Factors Affecting Forage Intake
by Range Ruminants: A Review
Item Type text; Article
Authors Allison, C. D.
Citation Allison, C. D. (1985). Factors affecting forage intake by range
ruminants: A review.Journal of Range Management, 38(4),
305-311.
DOI 10.2307/3899409
Publisher Society for Range Management
Journal Journal of Range Management
Rights Copyright © Society for Range Management.
Download date 09/08/2022 20:40:01
Item License http://rightsstatements.org/vocab/InC/1.0/
Version Final published version
Link to Item http://hdl.handle.net/10150/645491
Factors Affecting Forage Intake by Range Rumi-
nants: A Review
C.D. ALLISON
Abstract
Variation in voluntary forage intake is undoubtedly the major
dietary factor determining level and efficiency of ruminant produc-
tion. This variation is largest and least predictable for grazing
ruminants. Range ruminant productivity and efficiency is rela-
tively low due, in part, to intake limitations; therefore, productivity
could probably be increased most by increasing intake. Most
available literature points to digestibility and rate of ingesta pas-
sage and reticula-rumen fill as primary mechanisms of intake
regulation in range ruminants. Body size and physiological status
of ruminants appear to have the largest effect of animal-related
factors
in governing level of voluntary intake. Kind and amount of
supplementation, forage availability, and grazing intensity are
major management-controlled variables affecting intake by
domes-
tic range ruminants.
Animal nutrition has generally been recognized as being depen-
dent upon 4 basic factors: the animal’s requirements, nutrient
content of the feedstuff, digestibility of the feedstuff, and how
much the animal will consume.
Range ruminant nutrition has unique characteristics and prob-
lems. Nutrient requirements of range livestock are not known
because requirements can be altered by grazing activity, travel, and
environmental stresses such as temperature extremes. Nutritive
value and digestibility are also difficult to determine because anim-
als select their diet from various combinations of plant species and
plant parts. The most critical factor in meeting nutrient require-
ments of a grazing ruminant is knowledge of how much it will
voluntarily consume. Conceptually, if an animal could eat enough,
it could satisfy its nutrient requirements on low-quality forages.
But total intake is limited by physical factors of the animal and
plant, animal physiological factors, and management strategies of
the plant-animal interface.
Crampton (1957) felt the value of a forage in animal production
depends more on the amount consumed than its chemical composi-
tion. This concept led to the nutritive value index for forages, based
on their voluntary intake and digestibility (Crampton et al. 1960).
Reviews on methodology to determine forage intake by range
ruminants include those by Cordova et al, (1978) and Kartchner
and Campbell (1979). Reviews on methodology to determine range
herbivore diets are available by Theurer et al. (1976), Van Dyne et
al. (1980), Holechek et al. (198 1, 1982), Harris et al. (1967), Harris
(1968), and Van Dyne (1969).
This review compiles findings regarding physical, physiological
and management factors that are known to affect or regulate
voluntary intake of range livestock.
Regulation of Voluntary Intake
Control of feed intake and regulation of energy balance in
ruminants were extensively reviewed at the Third International
Ruminant Symposium (Arnold 1970a, Baumgardt 1970, Cam-
pling 1970) and more recently reviewed by Baile and Forbes (1974).
In a review by Baile (1975), several intake-controlling mechanisms
were discussed. Included were humoral factors, neural transmit-
ters, and chemical and hormonal mechanisms, as well as digestibil-
ity, reticula-rumen fill, and rate of passage. The effect of oral and
abomasal infusions of volatile fatty acids on feed intake has been
recently studied by Papas and Hatfield (1978).
The bulky, fibrous nature of most range ruminant diets, and
their relatively low content of digestible energy, lends emphasis to
the importance of the physical effect of gut distention in limiting
voluntary intake. Considerable evidence is available showing, with
predominantly roughage diets, voluntary intake is limited by
capacity of the reticula-rumen and by rate of disappearance of
digesta from this organ (Balch and Campling 1962, Ellis 1978).
