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<br />was limited to a total of 5.0 kg DM per 3-h grazing
<br />session, but the heifers had the opportunity to meet
<br />their NRC-projected levels of DM intake during the
<br />afternoon grazing session. It should be emphasized that
<br />these rates of intake were calculated for three consec-
<br />utive hours of grazing with a fresh plot of the same
<br />herbage allowance each hour. In Exp. 1, 6.4 kg would
<br />be ingested from 18.4 kg ofavailable Dtv4 for an overall
<br />utilization of 35%, while in Exp. [I, ~.0 kg would be
<br />ingested from 10.2 kg of available DA7 for an overall
<br />utilization of 4990. Intake usually declines when uti-
<br />lization exceeds 50%, but this is based on longer graz-
<br />ing periods and involves the extension of grazing time
<br />to meet intake requirements (Osbourn, 1980). The
<br />mean rate of biting for Exp. I over a 3-h session would
<br />be slightly over 22 bites min 'and the mean DM
<br />intake per bite would be approximately 1.6 g bite-'.
<br />In this case the initial and residual herbage DM masses
<br />did not appear to be limiting (4.66 and 1.39 Ntg ha ',
<br />respectively). in Exp II, the mean rate of biting would
<br />be in [he vicinity of 25 bites min-' and the mean
<br />intake 1.14 g bite ' across a 3-h grazing session. In an
<br />unpublished 1985 study, similar ingestive behavior was
<br />observed in steers grazing alfalfa at a young regrowth
<br />stage (25-27 days) and at a low herbage mass (1.50
<br />Mg ha ').
<br />While it is hazardous to extrapolate from observa-
<br />tions on highly conditioned and tethered experimental
<br />animals, a few general conclusions regarding grazing
<br />behaviorcan be made. The ingestion rate was maximal
<br />at the commencement of grazing when conscious se-
<br />lectivity can be assumed to be minimal. The decline
<br />in herbage intake within a grazing session may be at-
<br />tributed to combinations of canopy depletion and di-
<br />minishingallowances, alleviation of hunger, or induc-
<br />tion of satiety-regulating mechanisms (Baffle and
<br />McLaughin, 1987). When animals graze alfalfa ac-
<br />cording to current recommendations (Douglas, 1986),
<br />individual animals should ingest DM at about 2 kg
<br />h-' and should satisfy their intake demand (NRC) in
<br />4 h day-'. To minimize trampling, soil compaction
<br />and erosion (Brown and Evans, 1973), and fouling
<br />(Forbes and Hodgson, 1985b), beef cattle grazing could
<br />be limited to two 2- to 3-h sessions per day, with the
<br />remainder ofthe day spent in a loafing area. lvtaximum
<br />intake may occur when animals are allowed several
<br />shoe-term grazing sessions per day, with each grazing
<br />session being allocated a fresh ungrazed sward, i.e., a
<br />system of strip-grazing (Alder and Minson, 1963).
<br />Tethering cattle has many attributes that make it an
<br />attractive technique for studies ol- grazing behavior.
<br />There are several advantages in terms of experimental
<br />design, and, in this example, the experimental unit was
<br />an individual heifer and its grazing plot for one grazing
<br />session. 1Vhen used in combination with a balanced
<br />change-over design,the directand residual effects were
<br />estimable as were the cffectsofanimalsand days. Teth-
<br />eringalso enabled the measurement of herbage intake
<br />of individual heifers by difference methods for grazing
<br />sessions as brief as I h, while allowing considerable
<br />control over herbage allowance. fvlanagemcnt and
<br />preparation of wards was simplified, and grazing of
<br />atypical areas such as fence rows, crcekbeds, and shaded
<br />areas was prevented. Fencing and animal water supply
<br />HAGEx1.4NN WIE• HOEFT & CIHA: HERBICIDE TOLERANCE OF LUPIN
<br />1007
<br />needs in research pastures were eliminated. Among the
<br />disadvantages of tethered grazing in research appli-
<br />cations are considerable labor inputs during the train-
<br />ing and conditioning phases of animal management
<br />and during the experimental phases. After heifers be-
<br />came familiar with the practice, they did not challenge
<br />the tether and its anchor, and they appeared to exhibit
<br />normal grazing behavior, although normal grazing be-
<br />havior has never been defined.
<br />CONCLUSIONS
<br />The rate of intake of grazing beef cattle at the onset
<br />of a grazing session is dependent on the properties of
<br />an alfalfa sward and the hunger-satiety status of [he
<br />animal. The rate of intake progressively declines dur-
<br />ing grazing sessions independent of sward properties.
<br />Methodology employing tethered cattle and change-
<br />over designs has many attributes that make it attrac-
<br />tive for research in grazing behavior.
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<br />Agricultural Research Council. 1960. The nutrient requirements of
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<br />Baffle, C.A., and C.L. McLaughlin. 1987. Mechanisms controlling
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