Selection and dietary quality of beef cattle grazing smooth bromegrass, switchgrass, and big bluestem

Authors: Kirch, Brett; MOSER, L - UNIVERSITY OF NEBRASKA; WALLER, S - UNIVERSITY OF NEBRASKA; KLOFPENSTEIN, T - UNIVERSITY OF NEBRASKA; Aiken, Glen; Strickland, James

Submitted to: Professional Animal Scientist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/10/2007
Publication Date: 12/1/2007
Citation: Kirch, B.H., Moser, L.E., Waller, S.S., Klofpenstein, T.J., Aiken, G.E., Strickland, J.R. 2007. Selection and dietary quality of beef cattle grazing smooth bromegrass, switchgrass, and big bluestem. Professional Animal Scientist. 23(6):672-680.

Interpretive Summary: Two years of studies were done to evaluate the ability of cattle to enhance their diet by selection when grazing a single species of grass. Three ruminally fistulated crossbred steers were used to sample graze of smooth bromegrass (SB), switchgrass (SG), and big bluestem (BB) as grass matured. Each grass was grazed at early vegetative growth, mid-season elongation growth, and an early seeding stage. An additional grazing occurred after the grass had been grazed off and allowed to regrow. The calves were allowed large amounts of grass to maximize their ability to select only the portions of the plants they wished. To determine what the calves were grazing the animals had the entire rumen contents removed and then they were allowed to graze for 45 minutes and a sample was taken and then the original contents were replaced. This sample was compared to a sample clipped from the same grass the calves were eating. The animals were able to improve their diet crude protein compared to the clipped sample when grazing each of the grasses. The digestibility of the diet of the animals was improved for each of the grasses as the grass became more mature. The fiber content was also lower in the selected diet as the grass matured, especially noticeable for SG and BB. The regrazed grasses and diets of the cattle were comparable in quality to the mid-season elongation growth. The results of this study demonstrated the cattle are able to improve their diet quality over the quality of the grass available to them even when there is only one kind of grass available.

Technical Abstract: The ability of grazing animals to enhance quality of diet by selection is important in production. The study’s objective was to determine the effects of selection on dietary quality of cattle grazing monocultures of smooth bromegrass (Bromus inermis Leyss) (SB), switchgrass (Panicum virgatum L.) (SG), and big bluestem (Andropogon gerardii Vitman) (BB) as influenced by plant maturity. Three ruminally-fistulated steers (295 kg) strip-grazed SB, SG, and BB at vegetative, elongation, early reproductive, and a regrowth stage of development. Selection was maximized by providing cattle access to 40 kg DM hd-1d-1. Clipped samples were compared with diet samples accumulated during 45 min grazing following total rumen evacuation. Diet CP was enhanced 3-4% for SG and BB, whereas SB was enhanced by 8% (P<0.05). Diet IVDMD was enhanced at elongation and reproductive stages for SG and BB and vegetative and reproductive stages for SB. Diet NDF was 7-13% lower in SG compared with forage-on-offer, whereas SB and BB diets were unaffected(P<0.05). Diets of cattle grazing SG and BB had lower ADF than clipped forage at elongation and reproductive stages, while the SB diet was lower at the elongation phase(P<0.05). Diet lignin did not exceed 4% while the grass on offer was much higher. Regrowth produced forage and diets comparable to elongation. If adequate forage is available, the selection ability of cattle can provide a superior diet compared to forage-on-offer. When the quality of warm-season grasses has declined, animal selection allows for potentially higher animal gain when grass quality is not optimum.