Digesta kinetics, energy intake, grazing behavior, and body temperature ofgrazing beef cattle differing in adaptation to heat

The objective of this study was to determine whether digesta kinetics, ener gy intake (EI, kcal ME intake.kg(-.75).d(-1)), grazing behavior, or body te mperature differed by breed, lactational state, or season of the year among cattle presumed to vary in adaptability to the subtropics. Two-year-old la ctating and nonlactating Brahman x Angus (BA; n = 5, n = 5), Tuli x Angus ( TA; n = 5, n = 4), and Angus (A; n = 4, n = 4) cows were used. During both early (ES) and late summer (LS), lactating cattle vs nonlactating cattle ha d greater gastrointestinal tract load (CM2) and EI (P < .01), although pass age rate did not differ (P > .48). During LS, lactating cattle had decrease d early morning rectal temperatures (P < .05) and spent more time grazing d uring the day compared with nonlactating cattle (P < .001). Among breeds, A had the largest CM2 (P < .01 compared with BA and P = .068 compared with T A) and accumulated the greatest heat during the day (P < .05). Due to great er daytime shading (P < .01) and less daytime grazing (P < .05), A had lowe r (P < .05) early morning and comparable (P > .26) late afternoon rectal te mperatures compared with BA and TA. With data pooled over both grazing tria ls, BA cattle had the smallest CMS(P < .01), and in ES they spent the least amount of time in the shade (P < .001). The TA spent more time in the shad e than did BA (P < .001) during ES and less during LS (P < .001) and had si milar (P > .28) early morning rectal temperatures compared with BA during E S and. LS. During LS, TA spent more time in the sun and less time in the sh ade than did either A or BA (P < .001). During ES, EI did not differ among breeds (P > .50). During LS, EI for lactating A was greater than for BA and TA (P < .05), and EI for nonlactating BA was less than for A and TA (P < . 05). Bite rate per minute for lactating cattle during ES was reduced (P < . 03) by increased body condition score. Tuli x Angus cattle appear to be com parable to BA with respect to heat adaptation. It appears that EI demands a re greater in a hot environment.