Background and Preliminary Results Determination of Energy Expenditure
Energy expenditure, measured as oxygen consumption (Vo2), of farm animals has been determined mostly under controlled, confined conditions because direct methods of measuring Vo2 (Young and Webster, 1963; Corbett et al., 1971; Young and Corbett, 1972; Schoeller and Van Santen, 1982) have limited field application or are very expensive. Unfortunately, results from confined conditions do not necessarily reflect those of free-ranging animals or of commercial cattle in feedlots. Environmental conditions, feeding level, time spent eating and digesting, tissue and pelage conductance, production level, and season of the year may affect EE of animals (NRC, 1981). Estimation of EE of free-ranging large animals based on HR, which is positively correlated with Vo2, has been examined by several workers (Webster, 1967; Richards and Lawrence, 1984; Renecker and Hudson, 1985; Yamamoto et al., 1989). This method could be most useful as recent developments in microelectronics allow the use of small HR radio transmitters to measure HR of animals in their natural habitat.
Brosh et al. (1998) examined whether HR could be used to estimate EE in cattle. The objective of this study was to examine whether HR in cattle can be used to estimate EE under different nutritional and environmental conditions. Six Hereford heifers (345 ± 10.8 kg BW), 12-mo of age, were implanted with HR radio transmitters and maintained in individual pens under the following treatments: 1) shade or sun exposure, 2) high- or low-energy diet, and 3) feeding in morning or afternoon. The HR of animals was measured for 5 min every half hour during an 85-d study; measurements of oxygen consumption and HR were made simultaneously in the morning and in the afternoon while animals were resting and exercising. Average daily HR (52± 4 beats/min) and average daily EE (380 ± 9 kJ/kg0.75) in animals on the low-energy diet were lower than values in animals on the high-energy diet (94 ± 4 beats/min and 653 ± 9 kJ/kg0.75, respectively). For each animal and within each diet, linear regressions best described the relationship between HR and EE in resting animals whereas quadratic regressions best described this relationship for exercising animals. The quadratic equation for the exercising animals could also be used for resting animals. In addition, a constant value of EE per heart beat (EE pulse) for each individual resting animal was found and gave accurate estimations. This method was convenient because 1) no exercise equipment was needed to generate the regression equations. They concluded that HR, a relatively easily obtainable measurement, can be useful and accurate in estimating EE.
The largest changes in Vo2 result from exercise. Studies, with humans, examining the accuracy of HR as a measurement of EE show that EE is best described with two prediction equations, one with a slope for HR in the sedentary range and the other for HR involving significant locomotor activity. The break point between the two slopes for Vo2 appears to be around 80 beats per minute (Dauncey and James, 1979; Kalkwarf et al., 1989). Preliminary results from free-ranging cows on green pasture (A. Brosh, Y. Ahanoni, and Z. Hozer, unpublished data) suggest that the ranges in HR levels were 86 to 116% for cows on low and high stocking rate regimes (3650 and 980 kg DM ha-1 , respectively). These values were within the range for penned animals (Brosh et al., 1998) on low (80 - 175%) and high (70 -149%) energy diets. Interestingly this preliminary study also showed that the daily peak for HR for the low stocking rate occurred between 1000 and 1400 hr, when the cows were at rest after the morning grazing. This suggests, that under the management conditions studied, variation in HR was not due to physical activity associated with grazing. Whether these ranges in HR are typical for all types of free-ranging cattle under all grazing conditions is unknown. A better understanding of the impact of the environment and managment systems on the HR of free-ranging cattle is needed before predictive equations for EE can be developed for free-ranging cattle.
Measurements of Activity:
Vibracorder, Pedometer, and GPS
Grazing management systems alter both quantity and quality of forage available for the grazing animal. In response to this, grazing cattle, in an attempt to meet their energy requirement, can alter their grazing time up to biologic limits set by rumen capacity, rate of passage, and environmental stresses. Under tropical conditions, Stobbs (1970) reported Jersey cows grazed 10.8 h day-1 when grazing a tropical legume compared to only 9.5 h day-1 when grazing a tropical grass. At the Subtropical Agricultural Research Station (STARS), Hammond and Olson (1994) found, that during the summer under subtropical conditions, Senepol cows, a tropically adapted Bos taurus breed of cattle, grazed longer (10.6 h day-1) than did non adapted Hereford cows (9.3 h day-1). They attributed this difference to better heat tolerance of the Senepol compared to the Hereford cows. Bowers et al. (1995), also at STARS, found that under subtropical conditions, time of year had a similar effect the grazing time of both Angus and Brahman cows. In that study, time spent grazing was less in the spring (6.4 h day-1) than in the summer (9.0 h day-1). This difference was due to the higher nutritive value of tropical grasses during the cooler spring period. Under northern temperate conditions environmental changes also affected the activity of grazing cattle, cows were found to spend less time grazing and to travel greater distances as temperatures got colder (Adams et al., 1991). Thus grazing activity, in addition to being an important determinant of production (i.e., milk production, weight gain, pregnancy, etc.), is a sensitive indicator of differences between forage nutritive value and environmental stress on the animal.