Rate of disappearance depends on rate of passage and rate of
absorption.
Voluntary food intake is limited by physical conditions within
the gut and particularly by amount of digesta in the reticulo-
rumen. Studies concerning
effects
on voluntary intake of intrarum-
inal additions or removals of food and other materials, relation-
ship between rumen-fill and voluntary intake, and relationship
between rate of disappearance of digesta and voluntary intake,
support the previous statement and were reviewed by Campling
(1970).
Removing swallowed hay as it entered the reticula-rumen
showed hay accumulation in the rumen exerted an immediate
effect on termination of eating by cows. Cows could be encouraged
to eat for much longer than normal periods by removing swallowed
hay. Conversely, addition of digesta, consisting of recently ingested
hay, to the rumen of cows during a meal caused an immediate
decrease in hay intake (Campling and Balch 1961). Confirmation
of these results is provided by Weston (1966), who conducted
experiments with a sheep offered chopped roughages. In this study
Author is extension management specialist, New Mexico State University, Las
Cruces 88003.
This article is Journal Article
I
107, Agr. Exp. Sta., New Mexico State University,
Las Cruces 88003.
Manuscript accepted January 2, 1985.
JOURNAL OF RANGE
MANAGEMENT 38(4), July
1985
305
coarsely ground roughage, sawdust, and finely ground polyvinyl
chloride were introduced into the rumen.
In general, ruminants try to compensate for the inclusion of
finely milled inert material in concentrate diets by an increased
total intake (Baile and Pfander 1964, Boling et al. 1967).
When ruminants are offered roughages such as hay and dried
grass, there is evidence that cattle and sheep eat to a constant
rumen fill. Blaxter et al. (1961) showed that sheep offered poor,
medium, and good hays had similar dry matter contents in the
digestive tract. Ulyatt et al. (1967) provided further confirmation
of these findings. Campling and Balch (1961) removed and
weighed digesta in the reticula-rumen after meals of hay and dried
grass. Eating ceased when the reticula-rumen contained similar
amounts of dry matter. Quantity of each roughage eaten was
directly related to its rate of disappearance from the reticula-rumen.
lntraruminal additions of water during eating did not affect the
feed intake of cattle or sheep (Campling and Balch 1961) presum-
ably because water rapidly leaves the rumen. These findings are
important with regard to forage moisture content and its effect on
dry matter intake.
Forage moisture level has been studied as a possible determinant
of voluntary dry matter intake. Minson (1966), feeding either fresh
or dried or frozen forage to sheep, showed no significant differen-
ces in voluntary intake resulting from method of preparation.
Renton and Forbes (1973) observed no significant differences in
dry matter intake of barely supplement fed either as a liquid or dry
supplement. Likewise, the supplement moisture level had no effect
on intake of the hay being fed. Holmes and Lang
(1963)
concluded
that dry matter intake of cattle feeding on fresh forage is not likely
to be restricted by either a high internal water content in the forage
or rain water on the leaf surface. Moisture level may affect selectiv-
ity of grazing. More succulent plants will usually be grazed in
preference to drier, more mature plants. Jackson and Forbes
(1970) observed that higher moisture levels in silage depressed
voluntary dry matter intake in cattle. However, moisture levels
may be having a secondary role to organic acids or other substan-
ces which are found in higher moisture silage and which may
influence dry matter and intake in some manner.
Van Soest (1982) states intake is dependent on the structural
volume and, therefore, cell wall content. The relationship between
water content of forages and intake, therefore, may be a function of
structural volume if the plant water is contained within the cell wall
structure. The addition of water per se to the rumen has little effect
upon intake because it is largely absorbed and removed. However,
Van Soest (1982) believes water retention by the sponge effect of
coarse structural components of ingested forage can have an inhib-
itory effect on intake.
