Rumen sensors provide specific information to help understand rumen functioning in relation to health disorders and to assist in decision-making for farm management. This review focuses on the use of rumen sensors to measure ruminal pH and discusses variation in pH in both time and location, pH-associated disorders and data analysis methods to summarize and interpret rumen pH data. Discussion on the use of rumen sensors to measure redox potential as an indication of the fermentation processes is also included. Acids may accumulate and reduce ruminal pH if acid removal from the rumen and rumen buffering cannot keep pace with their production. The complexity of the factors involved, combined with the interactions between the rumen and the host that ultimately determine ruminal pH, results in large variation among animals in their pH response to dietary or other changes. Although ruminal pH and pH dynamics only partially explain the typical symptoms of acidosis, it remains a main indicator and may assist to optimize rumen function. Rumen pH sensors allow continuous monitoring of pH and of diurnal variation in pH in individual animals. Substantial drift of non-retrievable rumen pH sensors, and the difficulty to calibrate these sensors, limits their application. Significant within-day variation in ruminal pH is frequently observed, and large distinct differences in pH between locations in the rumen occur. The magnitude of pH differences between locations appears to be diet dependent. Universal application of fixed conversion factors to correct for absolute pH differences between locations should be avoided. Rumen sensors provide high-resolution kinetics of pH and a vast amount of data. Commonly reported pH characteristics include mean and minimum pH, but these do not properly reflect severity of pH depression. The area under the pH × time curve integrates both duration and extent of pH depression. The use of this characteristic, as well as summarizing parameters obtained from fitting equations to cumulative pH data, is recommended to identify pH variation in relation to acidosis. Some rumen sensors can also measure the redox potential. This measurement helps to understand rumen functioning, as the redox potential of rumen fluid directly reflects the microbial intracellular redox balance status and impacts fermentative activity of rumen microorganisms. Taken together, proper assessment and interpretation of data generated by rumen sensors requires consideration of their limitations under various conditions.

The objectives of this study were to investigate the individual variation, repeatability and correlation of methane (CH4) production from dairy cows measured during 2 different years. A total of 21 dairy cows with an average BW of 619±14.2 kg and average milk production of 29.1±6.5 kg/day (mean±s.d.) were used in the 1st year. During the 2nd year, the same cows were used with an average BW of 640±8.0 kg and average milk production of 33.4±6.0 kg/day (mean±s.d.). The cows were housed in a loose housing system fitted with an automatic milking system (AMS). A total mixed ration was fed to the cows ad libitum in both years. In addition, they were offered concentrate in the AMS based on their daily milk yield. The CH4 and CO2 production levels of the cows were analysed using a Gasmet DX-4030. The estimated dry matter intake (EDMI) was 19.8±0.96 and 23.1±0.78 (mean±s.d.), and the energy-corrected milk (ECM) production was 30.8±8.03 and 33.7±5.25 kg/day (mean±s.d.) during the 1st and 2nd year, respectively. The EDMI and ECM had a significant influence (P<0.001) on the CH4 (l/day) yield during both years. The daily CH4 (l/day) production was significantly higher (P<0.05) during the 2nd year compared with the 1st year. The EDMI (described by the ECM) appeared to be the key factor in the variation of CH4 release. A correlation (r=0.54) of CH4 production was observed between the years. The CH4 (l/day) production was strongly correlated (r=0.70) between the 2 years with an adjusted ECM production (30 kg/day). The diurnal variation of CH4 (l/h) production showed significantly lower (P<0.05) emission during the night (0000 to 0800 h). The between-cows variation of CH4 (l/day, l/kg EDMI and l/kg ECM) was lower compared with the within-cow variation for the 1st and 2nd years. The repeatability of CH4 production (l/day) was 0.51 between 2 years. In conclusion, a higher EDMI (kg/day) followed by a higher ECM (kg/day) showed a higher CH4 production (l/day) in the 2nd year. The variations of CH4 (l/day) among the cows were lower than the within-cow variations. The CH4 (l/day) production was highly repeatable and, with an adjusted ECM production, was correlated between the years.

