, thyroxine (T4) and cortisol. Shearing caused a drop in plasma prolactin, gave proportional increases (P < 0·05) of 0·47 and 0·16 in T4 and cortisol respectively and gave a 0·12 increase (P < 0·01) in the outflow rate of chromium-mordanted fish meal from the rumen. The increases in T4, and outflow rate were sustained for at least 2 weeks.

For experiment 4, 22 ewes each suckling two lambs were used in three trials to see if variation between ewes in milk production and in lamb growth rate was correlated with variation in outflow rate from the rumen. The only significant correlation (r = 0·68, P < 0·01) was between outflow rate and lamb growth rate for the eight ewes in trial 1. The results of all four experiments are discussed in relation to the mechanisms by which non-dietary factors, via their effect on the rate of outflow of digesta from the rumen, may alter the quantity of protein reaching the abomasum and the likely significance of this alteration on animal performance.

Effects of air temperature, air velocity and feeding level on apparent digestibility, water intake, water loss and growth in calves given a milk substitute diet

Six groups of eight 2-day-old calves were placed successively in a controlled environment chamber. Three groups were exposed to air temperatures of 10° and 25°C. A liquid diet of skimmed-milk substitute and 4 I/day of drinking (free) water was offered to each calf. Within each group, calves were allocated to either a low (<0·2 m/s) or a high (>3 m/s) air velocity and to either a low (30 g dry matter (DM) per kg M0·75per day) or a high (40 g DM per kg M0·75per day) feeding level. At 8 days of age the apparent digestibilities of DM at air temperatures of 10° and 25°C were 0·77 (s.e. 0·126) and 0·82 (s.e. 0·126) respectively (P > 0·05). The apparent digestibilities of DM were greater at the low feeding level with low air velocity than for either this feeding level with high air velocity or the high feeding level at both air velocities (P< 0·05) between which there was no significant difference (P > 0·05). At 8 days of age there were significant air temperature × air velocity (P< 001) and air velocity × feeding level interactions in the intake of free water (P< 005). There was a significant air temperature × feeding level interaction for total water intake (P< 0·05). Urinary water loss relative to total water intake was significantly greater at the low air velocity than at the high air velocity (P< 0·05).

In a further two groups of eight calves given 30 g DM per kg M 75 per day at 8 days of age, the apparent digestibilities of DM at air temperatures of 10° and 25°C were 0·71 (s.e. 0·020) and 0·90 (s.e. 0·013) respectively (P< 0·01). In the same calves given 40 g DM per kg M0·75per day at 20 days of age, the apparent digestibilities of DM at air temperatures of 10° and 25°C were 0·89 (s.e. 0·009) and 0·93 (s.e. 0·011) respectively (P< 0·05). The free and total water intakes, the ratios of (total water intake-faecal water loss): total water intake and the urinary losses of water were significantly greater at the air temperature of 25°C than at 10°C (P< 0·05). Live-weight gains were lower at 10°C than at 25°C (P< 0·01).

The results suggested that air temperature, air velocity and feeding level can affect the health and growth of calves less than 4 weeks of age.

Assessment of welfare from physiological and behavioural responses of New Zealand dairy cows exposed to cold and wet conditions

There is a need to assess the welfare of dairy cows that live outdoors under cold and wet conditions. This study combined a number of techniques to measure stress and make an assessment of welfare in this situation. Two groups of ten non-pregnant, non-lactating Holstein Friesian cows were exposed to a week of wind and rain (WR) or housed indoors (I) with pre- and post-treatment weeks indoors in a cross-over design. Wind and rain consisted of continual air movement (7.1 kph) using fans, water sprinkling for 15 min (3.0 mm) per hour, a mean temperature of 3.4°C and wind chill of –0.3°C. Internal body temperature was recorded every ten min and behaviour for 16 h per day. Blood, faeces and infrared temperatures were sampled at 0800h each morning during treatment weeks, and three times per week during pre- and post-treatment weeks. All cows were challenged with 2 ml Leptoshield Vaccine (CSL Animal Health, Australia) subcutaneously after 3 days of cold exposure to test immune responses. During WR, cows spent a greater proportion of time standing and less time lying down and eating than during I. Infrared temperatures were lower during WR than I in both dorsal and orbital (eye) regions. There was a distinct diurnal pattern of internal body temperature which had a greater amplitude during WR than I resulting from both a lower minimum and a higher maximum. The time of the minimum was 40 min later for WR than I. The overall mean body temperature was 0.07°C higher in WR than I. There were greater increases in plasma and faecal cortisol during WR than I, respectively. Total T4 was higher during WR than I. Non-esterified fatty acid concentration was higher in the week following WR than I. Total white blood cell numbers were lower during WR than I. No treatment differences were found for creatine kinase or for tumour necrosis factor, heat shock protein 90, interleukin 6 or interferon gamma expression in response to vaccination. In conclusion, this study applied a suite of stress measures to dairy cows exposed to extreme cold and wet conditions. Together, these measures indicated activation of the stress axis, physiological and behavioural adaptations to cold and a reduction in welfare. A number of these measures could be used to assess welfare under cold conditions on farms.