Rate of disappearance of digesta from the reticula-rumen
depends primarily on its rate of digestion and this, in turn, depends
on chemical and physical properties of the food consumed (Hun-
gate 1966). The rapidly fermentable fraction of roughage does not
occupy space in the reticula-rumen for long periods of time, com-
pared with structural components of roughage-cell wall fractions.
Presenting ruminants with roughage that is in a physical form
which allows ready passage out of the reticula-rumen is generally
associated with a greater voluntary intake than when the same
roughage is in a longer, coarser form (Minson 1963, Poppi et al.
1981a). These findings support the concept of physical limitation
on roughage intake imposed by limited size of the reticula-omasal
orifice.
Waldo (1969) theorized that, with certain forages, intake could
be limited by rumen capacity and rate at which undigested residues
left the reticula-rumen. Using this theory, Thorton and Minson
(1972) postulated that voluntary intake could be estimated from
rumen fill and rumen organic matter retention time. These
researchers believed that if fill was assumed constant, dry matter
intake and retention time would be inversely related. Thornton and
Minson (1973) tested this hypothesis with grasses and legumes fed
to sheep. They found a significant correlation (r
q
-0.93) between
intake of digestible organic matter and retention time of organic
matter in the rumen. It was concluded that, because rumen fill with
all forages tested had been relatively constant, voluntary intake
was primarily affected by retention time of the fibrous fraction in
the rumen. Because retention time is reciprocal of rates of passage
and digestion, the expression proposed is, in essence, intake
q
total
outflow from the rumen. Greater consumption of leaf material
versus stems in legumes (Hendricksen et al. 1981) and grasses
(Poppi et al. 1981b) was associated with shorter retention time in
the rumen and not by differences in digestibility as such. Poppi et
al. (1981~) concluded higher voluntary intake and shorter rumen
retention time of grass leaves over stems was associated with: (a) an
apparent higher rate of digestion of neutral detergent fiber (NDF)
in vivo, (b) higher rate of passage of the NDF from the rumen, and
(c) higher potential digestibility of the leaf. Poppi et al. (198
1
b,c)
found cattle had longer ruminal retention times for NDF and for
large particles than did sheep. Golding (1976) concluded ruminal
retention time of organic matter was theoretically a rational vari-
able to predict voluntary intake, but more indirect methods to
estimate rates of passage were needed.
It would appear variation in voluntary intake of forages over the
gut-fill range could be explained to a high degree by differences in
rumen retention time, independent of rumen fill. However, some
inherent problems should be considered when interpreting this
relationship. Rumen capacity can be affected by animal attributes
such as pregnancy (Jordan et al. 1973). Thornton and Minson
(1973) have suggested certain plant attributes, such as low protein
content, result in reduced intake and gut fill. Under these circum-
stances, nitrogen deficiency would be the primary factor affecting
intake. This point will be discussed in greater detail in a later
section. Results of Egan (1970) point out that the level of reticulo-
rumen fill is not constant between diets, but is influenced by other,
presumably nutritional, factors. One such factor is protein nutri-
tion of the animal. Campling (1966) noted, with roughages con-
taining up to 8 to lO%crude protein, intake is apparently limited by
reticula-rumen capacity and rate of disappearance of ingesta from
this organ whereas intake is limited by other metabolic factors with
forages containing more than 10% crude protein,
Voluntary intake is also related to forage digestibility. Rate of
passage through the reticula-rumen has been shown to increase
with increasing digestibility, even when rumen fill remains con-
stant (Blaxter and Wilson 1962). Although voluntary intake
increases with increasing digestibility, there is a point where further
increases in food digestibility will result in zero or negative
increase. Hutton (1963) noted a decline in voluntary intake of dairy
cows grazing forage above 70% digestibility. The digestibility level
above which energy intake remains static is not defined and varies
between 56% (Montgomery and Baumgardt 1965a) and 67% (Con-
rad et al. 1964, Conrad 1966). A study by Dinius and Baumgardt
(1970) showed little difference in voluntary intake when forage
energy digestibility was expressed on a weight or volume basis. Dry
matter intake increased as the digestible energy per gram increased
to 2.5 kcal but, above this level, dry matter intake decreased and
digestible energy intake remained static.
