The diurnal patterns of rumen fill in sheep grazing pure white clover of prairie grass swards : A dissertation presented in partial fulfilment of the requirements for the Bachelor of Agricultural Science (Honours) Degree at Lincoln College

Abstract

The physical regulation theory is thought to explain the intake achieved by animals grazing forage diets. This postulates that intake is controlled by the amount of digesta in the rumen and the retention time of the material in the rumen. The diurnal pattern of grazing and rumen fill was studied to assess whether the initiation and cessation of eating was associated with a particular level of rumen fill. The use of markers to predict rumen fill in the grazing animal was also studied. Four wethers (65 ± 1.8kg ) fitted with large rumen cannulae grazed either a pure Huia white clover sward or a pure Matua prairie grass sward for 18 - 21 days. The time spent grazing each hour was automatically recorded. Animals grazed longer on the prairie grass, ( 8.9 h versus 5.8h ) on the white clover. Three main grazing periods were observed - morning, afternoon and evening. although generally the animals spent more of each hour grazing in the afternoon. The volume,wet weight and dry weight of the rumen digesta were recorded by manually emptying the rumen through the cannula at times chose to represent the three main grazing periods. The times chosen were 1 am, 5 am ( start morning ) 9 am ( stop morning ), 1 pm (start afternoon ), 5 pm, 9 pm, (stop afternoon ). The rumen fill showed a diurnal pattern. On the grass the maximum fill was at 9 pm, (9.5g DM/kg body weight) and the minimum fill was at 9 am (5.2g DM/kg body weight). while on the clover the maximum fill was still at 9PM but it was only 7.0g DM/kg bodyweight and the minimum fill was at 1pm (3.7g KM/kg body weight). A similar pattern occurred for the volume of rumen contents. On the grass 109ml/kg body weight at 9pm and the minimum was 74ml/kg body ·weight at 9 am. These values on the clover were 97ml/kg body weight and 88ml/kg body weight. The markers did not provide a reasonable estimate of the quantity of didgeta in the rumen and do not provide a satisfactory alternative to manually removing the digesta and gravimetrically determining the quantity present. The significance of the diurnal pattern of rumen fill is that animals do not always eat to an observed maximum rumen fill and hence intake is less than that which is physically possible. Intake was not limited by the time available for grazing as the observed grazing times were less than the potential maximum grazing times. Factors other than rumen fill appear to be involved in intake regulation of grazing animals. A model incorporating physical and chemical regulators would appear useful.