Individual drinking patterns are a potential tool for disease monitoring in pigs. However, to date, individual pig drinking behaviour has not been described, and effects of external factors have not been examined. The aim of this study was to perform detailed quantification of drinking behaviour of growing pigs and to examine effects of period of day and effects of competition for access to the drinking nipple on the drinking behaviour, amount of water used and water wastage. In all, 52 cross-bred castrated male pigs (live weight 20.5±1.7 kg; mean±s.d.) maintained as either 3 (N3) or 10 (N10) pigs per pen and water nipple (four groups/treatment) were used. All pigs were fitted with a transponder ear tag. A radio frequency identification reader recorded and time stamped visits at the nipple. In each pen, water flow was logged every second. The drinking behaviour was recorded for 4 consecutive days and analysed using a linear mixed model. Overall, the pigs spent 594 s at the nipple during 24 h distributed among 44 visits. During this period, 5 l of water were used, of which >30% was wasted. Social competition did not affect the drinking behaviour over 24 h, except for the proportion of interrupted visits where pigs, kept with recommended nipple availability (N10), showed an increased proportion of interrupted drinking bouts compared with pigs kept at very low level of competition (N3) (0.18±0.01 v. 0.11±0.01; P<0.01). However, splitting data into 8-h periods (P1, P2, P3) starting from 0600 h revealed differences between treatments, showing that in N3, water use per visit was lower in P1 than P2 and P3 (110±10 v. 126±7 and 132±7 ml; P<0.05), whereas in N10, the water used per visit was higher during P3 than during the other periods (P1: 107±14 ml, P2: 112±10 ml v. P3: 151±10 ml; P<0.001). A similar pattern was found for visit duration. In N3, fewer nipple visits were observed in P2 than P1 (15.6±1.2 v. 22.0±1.2; P<0.001), whereas no difference was found between P1 and P2 in N10. The results demonstrate that growing pigs at the two levels of competition maintained a comparable level of 24 h water intake by changing behavioural variables involved in drinking. This dynamic characteristic of drinking behaviour means that if individual drinking patterns are to be used as disease monitoring tools, it is important to consider effects of external factors and include data on period level to allow rapid detection of behavioural changes.

Better understanding of how animals regulate their intake may be gained by analysis of feeding behaviour. This is often recorded in terms of feeding events, e.g. visits to feeders, which can be clustered into meals. This enables calculation of the probability of animals starting a meal in relation to time since the last meal, which is thought to give insight into food intake regulation. Starting probabilities are often calculated with pooled data but recent work suggests that pooling may strongly affect conclusions.

In this study we analysed feeding behaviour of cows to investigate how previous conclusions about feeding behaviour may have been affected by pooling. Using parameters derived from experimental data, we constructed simulation models to further explore under what circumstances pooling, either across day and night or across individuals, could affect the interpretation of starting probabilities. Data were simulated to explore the consequences of pooling as either the proportion of meals occurring during the day or the individual variation in their mean number of meals per 24 h changed. Simulation allowed us to extend the analysis of the consequences of pooling for the interpretation of starting probabilities.

Analysis of experimental data, collected with 16 dairy cows, showed that they ate a mean of six meals per 24 h. Individual variation resulted in a proportional CV of the individual mean number of meals per 24 h of 0·14. Cows ate a mean proportion of 0·59 of their meals during the day. Analysis of experimental data suggested that pooling, conducted in previous studies, has probably led to a quantitative underestimation of the increase in starting probability with time since the last meal but not a qualitative misinterpretation of the direction of change in the starting probability.

Simulation studies showed that pooling had no serious consequences when the mean number of meals per 24 h, or the variation about this mean, was low. However, as the number of meals per 24 h and variation increased, pooling led to conclusions that may wholly misrepresent both magnitude and direction of the change in starting probabilities calculated separately for the individuals or for day and night. This may explain why the results of some published studies seem not to agree with biological principles of food intake regulation.

Twelve 6-month-old growing female goats and sheep were used in this study. Blood samples were obtained in the morning before goats and sheep were given food and then at 2-h intervals for 24 h (part I). This procedure was repeated 2 weeks later (part II). Concentrations of osteocalcin (OC), activities of total (tAP) and bone-specific alkaline phosphatase (bAP), degradation products of C-terminal telopeptide of type-I collagen (CrossLaps™ CL), and carboxyterminal telopeptide of type-I collagen (ICTP) were measured in serum.

In both parts of the study, all bone marker concentrations were significantly higher in goats than in sheep. The OC concentrations in goats increased in the late afternoon/evening and decreased thereafter to reach values similar to those obtained at the beginning. The ICTP concentrations in goats slowly decreased until 14:00 h, increased, and decreased again. The concentrations in sheep decreased continuously but not significantly, towards the morning sampling. The CL concentrations increased in both sheep and goats during the night but at 06:00 h started to decrease to levels found at the beginning of testing. The bAP activities decreased in goats from 20:00 to 22:00 h. Changes in the concentrations of bone markers were mainly observed in goats of this study. As documented for bone resorption and formation in other species, circadian rhythms were evident for concentrations of ICTP, CL, bAP and OC. The present study indicates that growing goats may have a physiologically higher bone turn-over than growing sheep, because the bone marker concentrations were always higher.