In a review, Conrad (1966) suggested forage intake is controlled
by rate of passage up to about 66% digestibility but, above this
level, other factors are involved. However, Minson (197 1) observed
large differences in voluntary intake which were related to digesti-
bility, but had a different relation for many different forages (i.e.,
related to digestibility, but differed in intercept or slope). Mont-
gomery and Baumgardt (1965b) indicated digestibility-gut fill con-
trols may be influenced by particle size of forage fed. Other plant
attributes, such as leaf-stem proportion, also affect the relation-
ship. Laredo and Minson (1973) observed higher intakes of leaf
fractions than stem portions, despite similar digestibilities.
306
JOURNAL OF RANGE MANAGEMENT 38(4), July
1985
Factors Affecting Intake
vary over time, and body condition varies among individuals.
Major differences in nutritional regimes of grazing and housed
ruminants have been described by McDonald (1968) and Osuji
(1974). The type of food eaten will differ chemically and physically,
e.g., in water content, proportions of leaf to stem, type and concen-
tration of carbohydrates, and protein constituents. Food intake
will not be to appetite in grazing ruminants if available food is
difficult to harvest. It has been demonstrated that energy expendi-
ture and the requirement for nutrients is markedly affected by the
grazing animal’s environment (Osuji 1974).
Therefore, liveweight can be a poor index of energy demand and of
intake, even when differences in productivity are accounted for
(Arnold 1970a).
previously well fed.
Arnold et al. (1964) noted that as thin sheep become fat, intake
decreases, and intake and liveweight are negatively related. Lan-
glands (1968) and Allden (1968) reported that thin sheep grazing
with fat sheep make compensatory gain by increasing intake by
20% or more on a per unit of liveweight basis. Allden (1968) also
found young sheep compensated for previous periods of under-
nutrition by eating more per unit liveweight than sheep which were
Body Size
Voluntary intake of grazing animals has been related to body
size (Holmes et al. 1961) and to metabolic body size (Johnson et al.
1968). Energy demands are proportional to 0.75 power of body
weight (Klieber 1961), thus, energy needs per unit weight of smaller
animals are greater than that for larger ones. The rumen of young
animals is relatively smaller than in adults, and their increased food
requirement is usually met through increased appetite and faster
turnover rate of ingesta (Hungate 1966). It may be that younger
animals consume a higher quality forage, thereby causing a faster
turnover rate. Arnold (198 1) found 5-month-old sheep had a diet
higher in digestibility and in nitrogen content, and lower in fiber,
than that of older sheep. This may have been due to lambs being
deliberately more selective when grazing, but it may simply be, that
with smallerjaws, they can choose more precisely than older sheep.
Similarly, Horn et al. (1979) found calves tended to select forage
with higher crude protein level and lower acid detergent fiber
(ADF) and cellulose levels than did cows. Waldo (1969) felt it was
extremely important to express intake in relation to metabolic
body weight.
When abundant, good quality forage is available,
ad libitum
intake of grazing ruminants is influenced by energy demand.
Intake of cattle (Corbett et al. 1963) and sheep (Owen and lngleton
1963) is related to liveweight, liveweight change and milk production.
For house sheep (Blaxter et al. 1961) and cattle Blaxter and
Wilson (1962),
ad libitum
intake is proportional to metabolic size,
but varies with feed digestibility. It is frequently assumed that
intake by grazing animals also varies with some function of live-
weight, but it seems unlikely any single relationship will be gener-
ally applicable because liveweight differences may result from
differences in age, breed, and previous nutrition level (Langlands
1968). Langlands (1968) felt, within a breed, intake is more closely
related to age than liveweight. It appears different classes of cattle
do not have similar intakes, even when data are corrected for body
weight.
