3.5. Trial 4 3.5.1. Conditioning
Groups did not differ in straw intake during the first 3 days of conditioning Ž
. P 0.05; Fig. 2 . After day 3, intake of straw was higher for lambs infused with starch
Ž .
Ž . Ž
Group 1 than for lambs infused with water Group 2 Group by day interaction,
. P - 0.05; Fig. 2 . Average intake of straw across days was 44 g for lambs in Group 1
Ž .
and 27 g for lambs in Group 2 P 0.05; S.E.M.s 10 . 3.5.2. IndiÕidual tests
Lambs conditioned with starch ate more straw at higher rates than lambs conditioned Ž
. with water P - 0.10; Table 1 .
3.5.3. IndiÕidual tests without infusions Ž
. Ž
. Lambs treated with starch ate more straw P - 0.10 , at faster rates P - 0.15 , and
Ž spent more time feeding during the 5-min tests than lambs treated with water P - 0.10;
. Table 1 , even when starch infusions were suspended.
4. Discussion
4.1. PostingestiÕe feedback and ingestiÕe behaÕiour It has been proposed that the potential rate at which a food can be eaten is largely
Ž .
determined by the food’s physical characteristics Kenney and Black, 1984 . However, under most conditions, food structure and quality are confounded because foods with
high nutrient content are typically easy to ingest and highly digestible, whereas plants with low nutrient content are usually difficult to ingest and poorly digestible.
Thus, we sought to isolate the effects of a plant’s structure from its postingestive effects. We hypothesized that rates of food intake are not only determined by the
physical structure of a forage but also by its postingestive actions. We found that groups of lambs exposed to the same forage — wheat straw — for the same periods of time
showed quite different patterns of ingestive behaviour, dependent on the postingestive consequences during straw ingestion. Lambs that received intraruminal infusions of
Ž .
starch increased intake, intake rate, bite rate and intake per bite Table 1 . Lambs manifested this behaviour even when starch was no longer supplied during testing
Ž .
Ž Table 1 . Lambs conditioned with starch continue to prefer flavoured straw Villalba
. and Provenza, 1997a , even when starch is no longer administered, which suggests that
the postingestive effects of energy increase the hedonic value or liking and make preferences — and ingestive behaviour in this study — more resistant to extinction.
Most studies of grazing have focused on the influences of food structure on intake Ž
. rate Hodgson et al., 1999 , and they have manipulated sward structure rather than sward
nutritional quality. The consensus is that other factors — nutritional and toxicological Ž
. — being equal, intake rate is largely determined by bite mass Ungar et al., 1991 ,
Ž which in turn is influenced by canopy structure e.g., herbage height and herbage weight
. Ž .
per unit canopy volume Black and Kenney, 1984; Laca et al., 1992 . For instance,
sward height has been proposed to affect bite depth and consequently intake per bite Ž
. Gong et al., 1996; Hodgson et al., 1999 .
Ž In contrast, our study shows that when physical structure is held constant e.g., same
. forage, same height and density — and nutritional effects are unequal — intake rate,
bite rate and bite size are variable and largely influenced by the postingestive conse- quences of food ingestion. Rates of straw intake were low and variable when no starch
was infused in the lambs’ stomach, which supports previous work suggesting that
Ž .
animals forage at variable rates, often lower than their potential Owen-Smith, 1993 . Low rates of intake are more likely to occur when foods supply low amounts of
macronutrients and other foods of higher quality are available for consumption. Ž
. Rates of food ingestion are elastic
Owen-Smith, 1993 , and influenced by an Ž
. animal’s physiological state Newman et al., 1994; Villalba and Provenza, 1999a .
Fasted animals can compensate for increased nutrient demands by increasing bite rate Ž
. and bite size Paterson et al., 1998 . Even though larger bites require more mastication,
Ž cattle can reduce the amount of chewing to increase intake rate
Greenwood and .
Demment, 1988 . Increases in chewing due to increases in bite mass can also be Ž
compensated by using jaw movements for both cropping and chewing Laca et al., .
1994 . Besides the general effects of fasting, specific nutritional needs can also modify rates
Ž .
of food intake. Intake rates of an energy-dense food barley are higher in lambs after a Ž
. meal high in protein than after fasting for 15 h Villalba and Provenza, 1999a . This
response is likely due to an increased need to balance the excess nitrogen supplied by the protein-rich meal with rapidly fermentable carbohydrates in barley. In the current
study, energy-restricted lambs increased their rate of straw intake when straw was paired with starch. Collectively, these results suggest that rates of food intake are dynamic and
depend on the nutrient composition of the food and the animal’s physiological condition.
