meals was largely random and not tightly controlled by the animal reaching a threshold level of satiety. Although statistically significant, pre- and postprandial correlations were associated with very small proportions of the variation in meal size (Tolkamp et al., 2002). Preprandial correlations, although low (r ≈ 0.12) were about four times higher than postprandial correlations. This is a sign that feeding behaviour is determined more by satiety than by hunger mechanisms, and means that cows manage their daily intake by means other than inter-meal interval, i.e. feeding behaviour is flexible but, the longer the time over which meals are summed, the more stable and predictable is intake. Therefore, detailed studies of feeding behaviour are of little direct help in improving our understanding of what physiological mechanisms underlie the control of voluntary food intake on a longer timescale.
increased, chickens exhibit similar patterns of feeding to mammals, and similar functional relations between foraging cost, meal frequency and meal size, i.e.
meal frequency and, eventually, daily intake, decline with increasing difficulty of obtaining food. The authors conclude that: ‘Meal patterns do not reflect momentary fluctuations in the internal environment; rather, they appear to be a behavioural device that animals adjust to exploit the available resources in their current habitat efficiently’. Savory (1989) has suggested that feeding behaviour should be studied in conditions where birds have to do a modest amount of work to get food, as this provides more clearly defined meals than when food is easily available all the time.
Although broiler chickens eat almost twice as much as layers of the same age, they spend only half as much time eating and take more meals of shorter duration (Masicet al., 1974).
Feeding data of poultry have not yet been subjected to the rigorous analytical methods of Tolkamp and colleagues to define the critical inter-meal interval. However, analysis of a modest amount of data – from 11 individually caged male chicks of a layer strain, 9 weeks of age with an average body weight of 1115 g – has been performed (Hannah, 2001, using data from M.
Picard). The food provided 13.0 MJ ME and 190 g protein/kg. In every bird there were two peaks of interval frequencies, and an example is given in Fig.
2.8. Note that intervals of ⭐ 5 s have been filtered out before analysis, so the left-hand distribution is artificially truncated at 1.6 (natural logarithm of 5).
It is clear from these relative frequency distribution plots that two populations of intervals are present, i.e. intervals within and between meals.
These initial observations suggest that the log-normal model incorporating two Gaussian distributions can be used to analyse the data to obtain the critical interval time between meals in chickens, which was found to be 1 min 49 s for the bird shown in Fig. 2.8. The proportion of intervals in the first distribution was calculated to be 0.82. To calculate how many meals/day this bird was
Feeding Behaviour 25
Fig. 2.8. Relative frequency distribution of natural log-transformed intervals between feeding activity (interval length in s) of an individual chicken (from Hannah, 2001).
eating, the number of intervals/day between feeding activities must be multiplied by the proportion of intervals in the second distribution (1 – 0.82 = 0.18). The total number of intervals recorded for this chicken was 175/day, so the number of meals/day eaten was 175 ⫻(1 – 0.82) = 31.5. Ten other birds in the same study had critical inter-meal intervals of 18–163 s and took 30–70 meals/day.
The mean critical interval time between meals is therefore just under 3 min. The relative paucity of intervals in the middle of the distribution (between 1 and 20 min) means that changes in the critical interval within this range do not have a very large effect on the calculated number of meals.
Beak trimming
While beak trimming, sometimes carried out on laying hens to overcome fighting, does not affect daily food intake or meal number or size (Persynet al., 2004), beak-trimmed birds spent more time per day eating (3.3 versus 2.0 h/day), so ate more slowly (0.43 g/min-kg0.75 versus 0.79 g/min-kg0.75).
Trimmed hens seemed to be more selective as there was a higher proportion of large particles in the food refusals, suggesting that they found it more difficult to pick up particles, for which the tip of the beak is required. Because of this selection the food actually eaten by the trimmed birds had somewhat higher crude protein and mineral contents than for the untrimmed hens.
Pigs
Pigs grasp at food with their mouths and chew vigorously to mix it with saliva before swallowing the bolus. When kept outdoors, they spend a lot of time rooting in the soil with their snouts. Pregnant sows with nose rings to inhibit rooting ate pellets more slowly than did those without rings, especially when they had to root in turves for the food pellets, putting them at a disadvantage if a mixed group is given a limited amount of pelleted food (Horrellet al., 2000).