Physiological Status
Changes in intake are largely determined by alteration in physio-
logical requirements of the animal. Although dry, pregnant ewes
within breeds have exhibited similar dry matter intakes, lactating
ewes in the same flock required as much as 25 to 50% greater dry
matter intake (Hutton 1963). Similar results were obtained in
experiments utilizing dry vs. lactating sheep under grazing condi-
tions (Arnold and Dudzinski 1967a,b). Likewise, Arnold (1970b)
found greater digestible organic matter intake for pregnant and
lactating ewes than for dry ewes. Dijkstra (1971) and Allison et al.
(1981) found significant differences in average dry matter intake
between lactating and dry, pregnant cows, with lactating animals
consuming more than pregnant or dry cows and pregnant cows
consuming more than dry cows. Rosiere et al. (1980) found dry
2-yr-old heifers consumed only 67% as much forage as lactating
2-yr olds. Journet and Remond (1976) also found similar variation
in voluntary intake by cattle during lactation and pregnancy.
Supplementation
With the exception of Allden (198 I), who reviewed work on the
effect of energy and protein supplementation, most literature per-
taining to supplementation has been confined to liveweight
response. However, evidence is accumulating on the importance of
supplemental protein and energy on voluntary intake of forages.
Generally, it has been found that addition of readily available
carbohydrates to a roughage diet decreases voluntary intake (Elliot
1967a, 1967b; Cook and Harris 1968; Rittenhouse et al. 1970;
Lusby et al. 1967a, 1967b; Lake et al. 1974). Conversely, addition
of protein supplements to lowquality roughage diets increases
voluntary intake and digestibility (Elliot 1967a, 1967b; Cook and
Harris 1968; Lyons et al. 1970; Kartchner 1980). Increases in intake
associated with protein supplementation is generally attributed to
increasing rumen microbial activity and consequently rate of pas-
sage. There is evidence that intake responses to protein supplemen-
tation occur only when forages contain less than 8 to 10% crude
protein (Blaxter and Wilson 1963, Elliot and Topps 1963, Milford
and Minson 1965) although Weston and Hogan (1968) and Rit-
tenhouse et al. (1970) failed to show responses with forages of 6 to
8% crude protein.
Milford and Minson (1965) found forage intake by sheep
declined precipitously when diet crude protein levels fell below 7%.
However, intake and diet crude protein concentration were not
well associated when diet crude protein concentration was above
7%.
Apparently
diet
crude protein concentrations below 7%do not
meet the nitrogen needs of rumen microbial populations (Van
Soest 1982).
When pasture is sparse, provision of concentrations has less
effect on forage intake than when pasture is readily available.
Newton and Young (1974) reported substitution was greatest when
herbage was abundant and least when pasture was sparse. (Conclu-
sive research is lacking on the substitution of hay for pasture.
However, hay wastage appears to be substantial when pasture
forage is in good supply.)
Forage Preference
The degree of choice effect on intake has not been examined with
grazing animals. In pens, Reid and Jung (1965) reported higher
total hay intake when several hays were offered than when any one
hay was fed alone. A similar effect might occur in grazing
situations.
Strains of a species that differ in acceptability in a free choice
situation, but have comparable digestibilities, may give different
intakes when they are the sole feed. Comparisons of acceptable and
unacceptable strains of
Phalaris arundinacea
have produced
intake differences up to 36% in favor of acceptable strains (O’Don-
ovan et al. 1967). These results do not show why intakes differed,
but the authors implied odor was important. Arnold (1966) found,
on 5 of I1 pastures, intake was influenced by either taste, smell or
touch, with decreases in intake up to 61% and increases up to 35%
due to sensory stimuli. Evidence is accumulating that acceptability
Body Condition
of forage plants can strongly influence intake of grazing animals.