Peripheral concentrations of melatonin in the neonatal mammal are low and the diurnal rhythm is not apparent during the first days or weeks of life. Maternal milk provides a rhythmic although low source of melatonin to the newborn. Diurnal rhythm of serum melatonin was monitored in Friesian calves from birth to 4 weeks of age. Some of the calves under 1 week of age had no clear nocturnal elevation of serum concentrations of melatonin. Thereafter the nocturnal melatonin elevation was apparent in all calves and of the same amplitude as in adult cattle. The effect of oral intake of night-time milk or milk supplemented with low doses of synthetic melatonin was studied during day-time in a group of Ayrshire calves aged 1 to 5 weeks. Two and a half or 3 litres of night-time milk with a mean melatonin concentration of 40 pg/ml, providing 01 to 012 μg melatonin, did not cause any change in the day-time serum concentrations of melatonin, neither did addition of melatonin in doses up to 10 μg per animal. Only the highest dose of melatonin 200 μg in 2 l of milk (01 μg/ml) caused an elevation of day-time serum concentrations of melatonin. These studies show that the pineal gland of a very youg calf is functional and that the concentration of melatonin in blood does not reflect the concentration in milk at the time of feeding. The presence of melatonin in milk suggests, however, its potential significance for the neonate.

A series of 24-h video studies on four commercial Danish pig herds investigated the behaviour of pregnant sows kept in dynamic groups (72 to 200 sows) with electronic sow feeding (ESF). The herds mainly differed with respect to provision of a layer of unchopped straw as bedding material, the frequency of introduction/removal of animals, space allowance in the lying area, group size and number of feeding stations, and starting times for the feeding cycle. All herds had one feeding cycle per 24 h. Six 24-h video recordings in the most settled period with respect to rank relationships (2 to 12 days after the last introduction) were collected from each herd and analysed according to activity and aggressive interactions. In two of the four herds recordings were collected in two separate periods with different starting times for the feeding cycle. In all herds peaks of activity in the morning and in the afternoon were evident. Furthermore, the diurnal variation was overlaid by an increase in activity at the start of the feeding cycle. The activity and aggression in the feeding area as well as the duration per sow of the period with high feeder occupation were lowest in herds where a layer of straw bedding was provided. Changing feeding start from day time to night time was accompanied by a reduced feeder occupation in the period following start of the feeding cycle. However, in herd 1 (characterized by no litter, high stocking density and high frequency of grouping) the level of aggression was reduced, whereas it was increased in herd 2 (characterized by straw in lying area, low stocking density, low frequency of regrouping). Number of regroupings and space allowance apparently had no obvious effects on the average frequency of aggression or the aggression per sow at risk in periods between introduction of new animals, but space allowance may have improved social function by weakening the association between activity and aggression. Due to the small number of herds included the present results were descriptive rather than conclusive. However, the study supports the suggestions that provision of unchopped straw as bedding material and starting the feeding cycle in the evening for overnight feeding may improve behaviour in dynamic group housing systems with ESF. However, the benefits of starting the feeding cycle in the evening may depend on low disturbance in daytime from other management procedures.

Plasma leptin concentration shows pulsatility and diurnal rhythm in humans. However, there are few reports concerning the 24-h profile of circulating leptin levels in ruminants. Five crossbred ewes were housed in metabolism cages under a 1-h light-dark cycle. The ewes were offered alfalfa hay daily to meet their energy requirement. Blood samples were collected at 15-min intervals for 24 h. Plasma leptin concentrations were determined using a radioimmunoassay and the profile of plasma leptin levels was analysed by the PULSAR algorithmic program for detecting pulse. Plasma leptin concentration changed in a pulsatile fashion. The mean leptin concentration was 2·93 ng/ml. The mean pulse frequency was 4·8 pulses per day and the mean pulse amplitude was 0·67 ng/ml with an average pulse length of 1:13 h. Plasma leptin level was not affected by feeding or lighting cycle. These results indicate that plasma leptin level in sheep shows pulsatility but diurnal rhythm is not exhibited.