Stolba and Wood-Gush (1989) described the behaviour of domestic pigs in a semi-natural environment. Concentrates were given, as the paddocks were not large enough to support the groups with natural food. Even so, on average, 31% of the day was spent grazing and 21% rooting, i.e. over half of the time was spent in ingestive activity. The animals turned turves over to get at worms and prized certain types of root, especially tree roots and those of sedge.
Stereotypies and vices, common in indoor confinement, were absent.
Individually housed pigs have shorter and more frequent visits to the feeder than do pigs kept in groups. Nielsen et al. (1996) initially housed growing pigs in groups of ten for 14 days and then moved selected animals into individual housing. Contrary to expectations, the animals’ feeding behaviour did not appear to change greatly within this new social environment. The individual housing resulted in only a small increase in the frequency of feeder visits. The authors suggested that the feeding pattern of growing pigs may be less flexible than expected.
Cattle
Dulphy et al. (1980) and Dulphy and Faverdin (1987) have reviewed the feeding behaviour of ruminants in detail. A characteristic of grazing cattle is that they wrap their tongue around herbage and pull rather than biting cleanly.
They walk slowly forward, moving their head from side to side to take mouthfuls as they go.
The feeding behaviour of housed cattle depends to a large extent on the ratio of animal numbers to length of trough. If there is sufficient space for all animals to eat at once then this they will do, especially when fresh food has been given or when they have just returned from milking. At other times a proportion will be eating leisurely. If, however, there is insufficient space for all animals to eat simultaneously then a great deal of manoeuvring takes place, dominant animals displacing timid ones, and meals tend to be a succession of brief episodes interspersed with movement to other feeding positions. Those cows lower in the social dominance order are forced to eat at less popular times of day, including late at night.
Using a critical inter-meal interval of 9 min, it was found that Friesian- Holstein cows in early lactation ate an average of 8.7 meals/day, irrespective of whether the silage dry matter (DM) content was high (H, 449 g/kg) or low (L, 215 g/kg) or whether the supplementary compound food was high in starch or high in digestible fibre (Romney et al., 2000). The results show that chewing indices (time spent eating silage, ruminating and total time chewing/kg DM ingested) were significantly greater for the L silage, while time spent eating, and duration and number of meals were similar for either silage. The most likely explanation for the greater intakes of H is a faster particle breakdown in the rumen, allowing larger meal sizes before animals became constrained (see Table 2.1). Factors affecting silage intake are discussed in Chapter 14; the information given here is as an example of eating parameters in dairy cows.
In a similar experiment, intake reached a peak in mid-lactation, while the number of meals per day reached a nadir around week 15 of lactation (Jacksonet al., 1991). The daily intakes of silage for the two supplement types were 10.8 and 13.0 kg DM, respectively, eaten in 209 and 229 min. Thus the higher silage intake was achieved on the digestible fibre supplement by eating
Feeding Behaviour 27
Table 2.1. Food intake parameters derived from 48 h continuous recording of intake for dairy cows consuming low (L) or high (H) DM silage supplemented with 9 g/day concentrates (from Romneyet al., 2000).
Silage L H Significance
Silage intake (kg DM/day) 9.0 13.8 +++
Time spent eating (min/day) 183 200 nsa
Rate of eating (g/kg) 49 69 +++
Number of meals 8.4 8.6 ns
Intra-meal pauses (min/day) 27.6 36.8
aNot significant.
larger, longer meals. Figure 2.2 shows an example of a 24-h meal pattern of one cow. In this case, nine meals of silage were taken and three allocations each of 2 kg concentrates were dispensed. At 230 min, she tries to get concentrates but is not allowed and diverts her attention to eating silage. Some of the silage meals are protracted and fragmented (e.g. those at 650–750 min), while others are continuous (e.g. that starting at 840 min).
Dado and Allen (1994) have monitored feeding, ruminating and drinking behaviour in lactating cows offered a total mixed ration and have concluded that a Latin Square design with 12 cows monitored for 5 days would be sufficient to detect a 10% difference between means for feeding-related behaviour variables with a probability of 80%.
Sheep
From 2 or 3 weeks of age, lambs spend increasing amounts of time nibbling at food. After weaning they eat by prehending food with their lips and tongue, pulling it back into the mouth for extensive chewing.
Sheep can select well as they have a narrow bite. However, they have a blind area about 30 mm in front of the nose so they can’t see clearly what they are eating! Perhaps they use touch to decide exactly what to eat. When they are selecting actively, e.g. green material from a predominantly dead sward,