Supplementation and monensin effects on digesta kinetics. I. Cattle grazing summer range

Abstract

Sixteen ruminally cannulated beef steers grazing native summer range in the Northern Great PIains were assigned to 4 treatments in a 2 X 2 factorial arrangement. Main effects were barley grain and monensin. Rolled barley (RB) containing 7.5% molasses was fed at 0 and 1.36 kg head-1 day-1. Steers received no monensin (M) or M released at 101 mg/d via a ruminal delivery device. Forage intake and digestibiIity, ruminal fermentation, and ruminal passage rate were measured during trials in: (1) June, (2) July, and (3) August. Diet samples were collected from esophogeally fistuIated steers during each trial. Dietary crude protein was greater (P<0.05) during trials 1 (15.2%)and 3 (14.3%) than in trial 2 (10.2%). In vivo organic matter (OM) digestibiiity, ruminal fluid passage rate, and fermentation variables varied by trial (P<0.01). Forage OM intake was reduced (P<0.10) by RB, but was not influenced (P>0.10) by M or the M and RB combination. In vivo OM digestibility was increased (P<0.05) by M, while RB had no effect. Particulate passage was not affected by M or RB but gastrointestinal tract fill was reduced by monensin (P<0.05). Ruminal fluid passage rate was affected by the RB X M X Trial interaction (P<0.05). Within June and July, fluid passage rate was similar among treatments and ranged from 14.0 to 11.3 %/h, respectively. During trial 3, a RB X M interaction (P<0.05) increased fluid passage rate. Ruminal ammonia-N concentration was similar among treatments. Barleey lowered (P<0.05) ruminal pH and increased (P<0.10) total volatile fatty acids. A RB X M X Trial interaction (P<0.05) was noted for molar proportions of acetate, propionate, and butyrate. Within trials, RB, M, and their combination affected (P<0.01 to P<0.10) acetate, propionate, and butyrate. We conclude that barley, monensin, and forage quality infiuence ruminal fermentation, passage rate, and intake traits of steers grazing summer range.