Influence of exogenous triiodothyronine (T3) on fecal shedding of Escherichia coli O157 in cattle

Fecal prevalence of Escherichia coli O157 in ruminants is highest in the summer months and decreases to low or undetectable levels in the winter. We hypothesize that the seasonal variation of this pathogen is a result of physiological responses within the host animal to changing day length. The thyroid is an endocrine gland known to respond to changing day length. Two experiments were conducted to determine if a hyperthyroid status would initiate fecal shedding of E. coli O157 in cattle during the winter when shedding is virtually nonexistent (winter experiment) or influence cattle actively shedding E. coli O157 (summer experiment). Yearling cattle were group-penned under dry-lot conditions, adjusted to a high concentrate ration, and randomly assigned to treatment: control (1 mL corn oil injected s.c. daily) or triiodothyronine (T(3); 1.5 mg suspended in corn oil injected s.c daily). Cattle were individually processed daily for collection of fecal and blood samples. Treatment with exogenous T(3) produced a significant change in serum thyroid hormone concentrations indicative of a hyperthyroid status in both experiments. No differences (P>0.10) were observed in fecal shedding of E. coli O157 in the winter experiment. In the summer experiment, fecal shedding of E. coli O157 was decreased (P=0.05) by administration of T(3) during the treatment period (days 1-10), tended to be lower (P=0.08) during the following 7-day period of no treatment, and was lower (P=0.01) when examined across the entire experimental period. Results of this research indicate that the thyroid or its hormones may be involved in the seasonal shedding patterns of E. coli O157 in cattle.

Effects of growth-promoting agents and season on blood metabolites and body temperature in heifers1,2

To assess the efficacy of growth-promoting agents among seasons, triiodothyronine (T3), thyroxine (T4), plasma urea nitrogen (PUN), IGF-I, and tympanic temperature (TT) were measured in summer and winter studies. Heifers (n = 9/pen) were allotted to 12 pens in both December and June. Pens were assigned to 1 of 6 growth promotant treatments: control (no growth promotant), estrogenic implant (E), trenbolone acetate implant (TBA), E + TBA (ET), melengestrol acetate (MGA), and ET + MGA (ETM). Blood samples were collected from 4 heifers per pen per study on d 0, 28, 56, and 84 via jugular puncture. Near the midpoint of both studies, TT were obtained from the heifers. There was a season by sample day interaction for all blood metabolites (P < 0.05). During the winter, IGF-I levels peaked on d 28, whereas T3, T4, and PUN peaked on d 56. In the summer, IGF-I levels increased from d 0 to 28 and remained elevated throughout the study. Season by growth promotant interactions (P < 0.05) indicated that in the winter ET increased T3, whereas TBA alone decreased both T3 and T4, compared with control, or ET, and ETM treatment groups. Across seasons, treatments ET and ETM increased (P < 0.05) IGF-I and decreased (P < 0.05) PUN. However, E, TBA, and MGA alone had no effect on IGF-I or PUN concentrations. The maximum TT was greater (P < 0.01) in the summer than in the winter, whereas the minimum TT was lower (P < 0.01) in the summer. Mean TT did not differ among growth-promoting treatments. However, in the summer and over both seasons, the maximum TT was lower (P < 0.05) in E-, MGA-, and ETM-treated heifers. Although limited growth promotant by season interactions existed, changes in blood metabolite levels resulting from the use of growth promotants do not appear to influence seasonal changes in body temperature as measured by TT.