The plasticity of rates of food intake are also manifest through their dependence on an animal’s past history. Foraging skills, acquired through experience, have a significant
impact on foraging behavior by increasing the efficiency of food consumption and by Ž
enhancing rates of forage intake Provenza and Balph, 1988; Flores et al., 1989;
. Ortega-Reyes and Provenza, 1993 .
4.2. PostingestiÕe feedback and food choice Food preferences also have been regarded as a function of the rate at which foods can
be ingested. Other factors being equal, plants that can be eaten quickly are likely to be Ž
. preferred to tougher species Wilman and Asiedu, 1983; Kenney and Black, 1984 .
Ž .
Ž .
Cattle Distel et al., 1995 and sheep Black and Kenney, 1984 select those patches that allow higher intake rates from swards that differ in height and density. Thus, in
horizontally heterogeneous pastures of similar quality, animals may be able to assess the Ž
. reward i.e., postingestive consequences from a patch by sensing the rate at which food
Ž can be obtained from the patch, and modify their preferences accordingly Distel et al.,
. 1995 . However, when sward quality is variable, nutrient composition can be even more
Ž .
Ž important than structure in determining preference. Tall Bazely, 1990 or short e.g.,
. Illius et al., 1987 swards patches may be preferred when their nutritional quality is
higher. Foods that yield lower intake rates can be preferred by lambs fed unbalanced basal diets when those foods provide ratios of energyrprotein closer to the lambs’
Ž .
nutritional needs Villalba and Provenza, 1999a . Thus, rates of food intake should be considered an emergent property of the interaction between the structure and biochemi-
cal composition of food. A trade-off between diet quality and intake rate has been found for ewes grazing
Ž .
cocksfoot swards. In order to consume higher quality components leaves , ewes must take smaller bites which promotes lower intake rates; less selective ewes take larger
Ž .
bites and have higher intake rates to the detriment of diet quality Prache et al., 1998 . Thus, degree of selectivity can influence rates of food intake. Rates of food intake are
then a result, rather than a cause, of food selection. We have shown that lambs ingest wheat straw at rates lower than their potential — when offered straw paired with water,
presumably because of a lack of reward during and after straw ingestion. Conversely, lambs increased straw ingestion — bite rate, bite size, number of bites — when they
received infusions of starch. If the food preferences of ruminants are controlled by specific associations between a food’s flavour and the food’s postingestive actions
Ž
. Provenza, 1995a,b, 1996; Villalba and Provenza, 1996, 1997a,b , then it is likely that
once a particular food is preferred, postingestive feedback from nutrients may enhance rates of food consumption. In contrast, when preferred items become less accessible,
intake per bite will decrease and animals may lower their intake rate.
4.3. PostingestiÕe feedback and palatability Palatability is classically defined to be a function of the taste, odour and surface
Ž .
characteristics of foods Arnold, 1981 . Within this context, it has been proposed that palatability is probably not an important determinant of intake for better-quality
Ž roughages, but it may limit intake of poor-quality roughages like straw Greenhalgh and
. Reid, 1971 . This was concluded from studies where sheep were fed straw while straw
was put through a fistula in the rumen. Under these conditions, sheep were extremely reluctant to eat straw, but they significantly increased straw intake when grass hay, a
Ž more nutritious food, was put into the rumen replacing straw Greenhalgh and Reid,
. 1971 .
We also found that straw intake was enhanced when lambs had an increased supply Ž
. of macronutrients to the rumen starch . However, we differ in the interpretation of the
results. Palatability is best understood as a compound phenomenon emerging from the Ž
. interrelationship between the chemical senses taste, smell and postingestive feedback
Ž .
Provenza, 1995a,b, 1996 . Thus, palatability per se is probably not the main reason for the low consumption of straw displayed by sheep, but an outcome of the association
Ž .
between organoleptic characteristics of straw e.g., flavour and the low amount of nutrients supplied to the rumen during and after straw ingestion. Decreases in preference
for straw due to a lack of nutrient feedback can explain why sheep become increasingly Ž
. reluctant to eat straw
Greenhalgh and Reid, 1971 , and why lambs given water displayed lower intake rates as exposure to straw increased. Conversely, lambs increase
Ž .
straw intake and preference for flavoured straw Villalba and Provenza, 1997a, 1999b
when straw ingestion is followed by intraruminal infusions of starch. This increase can be interpreted as an enhanced incentive or drive to eat straw due to its association with a
readily available source of energy.
5. Conclusions