Intake is related to body condition as well as to body size. Body
Experience can also affect intake. Intake of sheep inexperienced
condition often varies more in grazing animals than in penned
on pasture and in the environment may be depressed by 50% for as
animals. In a grazing herd or flock, liveweights of mature animals
long as 10 months (Arnold 1970a).
JOURNAL OF
RANGE MANAGEMENT 38(4), July 1985
307
Intakes of broadleafed plants can differ from those of grasses.
Considerable research is available showing higher intakes for
legumes than grasses when digestibilities are comparable (Ulyatt
1981). A review of forage class influences on intake of range
ruminants is provided by Holechek and Vavra (1982). Leafy mate-
rials from forbs and shrubs usually have more rapid digestion rates
than grasses at comparable stages of phenology (Short et al. 1974,
Wofford and Holechek 1982). There is limited evidence that leafy
materials from forbs and shrubs may have a faster passage rate
than material from grasses (Milchunas et al. 1978).
Information is lacking on associative effects between forages on
intake. However, associative digestibility may play an indirect role
in increasing intake. For example, browse species in the diet may
increase digestibility of grasses, increasing overall digestibility of
the total diet with a corresponding increase in intake (Milchunas et
al. 1978). During winter, shrubs with a higher crude protein con-
tent such as fourwing saltbush (Cordova and Wallace 1975) could
improve the intake of grasses with crude protein levels below 7% by
providing rumen microbes with a source of nitrogen.
Forage Availability
Arnold (1964), Arnold and Dudzinski (1966) Greenhalgh et al.
(1966) and others have demonstrated that yield and physical pres-
entation of available forage to grazing animals may have marked
effects on feed intake under intensive pasture conditions, but may
have no measurable effect on extensively managed pastures.
Even for pastures of a single plant species, there is rarely, over a
short time span, a simple relationship between intake and pasture
yield (Wheeler et al. 1963). The extent to which intake is kept below
that determined by energy demand, and the chemical and physical
attributes of the diet, depends on the adaptability of grazing behav-
ior. A simplified model of intake, grazing behavior and pasture
condition was presented by Arnold (1970a).
Homeostasis of intake with changing availability of forage is
maintained by altering grazing time, bites per minute, and amount
per bite. There is no set pattern of adjustment to meet a particular
energy demand
under different pasture conditions (Arnold 1970a),
although relationships have been obtained between these variables
and pasture yield in specific situations (Arnold and Dudzinski
1966). It is interesting to note that sheep with different energy
demands (due to age, size or reproductive state) maintain the same
intake differences over a wide range of pasture conditions (Arnold
1970a).
Allden and Whittaker (1970) defined herbage intake by an
animal to be the product of eating rate and grazing time. These
workers examined certain pasture attributes that determine ease of
prehension of herbage. A close relationship was found betwen rate
of intake and herbage availability. At herbage availabilities greater
than 3,000 kg/ ha, both grazing time and intake rate were relatively
constant. As herbage dry matter decreased from 3,000 to 500
kg/ ha, there was a four-fold reduction in the rate of consumption
and a two-fold increase in time spent grazing. ,Allden and Whit-
taker (1970) speculated that, as amount of herbage decreases, a
point is reached when herbage availability apparently imposes
limitations on the rate at which animals can ingest feed, but this is
compensated for by increased grazing time. Thereafter, animals
extend their grazing period further, but compensation becomes
progressively more incomplete, and total intake would be expected
to fall drastically.
In work with dairy cattle grazing temperate pastures, Johnstone-
Wallace and Kennedy (1944) observed consumption increases as
herbage yield increased. Unlike Allden and Whittaker
(I
970) who
showed the bite size of sheep increased linearly with increasing
plant height or tiller length, Stobbs (1973a) found these factors did
not exert a major influence upon bite size. Rather, sward bulk
density (kg/ ha/ cm of herbage height) incorporating a low stem
count and a high leaf/ height ratio appeared to be the major factor
affecting bite size of cattle.