Activity of the hypothalamic-pituitary-thyroid axis during exposure to cold

It seems clear from the studies reviewed here that there is adequate evidence to support the concept of a biphasic response of the thyroid gland to cold as first postulated by Moll et al. (1972). The initial response to acute exposure to cold begins at the level of the hypothalamus as a result of either neural stimuli from skin and other areas and/or blood of somewhat lower than normal temperature reaching the hypothalamus (Andersson et al., 1963). As a result, the secretion of norepinephrine and/or dopamine may increase, and serotonin and/or somatostatin may decrease. The net result of these is an increase in the release of TRH from the hypothalamus. This, in turn, stimulates the cascade for the release of TSH from the anterior pituitary gland and thyroid hormone from the thyroid gland. Moll et al. (1972) postulated the lack of a feedback limb in this acute phase, and, indeed, this may be the case. It is possible, however, that certain hormones, such as somatostatin, norepinephrine, T3, and T4 could act in the capacity of feedback inhibitors. Additional experiments will be required to assess this possibility. The transitional link between the acute (less than 1 day) and chronic (greater than 1 day) phases of the response of the thyroid gland to cold could be T4 itself. An increase in the concentration of T4 in plasma has been reported to increase peripheral deiodination of T4 to T3 by kidneys and liver of rats. There are no studies at present to indicate that hepatic conjugation can be increased by elevation of plasma levels of T4 and T3. If it can, these responses would provide adequate reasons as to why peripheral metabolism of thyroid hormones increases during chronic exposure to cold. The time-course for these changes to occur needs to be studied in greater detail to establish the sequence of events following acute exposure to cold. The latter may also increase urinary excretion of T4 and T3 in man, but not the rat. This suggests that another aspect of exposure to cold needing additional study is measurement of the binding affinities of T4 and T3 for their transport proteins during exposure to cold as compared to affinities prior to exposure to cold. If binding affinities are reduced, the amount of free hormones would increase and, consequently the likelihood of being excreted into urine and conjugated by the liver would also increase.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of shearing on the energy metabolism of the pregnant ewe

1. Metabolizable energy (ME) intakes, heat production, non-protein respiratory quotient (NPRQ) and the plasma concentrations of glucose, non-esterified fatty acids (NEFA), 3-hydroxybutyrate, insulin, growth hormone (GH) and cortisol were measured in shorn and unshorn pregnant ewes. 2. Lamb birth-weight was 17% higher from shorn ewes despite similar ME intakes in the two groups. Shearing resulted in a significant decrease in the digestibility of dry matter and energy. 3. Both shorn and unshorn ewes were found to be in positive nitrogen balance and negative energy balance. Heat production was 28% higher in shorn ewes. This increase in heat production in the shorn group could be completely accounted for by an increase in the oxidation of fatty acids as measured using the NPRQ values. 4. Despite an apparent increase in the use of fat as an energy source there were no effects of shearing on the mean plasma concentrations of NEFA, 3-hydroxybutyrate, GH and cortisol. 5. Measurements made at 1 h intervals for 24 h indicated a tendency for the concentrations of glucose to be increased and insulin decreased in shorn ewes, particularly, between 6 and 11 h after feeding. 6. It is concluded that shearing pregnant ewes at 8 weeks before lambing results in a chronic increase in energy requirements which are met by oxidizing body fat depots. The cold stress induced by shearing may also inhibit insulin secretion resulting in increased plasma glucose concentrations. The effects of shearing on energy metabolism in the ewe are discussed in relation to the nutrient supply for the developing fetus.

Insulin response to secretogogues in sheep exposed to cold

1. The effects of cold exposure on insulin secretion in response to intravenous injections of glucose, butyrate, arginine, glucagon and tolbutamide were examined in ten sheep exposed to a temperature of 0 degree C for 4-13 days. 2. Cold exposure produced a marked decrease in insulin responses to glucose and butyrate to a level similar to that caused by arginine administration. Cold exposure brought about a significant but only slight decrease in the insulin response to arginine. 3. Glucagon injection resulted in a markedly lower secretion of insulin in the cold than in the warm environment. 4. In the warm environment tolbutamide caused a significant increase in insulin secretion, but in the cold environment did not produce any statistically significant increase in plasma insulin. 5. It is concluded that cold exposure decreases insulin secretion in response to a variety of stimuli.