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Title: Steer Performance, Dry Matter Intake, Digesta Kinetics, and Pasture Characteristics of Switchgrass at Three Forage Masses

Author
item Burns, Joseph
item Fisher, Dwight
item POND, K - Texas Tech University

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/2010
Publication Date: 12/15/2010
Citation: Burns, J.C., Fisher, D.S., Pond, K.R. 2010. Steer Performance, Dry Matter Intake, Digesta Kinetics, and Pasture Characteristics of Switchgrass at Three Forage Masses. Agronomy Journal. 103:337-350.

Interpretive Summary: Switchgrass has generated considerable interest as a source of biomass for the conversion to biofuel. Switchgrass, however, is considerably more diverse having potential also as a perennial forage, to be utilized in animal production systems as a pasture, hay or silage. This study examined switchgrass as a pasture forage when continuously stocked at Short, Medium and Tall forage mass to determine the optimum forage mass for grazing. Switchgrass was readily consumed by grazing steers regardless of the range in forage. During the more favorable (spring to early summer) part of the grazing season, steer ADG exceeded 1.0 kg d-1 and variation in canopy characteristics among the Short, Medium and Tall FM treatments did not alter diet nutritive value or pasture productivity. The erect, bunch type growth habit of switchgrass resulted in steers grazing across the pasture canopy, but differentially extending their grazing time on the Short forage mass during the mid-day and evening grazing sessions. This shift apparently enabled steers to consume an adequate quantity of forage from each of the three canopy types to produce acceptable and similar steer daily gains. In the latter, more stressful (July –August), portion of the grazing season switchgrass canopies varied more widely among forage mass treatments than in the early portion of the season and steer performance was variable. As noted in early season, steers differentially extended their grazing time on the Short forage mass during the mid-day and evening grazing sessions. Continuous stocking appears feasible from the plant perspective when pastures are maintained above 30 cm or have a forage mass of about 2200 kg ha-1. Canopies grazed below 30 cm thinned and stand loss was evident after the third summer of grazing. The use of continuous stocking prior to mid-summer appears feasible, however, after mid-summer it may be best to rotationally graze the pastures to ensure adequate leaf availability (proportion and mass).

Technical Abstract: Management strategies to accommodate switchgrass (Panicum virgatum L.) in production systems have been reported but the impact of variation in forage mass (FM) on animal responses and pasture stands are lacking. This 4-yr study assessed canopy and stand characteristic and steer responses when ‘Kanlow’ switchgrass was continuously stocked to produce Short (10 to 16 cm), Medium (20-26 cm) and Tall (35 to 41 cm) canopy heights. From April to mid-July (Period 1) FM for Short (S), Medium (M), and Tall (T) average 818, 2164 and 3294 kg ha-1, respectively. Canopies had similar IVOMD (626 g kg-1) but increased in NDF from S (645 g kg-1) to T (665 g kg-1). Diets selected decreased in IVOMD from S (744 g kg-1) to T (706 g kg-1). Digesta kinetics (passage rate, fill, and fecal output) were not altered and steer ADG was similar among FM (1.11 kg d-1). From mid-July to late September (Period 2) FM averaged 465, 1282 and 2388 kg ha-1, respectively, for S, M, and T. Whole canopy IVOMD increased from S (484 g kg-1) to T (538 g kg-1), whereas NDF was not altered (696 g kg-1). Steers selected a diet similar in IVOMD (685 g kg-1) and NDF (667 g kg-1) among the three FM. Estimates of DMI (kg 100-1 kg body weight) increased from S (1.75) to T (2.54). Tiller density decreased from T to S. Continuously stocked switchgrass at canopies of 20-40 cm remained vegetative with acceptable stands after 3 yr of grazing.