Until recently methods for measuring grazing activity were limited to direct observation and vibracorders (Stobbs, 1973), both of which have their limitations. Direct observations require considerable manpower and are usually are limited to the day light hours, which can result in erroneous conclusions. Under subtropical conditions during the summer, non adapted breeds of B. taurus cows (Angus and Hereford) were found to compensate for reduced grazing activity during the hotter parts of the day by increasing time spent grazing at night (Bowers et al., 1995; Hammond and Olson, 1994). Vibracorders do not have the limitation imposed by daylight, but they require manual, subjective interpretation, are difficult to keep in proper position on the animal, and expensive to maintain. Pedometers, electronic devices used to count foot falls, particularly when used in conjunction with GPS and continuous measurement of animal body temperature, may be useful tools for determining grazing activity.
Pedometer readings are based on changes in angular movement that activates a switching mechanism. In their simplest form, footfalls over a given period of time can be determined. Pedometers were first applied to animal agriculture as a means of detecting estrous activity in dairy cattle. During estrous, activity level markedly increases. This activity spike has been shown to predict time of ovulation as well as if not better than with conventional heat monitoring procedures. Improvement in pedometer technology, which includes greater distances for remote downloading of stored data and inclusion of preprogramable time periods for recording movement between downloading periods, has enhanced the usefulness of this technology for quantitatively defining differences in cattle behavior under grazing conditions.
Under Mediterranean conditions, preliminary studies (M. Rosen and M. Gutman, unpublished data) have shown that pedometer measurement of cattle activity is sensitive to changes in supplementation levels on pasture. When cattle were switched from a full amount of supplement (a concentrate-poultry litter mixture) to one-half the amount of supplement,number of steps increased an average of 423 steps day-1. In another study looking at the effect of residual biomass on grazing activity and supplement consumption (dry straw), they found that numbers of steps decreased (-625 steps day-1) on low residual biomass and supplement consumption increased when cattle were switched from high residual biomass to low residual biomass pastures. Their data suggest that pedometer readings are also sensitive indicators of environmentally mediated activity of grazing cattle. They found that temperature-humidity index during the night was correlated with pedometer activity regardless of residual biomass.
In addition to grazing activity, movement of free-ranging cattle can also vary due to spatial arrangement of forage resources within pastures and the proximity of water, minerals, and shade to grazing sites. Under humid, tame pasture situations, this may not constitute a significant portion of the total activity per day, but under extensive range situations this additional movement may be significant. Adams et al. (1991) used pedometers to study the interaction of body condition and supplement intake on the grazing activity of cows in northern temperate range sites (Miles City, MT). The found that as ambient temperature decreased, grazing time decreased and walking activity increased. Whether this increased walking activity was due to environmental effects on the animal or due to declining availability of desirable forage species as the grazing season progressed was not clear and is an example of the need for more integrated studies into the factors that affect grazing behavior. Additionally, the effect this increase walking activity might have on EE of the cattle is unknown.
Global positioning systems make use of the Navstar satellite system that consists of 24 active satellites that constantly transmit signals to earth. The position of a receiver on earth can be calculated based on the time it takes for the signal from three to four satellites at one time to travel to a receiver. Initial commercial applications of GPS included airplane and ship navigation and survey systems. In recent years there has been ever increasing use of GPS in agriculture, environmental studies, and wildlife research in such areas as precision farming systems, environmental changes (i.e., vegetation changes, water quality, soil erosion, etc.), and to track the location of wildlife. When combined with the analysis capabilities of GIS (Geographic Information Systems) software, spacial relationships can be determined and changes in time and space monitored.
Previously, most civilian GPS applications have had a positioning accuracy of only 100 m (95% of the time) due to selective availability (SA), which is an intentional degradation of the accuracy for military concerns. Although SA positions may be adequate for wildlife studies, with cattle, particularly under tame pasture situations where pasture sizes are relatively small, this degree of accuracy is unacceptable. Use of differential GPS technology has improved position accuracy to within 5 m (95% of the time) using commercially available satellite frequency, which now allows the use of GPS to monitor the behavior of free-ranging cattle. Use of this technology will provide a better understanding of the impact of management systems have on both animal and plant productivity by monitoring such things as spatial changes in forage availability and nutritive value, relative location of water and shade sites, and even the proximity of other animals.
Cost of GPS equipment remains a bottle neck for its use in grazing systems research. Even with the advent of smaller systems in the early 1990s, individual units currently run between $4000 to $5000 (US$) with battery costs running between $700 to $1000 per unit per trial period. In cattle grazing research, more than one animal per treatment must be monitored to take into account animal to animal variation. As a result the initial equipment cost for studies using GPS can be quite high. The proposed collaborative research, in addition to providing grazing behavior information under diverse environmental conditions and management conditions, allows year-long utilization of GPS equipment on up to 16 head of cattle at one time. Additionally the simultaneous measurement of GPS combined with both vibracorder, pedometer, and HR monitoring provides an unique opportunity to test the effectiveness of individual or combinations of these measurements for determining the effect of managment systems on the grazing behavior and performance of lactating beef cows.