REV103415 - Laserfiche WebLink

Home Browse Search

' t, IOO6 AGRONOAIY JOURN.4 L. VOL. 79. NOVEh1BER-DECEh1BER 1987 Table 6. Least squares means (grand mean plus direct effecll of variables associated with grazing IExp. 111 Variable Units 0 Treatment Ihl 1 2 SE7 Linear P~V aloe Quadratic Intake kg h" 2.35 1.47 1.20 0.225 0.00 0.13 Intake kg 1100 kg LWI" h" 0.62 0.39 0.32 0.059 000 0.14 Biting rate bites mm ' 34.4 21.7 17.6 1.67 0 00 0.00 Bite size g bite ' 1.16 1.17 1.19 0.192 0.81 0.98 Bite size mg Ikg LWI" 3.00 3.10 3.13 0.51 0.79 0.94 RDM1t M11g ha' 0.432 0.929 1.089 0.120 0.00 0.12 Utilization % 746 46.9 36.9 6.75 0.00 0.14 Sward height cm 24.3 30.6 33.9 2.16 0.00 0.44 i Standard error of difference Table 7. Summery of selected mean daily weather vsriebles for days during the measurement phases of Exp. I end Exp. 11 from the Spindle[op weather elation. Temperature Relative humidity Precip- Evepo- Date High Low High Low itetion ration - °C 9a mm 10 June 29.4 21.1 94 67 0.8 5.8 11 30.0 23.3 96 66 1.0 6.1 12 26.7 22.2 94 52 2.5 8.9 13 22.8 15.6 95 64 0.0 3.3 17 25.6 20.0 94 33 0.0 9.1 18 26.7 11.7 90 31 0.0 7.6 19 31.7 11.1 96 26 0.0 7.9 20 32.8 18.3 91 47 0.0 5.3 location are in concurrence with our data (T.D.A. Forbes, 1985, personal communication). It is the con- sensus of researchers in the area of ingestive behavior ofgrazing animals that properties o(the sward regulate the ingestive process of a particular size class of ani- mals (Hodgson, 1982b). Favorable swards permit an- imals to ingest relatively large amounts of herbage per bite, and bites of the mass taken in this experiment indicate very favorable swards. Alfalfa used in Exp. II was inferior in terms of herbage intake since this sward had much lower herbage mass, was 13 cm shorter, and herbages allowances were about one half those in Exp. I (Table 2). In Exp. 1 the herbage mass was depleted to I.5 Mg ha-' and 29 cm after the first hour of grazing (Table 4), while in Exp. If the alfalfa was grazed down to 24 cm and 432 kg ha-' of herbage D1v1 mass (Table 6). It has been suggested that swards of temperate spe- cies become limiting in terms of intake when herbage DM mass declines below 1000 kg ha ~' (Minson, 1983) and, in forage legumes such as alfalfa, when the pro- portion of more mature basal stem tissue becomes sig- nificant (Alder and Minson, 1963). Both mass and al- lowance of herbage evidently limited intake in the generally accepted fashion (Gibb and Treacher, 1976). The highest rates of ingestion were recorded during the first hour of grazing. This high rate of herbage intake reflects favorable sward conditions in terms of herbage mass, in proportions of leaf and stem, and in terms of younger, rather than older, tissues (Hodgson, 1982b). During this first hour ofgrazing it is possible that grazing was essentially nonselective, while later in the grazing session, grazing slowed down as hunger was alleviated and satiety mechanisms were activated and heifers became more selective (Chacon and Stobbs, 1976). This explanation may account for the observed declines in the rate of biting and the rate of herbage intake per bite as the grazing sessions progressed. In both experiments nearly 474'0 of herbage ingested in the 3-h session following an overnight fast was eaten in the first hour, with 29% in the second hour and 249'0 in the third hour. It would be interesting to know if the rate of intake varied within the first hour, but it is impractical to use this tethered grazing technique to determine herbage intake by differences in herbage mass before and after grazing for grazing sessions of less than I h. The simulation model of 1\lertens and Ely (1979) predicts asteady-state condition in which the rate of DM intake equals the rate of Dlvt disappearance from the reticulo-rumen. Our data indicate that the rate of intake may have approached steady-state during the second and third hours ofgrazing in both experiments (Tables 4 and 6). This was more evident in Exp. 1 when swards supported DM intake rates approaching 3 kg h-' (Table ~). The small particle size (Troelsen and Campbell, 1968) and small fraction ofcell walls (Hacker and Minson, 1981) of ingests from grazed alfalfa en- sure high rates of Dlvt intake through their combined effects on the rate of DM disappearance from the re- ticulo-rumen (Mertens and Ely, 1979). The declining rates of intake observed over [he 3-h grazing session cannot be attributed wholly to diminishing swards (Forbes and Hodgson, 1985x) or declining allowances (Marsh, 1979), nor can they be wholly ascribed to in- duction of the gut-fill mechanism of regulation of in- take. Because of the high digestibility of the upper canopy of alfalfa, a chemostatic mechanism cannot be eliminated (Waldo, 1986). Baile and McLaughlin (1987) considered the possibility that specific neuro- hormones associated with hunger mediate the onset of grazing while other neurohormones are concerned with the regulation of satiety. It is conceivable that rile decline in the intensity of grazing observed in Exp. I is an expression of satiety since swards did not appear to be limiting intake. Note that heifers ingested 6.4 kg of DAt during the 3-h morning grazing session. According to the most recent National Research Council (NRC) (1984) equa- tion that estimates DM intake, heifers of the frame size and liveweight used in this study should ingest about 8 kg day-' of alfalfa Dtvt of the approximate quality of the pastures used in Exp. I. These daily herbage intake values appear to be lower than neces- sary to support reported levels of liveweigllt gain of steers grazing alfalfa (Douglas. 1936). As all heifers had free-range access to these alfalfa fields for at least 3 h each afternoon, it can be assumed that they had the opportunity to saUSfy their demand. In Exp. 11, where herbage mass was more restrictive, the heifers' intake