DETAILED DESCRIPTION OF THE RESEARCH PLANDescription of the Problem
The most economially important trait in beef cattle production is reproductive performance (Wiltbank, 1994). Although some improvement in reproduction can be made by genetic changes, this method of change will be slow because the heritability of reproductive traits is low, around 10% (Wilham, 1973). The majority of improvement in reproductive performance in beef cattle will come from changes in the management environment (Wiltbank, 1994). In disease free herds, studies have shown that one of the largest losses in potential calves occurred because cows did not get pregnant during the breeding season (Wiltbank, 1983). The major factor regulating time of conception postpartum is weight or more accurately, body condition (BC) of the cow (Whitman, 1975; Kunkle et al., 1994). Whitman (1975) reported that 96% of the cows in good BC would be expected to breed within the first 20 days of the breeding season, compared to 79% for cows in moderate BC and 55% for cows in thin BC. In addition to the BC of cows at the start of the breeding season, weight changes, either gains or losses, during the breeding season also greatly affect the conception rate. In one study, only 43% of the cows losing weight at the start of the breeding season conceived after one service compared to 67% of the cows which experienced no weight change during the breeding season (Wiltbank et al., 1962). Conversely, 72% of dry cows in thin to moderate BC at the start of the breeding season conceived when fed a high level of energy compared to only 44 to 54% of the cows not supplemented with energy (Smith et al., 1980). On average, when using bulls of known fertility, 70 to 80% of the cows will conceive at the first service if the cows are gaining weight at the time of breeding (Wiltbank, 1994).
In beef cattle production systems world wide, the majority if not all of the energy requirement of the cow herd comes from forages, either native or tame. Grazing management decisions determine the available energy supply for grazing cattle by influencing such variables as forage availability, sward botanical composition, and nutritive value. Lactating cows are perhaps the most sensitive class of cattle to deficiencies or imbalances in nutrient availability supplied by pastureland because of their combined energy requirements for maintenance, lactation, and concurrent pregnancy. Determining the effect of management decisions that may alter the nutritional quality or quantity of forage species in different production environments has classically required a minimum of 3 to 5 years worth of treatment imposition. Additionally, relatively large numbers of animals are needed to evaluate the impact of management changes on the production of cow/calf systems because the long term effect on reproduction needs to be considered. A better understanding of the effect of grazing management on grazing behavior as it relates to energy expenditure (EE) of lactating beef cows should provide more precise and timely information for both production systems researchers and grazing system and animal modelers.
Objectives:
Objective 1. Determine the effect of grazing activity in subtropical and Mediterranean climates on heart rate (HR) of free ranging lactating beef cattle.
Objective 2. Determine the ability of animal activity measured by pedometers and/or global positioning systems (GPS) in conjunction with continuous body temperature monitoring to predict grazing behavior of lactating beef cows in Mediterranean and subtropical environments.
Objective 3. Determine the usefulness of pedometers and/or GPS as a substitute for HR monitors for predicting the EE of grazing lactating beef cows.
Hypotheses
1. Lactating beef cows are the most sensitive class of beef animal to changes in nutritive quality and availability due to the combined energy requirements of maintenance, lactation, and concurrent pregnancy.
2. Grazing management alters the quantity and nutritive value of available forage.
3. Subsequent changes in quantity and nutritive value of available forage in conjunction with environmental conditions, particularly temperature, affects the grazing behavior of lactating cows by reducing or lengthening the time and amount of grazing activity, which affects their ability to meet their dietary requirements.
4. Changes in grazing time affect the EE (as reflected in HR) of lactating beef cows which are reflected in weight and BC gain or loss.
5. Movement of lactating beef cows as determined by daily pedometer measurements is correlated with grazing behavior determined by vibracorder measurements and animal body temperature.
6. Diurnal and seasonal changes in relative geographic location of lactating beef cows in pasture or range sites are also correlated with grazing behavior determined by vibracorder measurements and animal body temperature.
7. Movements of grazing lactating beef cattle determined by pedometer and/or GPS can affect HR and thus predicted EE of grazing lactating beef cattle.