title: Digesta kinetics, energy intake, grazing behavior, and body temperature of grazing beef cattle differing in adaptation to heat.
abstract: The objective of this study was to determine whether digesta kinetics, energy intake (EI, kcal ME intake x kg(-.75) x d(-1)), grazing behavior, or body temperature differed by breed, lactational state, or season of the year among cattle presumed to vary in adaptability to the subtropics. Two-year-old lactating 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 had greater gastrointestinal tract load (CM2) and EI (P < .01), although passage rate did not differ (P > .48). During LS, lactating cattle had decreased early morning rectal temperatures (P < .05) and spent more time grazing during 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 TA) and accumulated the greatest heat during the day (P < .05). Due to greater daytime shading (P < .01) and less daytime grazing (P < .05), A had lower (P < .05) early morning and comparable (P > .26) late afternoon rectal temperatures compared with BA and TA. With data pooled over both grazing trials, BA cattle had the smallest CM2 (P < .01), and in ES they spent the least amount of time in the shade (P < .001). The TA spent more time in the shade than did BA (P < .001) during ES and less during LS (P < .001) and had similar (P > .28) early morning rectal temperatures compared with BA during ES and LS. During LS, TA spent more time in the sun and less time in the shade 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 comparable to BA with respect to heat adaptation. It appears that EI demands are greater in a hot environment.
authors.affiliation: Texas Agricultural Experiment Station, Uvalde 78801, USA. sprinkle@ag.arizona.edu
authors.forename: J E
authors.initials: JE
authors.lastname: Sprinkle
authors.forename: J W
authors.initials: JW
authors.lastname: Holloway
authors.forename: B G
authors.initials: BG
authors.lastname: Warrington
authors.forename: W C
authors.initials: WC
authors.lastname: Ellist
authors.forename: J W
authors.initials: JW
authors.lastname: Stuth
authors.forename: T D
authors.initials: TD
authors.lastname: Forbes
authors.forename: L W
authors.initials: LW
authors.lastname: Greene
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D001831:Body Temperature; D002417:Cattle / Q000502:physiology*; D004063:Digestion; D002149:Energy Intake; D005247:Feeding Behavior; D005260:Female; D006358:Hot Temperature; D006813:Humidity; D007774:Lactation; D016018:Least-Squares Analysis; D008297:Male; D012007:Rectum; D012621:Seasons