For the maintenance of its caloric and nutritional balance, an animal must consume food. The level of intake ultimately depends upon the factors that govern onset and cessation of the successive feeding spells. These factors may be studied either at the purely behavioural level, or at the level of the underlying physiological mechanisms. However, in both cases a detailed specification of the overt behaviour is a prerequisite. This study attempts to present such a specification of feeding behaviour in cattle. However, for explaining the feeding pattern, rumination behaviour has been considered as well.
Units of feeding and rumination behaviour were defined at the level of 'bouts', i.e. uninterrupted performances of these behaviours. In the case of feeding behaviour, clusters of bouts formed higher order units, termed 'meals'. All other behaviour was only considered in terms of 'intervals' between the feeding and rumination bouts. For the analysis of durations of the behaviour units, the first- order Markov process was used as the random model. Deviations from randomness were derived from the frequency distribution of the durations.
The concept of 'state variables' was introduced to refer to the conditions in the animal that determine, together with external stimuli, the likelihood of occurrence of a given behaviour. As working hypothesis it was assumed that (1) there are state variables that specifically promote feeding, and (2) state variables that specifically promote rumination.
The first part of the investigation concerned an analysis of the feeding and rumination patterns of cows fed ad lib. with hay wafers (Chapter 2, 3 and 4). The feeding rhythm in cattle is characterized by an alternation of 'meal' and 'interval' states. In the meal state, the animal is likely to eat, but this may be interrupted by relatively short spells of non-feeding behaviour. In contrast, in the interval state feeding is unlikely. To make a distinction between these two states as sharp as possible, a meal criterion of 20 min seems preferable.
The feeding rhythm is subject to strong diurnal rhythmicity. During the greater part of daylight, the main feeding period, meal size is positively correlated with length of the preceding interval, suggesting that meals tend to stop once some fixed level of food repletion is reached. On the other hand, meal size is not correlated with the length of the interval to the next meal. Therefore, it is improbable that cattle do not start meals until the food ingested at the previous one has been used up. However, the correlations between meal size and length of the adjacent interval change markedly towards the end of the day, probably due to the strong changes in the motivation for feeding at that time.
Rumination occurs in prolonged bouts, which are clearly spaced out in time. Apart from interference by feeding, the rumination rhythm maintains a rather constant periodicity over the day. Furthermore, duration of a bout does not affect the duration of the interval till the next bout, but a weak tendency exists for the latter to become longer when it starts earlier.
The ad lib. data strongly suggest that ongoing rumination may be broken off under the influence of the state variables for feeding, and further that renewed rumination is prohibited as long as the feeding rhythm stays in the meal state. After feeding has interfered in this way, the phase of the rumination rhythm is reset. The resumption of rumination is rather independent of the size of the foregoing meal.
The percentage of time spent on rumination during a between-meal interval does not affect size of the subsequent meal; nor does meal size affect the amount of rumination during the next interval. However, daily food intake and total rumination time are positively correlated, suggesting that facilitatory relations between the two behaviours do exist on a longer time scale.
On the basis of these results obtained under ad lib. conditions a preliminary model of feeding and rumination in cattle was put forward (Paragraph 4.4). On aim of the following experiments was to assess to what extent the assumptions in this model hold for a wider range of conditions.
In the first experiment it was established that the prevention of rumination during a three-hour period of food deprivation results in an increase of rumination in the subsequent period, but does not change the feeding pattern at all (Chapter 5). This supports the assumption that feeding has priority over rumination whenever the causal factors of the two behaviours are in conflict. Moreover, it indicates that the state variables for feeding are wholly independent of the amount of rumination in the foregoing period of at least three hours.
Subsequent experiments involved the restriction of feeding to two periods of two and a half to three hours a day (Chapter 6). In one experiment the amount of food given was additionally restricted to different levels, both on a hay wafer and long hay diet. The results reveal that the state variables for rumination are strongly dependent on food intake, presumably the intake integrated over the foregoing 24- 48 hours. However, amount of food consumed and nature of the diet have different influences on the state for rumination (for, whereas on the hay wafer diet amount of food eaten and length of the intervals between rumination bouts are negatively correlated, a minimum interval length is already reached at a very low level of intake of long hay, the coarser diet).
During the prolonged non-feeding periods, the rumination rhythm maintained a fairly constant periodicity, irrespective of the level of food intake. This indicates that the need for rumination wanes only slightly with the performance of this behaviour. Further, the experiments confirm that overt rumination is inhibited whenever the feeding rhythm is in the meal state. Evidence has also been found that this inhibition is promoted especially by the presence of food stimuli, but external cues informing the animal that food will become available may be effective as well. Finally, the results suggest that after overt feeding has interfered, the rumination rhythm is reset to a fixed phase.
This study was concluded by putting together the final model of feeding and rumination in cattle (Chapter 7). Various predictions that can be made by combining the assumptions in this model fit the data of the described experiments very well. On the other hand, it has been emphasized that the assumption of absolute priority of feeding over rumination needs further verification.