Distribution of herbage in the canopy, particularly leaves, can
influence theease with which herbage is removed. Stobbs (1973a,b)
found the ratio of sward leaf density and stem density in the
uppermost layers of the sward had the highest correlation with bite
size. These studies (Stobbs 1973a,b) emphasized that consideration
of the sward as one dimension is inappropriate. Stobbs (1975)
suggested that nitrogen fertilization of regrowth pastures increased
bite size by presenting higher leaf yield to the animals.
Arnold and Dudzinski (1967b) studied the effect of herbage
availability on intake of pregnant, dry and lactating ewes. These
researchers found about 40% of the variability in digestible organic
matter intake was accounted for by total dry matter available per
acre.
Hand1 and Rittenhouse (1972) working with steers on crested
wheatgrass pasture in eastern Oregon, found dry matter intake was
not limited when herbage availability equalled or exceeded 135
kg/ ha. In a later trial, dry matter intake was not limited at herbage
production levels equal to or greater than 92 kg/ ha or I76 kg/ ha,
using estimates of dry matter digestibility from clipped or dietary
samples, respectively.
Conversely, many other workers have found high degrees of
correlation between herbage availability and intake (Harkess et al.
1972, Langlandsand Bennett 1973, Greenhalgh et al. 1966, Green-
halgh et al. 1967, Greenhalgh 1966, Marsh 1977). In strip-grazing
experiments, Greenhalgh et al. (1967) allowed herbage to be avail-
able in amounts of 25,35, and 45 pounds of dry matter per cow per
day for a 3-month period. Higher allowances were not used
because earlier experiments (Greenhalgh et al. 1966) indicated
larger allowances were outside the critical range where herbage
availability and intake were closely related. Herbage allowances of
25, 35, and 45 pounds of dry matter per cow per day resulted in
mean intakes at 23.9, 25.6, and 26.4 pounds of organic matter per
cow per day, respectively. These workers also noted the differences
in digestibility between treatments were small.
Hull et al. (1961) using 700-pound steers grazing irrigated pas-
tures, allowed 8 to 54 pounds of dry matter per head per day.
Animals in this experiment ate all they were offered up to an
allowance of 16 pounds of dry matter per day (of which they ate
about 15 pounds), but consumed small proportions of further
increment increases. Intake at a maximum allowance of 54 pounds
per head per day was 16 pounds.
Broster et al. (1963) allowed 400-pound heifers to graze at three
allowances: 2.67, 3.20, and 3.93 pounds of dry matter offered per
100 pounds of liveweight per day. In this experiment, intake
increased linearly, the response being about 0.2 pound per
1
pound
increment increase in amount offered. With the smallest allo-
wance, 88% of the herbage was consumed and, with the largest,
64% was consumed.
Greenhalgh et al. (1966) stated the relationship between herbage
consumption and herbage allowance is probably a curvilinear
relationship. When less herbage is offered than animals consumed
voluntarily, increment increases in herbage allowance are likely to
produce increments of almost equal magnitude in herbage con-
sumed. As allowance increases further, response is likely to become
progressively smaller, and a point will be reached beyond which
further increases have no effect on intake. Greenhalgh et al. (1966)
emphasized that an increase in the allowance may affect quality as
well as quantity of herbage consumed, because opportunities for
selective grazing are increased.
Reardon (1977) allowed steers 10, 15, 22.5, and 33.8 kg dry
matter per head per day, and dry matter intake was equated with
dry matter disappearance in the standing crop. Results of this
experiment ran contrary to those from other experiments, in that
at a given level of herbage allowance, dry matter intake decreased
with increasing pasture yield. This was attributed to confounding
yield with plant maturity. However, it is also probably the result of
the method used to estimate intake. Herbage disappearance is
subject to many sources of bias. Among these are regrowth of
grazed forage, trampling damage and weathering losses, as well as
308
JOURNAL OF RANGE MANAGEMENT 38(4), July 1985