Browsing Journal of Range Management, Volume 46, Number 3 (May 1993) by Subjects
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Influence of ruminally dispensed monensin and forage maturity on intake and digestionEight ruminally cannulated crossbred steers (average weight 336 kg) grazing native blue grama (Bouteloua gracilis [H.B.K.]) rangeland were used in a repeated measures design to evaluate effects of monensin ruminal delivery devices (MRDD) and forage phenology on ruminal digestion. Three periods were assessed: mid-August (Aug.), early October (Oct.), and mid-November (Nov.). One MRDD was placed in the reticulum of 4 steers via the ruminal cannula 21 days before each period. Intake was estimated using total fecal collections. Diet samples were collected using 3 esophageally fistulated steers. Ruminal fill was measured by ruminal evacuation; rate and extent of in situ ruminal neutral detergent fiber disappearance were estimated before ruminal evacuations. Ruminal passage rates, retention time, and apparent total tract organic matter digestibility were estimated using indigestible neutral detergent fiber. In vitro organic matter disappearance of esophageal masticate did not differ (P > .05) in Aug. and Oct. (average of 53.7%), but declined (P < .05) in Nov. (48.7%), whereas organic matter digestibility was greater (P < .10) in Aug. (62.3%) than in either Oct. (55.2%) or Nov. (53.9%). Release of monensin from the bolus (68 mg day-1) was less than expected (100 mg day-1). Intake, organic matter digestibility, ruminal passage rates, retention time, pH, and ammonia were not affected (P > .10) by MRDD. In situ neutral detergent fiber disappearance at 96 hours was decreased (P < .10) by MRDD (68 vs 65% for control and MRDD, respectively). As the grazing season progressed, intake declined (P < .10), whereas ruminal fill and retention time increased (P < .05), and passage rate of indigestible neutral detergent fiber decreased (P < .05). At 48 hours in situ neutral detergent fiber was greatest (P < .05) in Aug. and least (P < .05) in Nov.
Nutrient composition of whitetopWhitetop or heart-podded hoary cress (Cardaria draba (L.) Desv.) is an Eurasian native of the Cruciferae actively invading rangelands throughout western North America. The plant is reported to be unpalatable to livestock and contain potentially toxic glucosinolates. Practical experience indicates sheep consume whitetop during its early growth stages and cattle ingest large quantities of seedheads. Chemical analysis of whole plants from rosette to hard seed, respectively, indicated the following trends: crude protein (28.8 to 7.9%), neutral detergent fiber (13.1 to 52.8%), acid detergent fiber (12.0 to 41.8%), cellulose (9.9 to 32.1%), lignin (1.9 to 9.4%), ether extract (1.6 to 2.4%), in vitro organic matter digestibility (77.3 to 49.1%), digestible energy (2.9 to 1.8 Mcal/kg), and total glucosinolates (28.4 to 84.0 micromoles/g). Leaves were higher than stems in crude protein, ether extract, in vitro organic matter digestibility, and digestible energy. Analysis of 11 micro- and macroelements revealed sulfur (S) levels ranged from 0.73 to 2.69% and were therefore higher than the reported maximum tolerable level (0.4%). High S levels likely reflected the S moiety of glucosinolates and their hydrolysis products. Whitetop has some forage value, but until further research establishes the toxicity of this species to sheep and cattle, prudence suggests managers use caution when allowing animals to graze whitetop-infested rangelands by providing supplemental iodine, utilizing mature and nonlactating animals, and reducing opportunities for animals to consume the plant.