Global warming over the next century is likely to be associated with a change in the extent to which atmospheric and soil temperatures fluctuate, on both a daily and a seasonal basis. The average annual temperature of the Earth's surface is expected to increase, as is the frequency of hot days. In this review, we explore what effects short-term and long-term changes in temperature are likely to have on root respiratory metabolism, and what impacts such changes will have on daily, seasonal and annual CO2 release by roots under field conditions. We demonstrate that Q10 values, and the degree of acclimation, differ between and within plant species. Changes in the temperature sensitivity of respiration with measuring temperature are highlighted. Temperature-dependent changes in adenylate control and substrate supply are likely to control the Q10 and degree of acclimation of root respiration. Limitations in respiration capacity are unlikely to control respiratory flux at most temperatures. The potential role of nonphosphorylating pathways such as the alternative oxidase in controlling Q10 values is highlighted. The possibility that potentially rapid changes in adenylate control might underlie the acclimation response (rather than slow changes in enzyme capacity) has implications for the total amount of CO2 respired by roots daily and annually. Our modelling suggests that rapid acclimation will result in near-perfect homeostasis of respiration rates and minimize annual CO2 release. However, annual CO2 release increases substantially if the speed of full acclimation is lower. Our modelling exercise also shows that high Q10 values have the potential to increase daily and annual CO2 release substantially, particularly if the frequency of hot days increases after global warming.

Photosynthetic rates of plants grown in natural systems exhibit diurnal patterns often characterized by an afternoon decline, even when measured under constant light and temperature conditions. Since we thought changes in the carbohydrate status could cause this pattern through feedback from starch and sucrose synthesis, we studied the natural fluctuations in photosynthesis rates of plants grown at 36 and 56 Pa CO2 at a FACE (free-air-CO2-enrichment) research site. Light-saturated photosynthesis varied by 40% during the day and was independent of the light-limited quantum yield of photosynthesis, which varied little through the day. Photosynthesis did not correspond with xylem water potential or leaf carbohydrate build-up, but rather with diurnal changes in air vapor-pressure deficit and light. The afternoon decline in photosynthesis also corresponded with decreased stomatal conductance and decreased Rubisco carboxylation efficiency which in turn allowed leaf-airspace CO2 partial pressure to remain constant. Growth at elevated CO2 did not affect the afternoon decline in photosynthesis, but did stimulate early-morning photosynthesis rates relative to the rest of the day. Plants grown at 56 Pa CO2 had higher light-limited quantum yields than those at 36 Pa CO2 but, there was no growth–CO2 effect on quantum yield when measured at 2 kPa O2. Therefore, understory plants have a high light-limited quantum yield that does not vary through the day. Thus, the major diurnal changes in photosynthesis occur under light-saturated conditions which may help understory saplings maximize their sunfleck-use-efficiency.

Rate of passage of digesta out of the rumen is generally assumed to follow first order kinetics but this assumption may not hold within 24-h feeding cycles. Four lactating cows offered a mixed forage/concentrate diet ad libitum from 08.00 h to 00.00 h and a protein-rich meal at either 08.30 or 00.30 h were used to investigate diurnal patterns in rumen volume and composition of digesta flowing into the duodenum. Cows were fitted with large rumen cannulae to permit complete emptying of rumen digesta and T-piece duodenal cannulae with inserts to enable collection of representative samples of digesta. Chromium-mordanted lucerne hay manually dosed to the rumen at 2-h intervals and a continuous infusion of cobalt-EDTA were used as digesta flow markers. Duodenal digesta was sampled over three alternate days to obtain samples representing every hour of the 24-h day, which were analysed individually. Use of either of the two markers gave similar results for mean daily flow but not for hourly flow and thus hourly flow data were not considered reliable. The content of crude protein in duodenal digesta varied within one feeding cycle from 200 to 320 g/kg in cows offered protein at 08.30 (‘day’) and from 240 to 300 in cows offered protein at 00.30 h (‘night’). On a daily basis, fractional passage rates (calculated from duodenal flow divided by rumen pool size) were significantly (P = 0·016) lower for protein for ‘night’ (0·0750 per h) versus ‘day’ (0·0824 per h) cows. The observation that marked diurnal patterns in the composition of protein flowing into the duodenum can apparently be ‘smoothed’ simply by altering feeding sequence, which was correlated with an increased milk fat yield, suggests there is potential for improving production through manipulating the way in which dietary components are offered within a day.

