L. M. Lauriault

Ingestive behavior of beef heifers within grazing sessions

Abstract Two field experiments were used to measure the net effects of changing hunger and satiety on the ingestive behavior of tethered beef cattle ( Bos taurus ) within grazing sessions that followed overnight fasts. Angus heifers [liveweight (LW), 364±8 kg] were grazed on a vegetative tall fescue ( Festuca arundinacea Schreb.) pasture with a herbage mass of 4.5 t ha −1 of dry matter (DM) and herbage DM allowances of 5.5 kg per heifer h −1 . During their first hour of grazing, heifers ingested herbage DM at 0.65% LW h −1 and took bites of 1 g DM at 41 bites min −1 . After 45 or 90 min of grazing, heifers were consuming herbage at 0.51% LW h −1 and forming bites of 0.9 g at 37 bites min −1 . In the second experiment with twice the herbage DM allowance (11 kg per heifer h −1 ) and slightly higher herbage DM mass (5.6 t ha −1 ), heifers ate herbage at 0.76% LW h −1 and took bites averaging 1.2 g (DM) at 36 bites min −1 during the first hour of grazing. Heifers in their second or third hour of grazing ate herbage at 0.48% LW h −1 and took bites of 1 g at 29 bites min −1 . The decline in rate of intake within grazing sesssions was attributed to declining hunger and increasing satiety. Rates of herbage intake of animals grazing temperate swards unrestricted by allowance and availability of herbage are ∼ 0.5% LW h −1 after hunger is alleviated.

Herbage allowance and intake of cattle

Rates of intake of herbage and grazing time of beef cattle are essential components of simulation models of grassland agroecosystems. We studied the effects of herbage allowance on rates of intake and ingestive behaviour of twelve 2-year-old Angus heifers ( Bos taurus )(364 ± 12 kg) grazing pastures of tall fescue ( Festuca arundinacea Schreb.). A balanced change-over design and a novel tethering technique were used to estimate direct, residual and permanent effects of three herbage allowances on intake per bite, rate of biting and rate of intake. Herbage dry matter (D.M.) allowances, which were established by varying tether lengths, were 5, 9 and 13 kg (1·4, 2·5 and 3·6 kg/100 kg of live weight) and made available in circular plots of 15, 27 and 38 m 2 , respectively, for a single measured 2 h grazing session each day. Heifers grazing Kenhy tall fescue swards, composed of vegetative tillers and free from the endophyte Acremonium coenophialum , with herbage D.M. masses (> 5 cm) of 2000 kg/ha and D.M. allowances of 9 and 13 kg/2 h per heifer, ingested D.M. at 1·9 kg/h by taking bites averaging 800 mg D.M. at 38 bites/min. Allowances of 5 kg/2 h per heifer slowed the rate of intake to 1·3 kg/h by limiting D.M. intake per bite to 654 mg and biting rate to 35/min. Rate of D.M. intake of cattle grazing vegetative temperate grass swards appears to be ca. 0.5% of live weight per hour when allowance and availability of herbage are not limiting.

Accessibility of Herbage Allowance and Ingestive Behavior of Beef Cattle

Rate of intake may be accelerated when grazing cattle (Bos taurus) are given access to fresh pasture. Two experiments using a balanced change-over design and tethered adult cows of 471±18 kg liveweight were used to measure the effect of accessibility of alfalfa (Medicago sativa L.) on ingestive behavior. In the first experiment, herbage dry matter (DM) allowances of 10.5 kg were available for 90 min in one, two or three equal portions by area and time. These treatments had little effect on ingestive behavior and cattle ate DM at a mean rate of 2.25 kg h−1 by taking bites of 1.5 g DM at 25 bites min−1. In a second experiment, cows had access to 8.8 kg herbage DM allowances as a whole (T1), in two equal portions (T2) in time and area within the same plot, or in two separate plots (T3). A mean rate of DM intake of 2.2 kg h−1 was attained by forming bites of 1.6 g at 23 bites min−1. Accessibility of allowance had no significant (P<0.05) effect on ingestive behavior. Rates of biting immediately before and after the change-over were 26 and 20, 17 and 33, and 16 and 28 for T1, T2, and T3, respectively. It was apparent that rates of biting accelerated when cows were exposed to fresh swards but the effects were transitory and contributed little to overall rates of DM intake.