Rate of passage of digesta out of the rumen is generally assumed to follow first order kinetics but this assumption may not hold within 24-h feeding cycles. Four lactating cows offered a mixed forage/concentrate diet ad libitum from 08.00 h to 00.00 h and a protein-rich meal at either 08.30 or 00.30 h were used to investigate diurnal patterns in rumen volume and composition of digesta flowing into the duodenum. Cows were fitted with large rumen cannulae to permit complete emptying of rumen digesta and T-piece duodenal cannulae with inserts to enable collection of representative samples of digesta. Chromium-mordanted lucerne hay manually dosed to the rumen at 2-h intervals and a continuous infusion of cobalt-EDTA were used as digesta flow markers. Duodenal digesta was sampled over three alternate days to obtain samples representing every hour of the 24-h day, which were analysed individually. Use of either of the two markers gave similar results for mean daily flow but not f or hourly flow and thus hourly flow data were not considered reliable. The content of crude protein in duodenal digesta varied within one feeding cycle from 200 to 320 glkg in cows offered protein at 08.30 day’) and from 240 to 300 in cows offered protein at 00.30 h (‘night’). On a daily basis, fractional passage rates (calculated from duodenal flow divided by rumen pool size) were significantly (? = 0·016) lower for protein for ‘night’ (0·0750 per h) versus ‘day’ (0·0824 per h) cows. The observation that marked diurnal patterns in the composition of protein flowing into the duodenum can apparently be ‘smoothed’ simply by altering feeding sequence, which was correlated with an increased milk fat yield, suggests there is potential for improving production through manipulating the way in which dietary components are offered within a day.

Photopic electroretinograms (ERGs) elicited by light flashes were recorded for three normal human subjects who were exposed, throughout, to natural ambient light cycles over 24-h day–night periods. ERGs were recorded either with the adaptation state of the eyes maintained at the level set by the natural ambient lighting, or after 10 min dark adaptation. The amplitudes and implicit times of both the a- and b-wave components of the ERG were examined and of these, only the b-wave implicit time exhibited significant diurnal variation, such that the nighttime values were 20–40% greater than those recorded during daytime. Such diurnal variations were observed under both recording conditions and cannot, therefore, be attributed to diurnal changes in the adaptation state of the cone photoreceptors. ERGs were recorded at midday and midnight during 24-h exposure to the natural light cycle, but during the recording period, the short-term adaptation state of the eye was controlled by exposure to rod saturating background field, so that visual sensitivity was the same at both recording times. The b-wave implicit times recorded at midnight were, nonetheless, greater than those recorded at midday. This difference is not, therefore, determined by the short-term state of retinal adaptation, but reflects long-term light history. Measurements performed under 24-h continuous light exposure showed no variation in the b-wave implicit time, whereas some measurements made during extended dark adaptation provided limited evidence for implicit time changes. By controlling the wavelengths to which the eye was exposed during the daylight phase of the diurnal cycle, it was shown that the shifts in b-wave implicit time associated with the change from dark to light are triggered by the rod system, although they are most clearly observed in the cone-dominant responses to long-wavelength light. The results demonstrate a diurnal variation in the temporal responses of the post-photoreceptoral cone pathways of the human retina, which is triggered by activation of the rod photoreceptors.

Eighteen multiparous Holstein-Friesian cows were offered grass silage ad libitum and 3,6 or 9 kg dry matter per day of concentrates. Triplicate milk samples were taken at each of two daily milkings over a 28-day period and analysed for fat protein and lactose concentration. Milk yield and fat concentration were more variable than protein and lactose concentrations both between animals and between days. Sampling variation was small for all component concentrations. The main source of variation for all traits was time of milking (morning or afternoon) followed by variation between cows and by day-to-day variation. There was little linear trend with time in the data due to cows being close to peak yield. There was no evidence of cyclical variation over time for any trait. Mean weekly or monthly yields and concentrations were better predicted using samples from four rather than from two consecutive milkings but there was no further advantage from using six samples.

An analysis of the variation of fore-, mid- and hindgut contents in third- to sixth-instar nymphs of the African grasshopper, Zonocerus variegatus, freshly caught every hour in the field, and also left without food for 1, 6 and 12 hr, showed that gut motility and the movement of ingesta through the gut was maximal at 08.00–10.00, 13.00–14.00 and 16.00 hr. Movement of ingesta was very slow at night. A diurnal rhythm therefore governs gut emptying. Nymphs, confined in jars with leaf discs, fed from 07.00 to 19.00 hr over the temperature range 17–34°C. Most food was eaten after 13.00 hr when it is warmer. Field insects feed during the night as well. Midgut α- and β-glucosidase activity was detected at all times of testing (06.00, 12.00, 19.00 and 24.00 hr), but the levels of activity were lowest at 12.00 and 19.00 hr.