Article: Influence of Grazing Management on Intake and Composition of Cattle Diets
Authors: Hirschfeld, D.J. Kirby, D.R. Caton, J.S. Silcox, S.S. Olson, K.C.
Journal: Journal of Range Management 49(3), May 1996
This research was conducted in central North Dakota at the Central Grasslands Research Center; 70% of the 44.1 cm of annual rainfall occur during the summer months, a seasonal distribution characteristic of brittle environments. The plant communities are described as blue-grama needle and thread-sedge (Bouteloua gracilis, Stipa comata, Carex heliophila) and western snowberry-Kentucky bluegrass (Symphoricarpos occidentalis, Poa pratensis).
The two research treatments were seasonlong grazing (continuous single paddock at 65 ha) and short duration rotational grazing, where the herd was moved through eight equally divided paddocks (16.25 ha) every three to seven days. The study was done over a period of two years, sampling four times a year for a total of eight sampling events.
Six steers and five heifers were ruminally cannulated and used for sampling on each of the two treatments. Ruminal cannulation is a surgical procedure that allows access to the rumen for sampling via removal of grazed biomass. To learn more about this procedure, read this paper from Idaho State University’s Animal Care and Use Committee. Fecal output was also measured using a controlled chromic oxide release intra-ruminal device.
Animals were sampled in the early morning, first restrained to remove all biomass from the rumen, “then allowed to graze one of the treatments for 60 to 90 min. Ruminal masticate samples were collected, mixed, sub-sampled, labeled, and immediately placed on ice.”
Samples were analyzed in a laboratory for the following information on fiber and similar compounds: acid detergent fiber, neutral detergent fiber, acid detergent lignin, and in vitro organic matter digestibility. The following nitrogen information was acquired via lab analysis: total nitrogen, soluble nitrogen, available nitrogen, and acid detergent insoluble nitrogen. This data was then analyzed using paired t-tests to determined differences between treatment means.
The most significant trends were in the nitrogen category. Early on in the research, mainly during the first two sampling events, few statistically significant differences were found. By the second sampling event, the season long continuous showed significantly higher nitrogen content in the collected forage samples. By the third sampling event, this trend reversed, and most categories of nitrogen (total N, available N, and soluble N) showed statistically significant differences with almost universally higher values for the short duration rotational grazing treatment. Interestingly, acid detergent insoluble nitrogen showed no such differences.
In the latter half of the study, an opposite trend characterized by statistically significant differences was observed in the fiber category, specifically acid detergent fiber and neutral detergent fiber.
The authors distill some of the implications of this research:
Chemical composition of cattle diets generates an approximation of the quality of the diet. In this study, cattle grazing under short duration management were allowed a higher quality diet than those grazing under seasonlong management. Quality, in terms of nitrogen content and digestibility of the forage in the diet, was enhanced under the short duration treatment especially in the latter portion of the grazing season…This study would suggest…that a rotational grazing system could improve diet quality.
The authors also note that the results of this research contradict other studies that have looked at this issue:
Species composition of the range, environmental conditions, stocking rates, and livestock management are highly variable from study to study and may explain some of the discrepancies between studies.
Additionally, methodological approaches are often highly variable, making cross comparison of results difficult, if not impossible.
Though this paper shows a strong relationship between short duration rotational grazing and improved diet quality, the mechanisms and factors at work are not discussed and are unclear. To be useful for managers, we must understand some of the forces at work to better apply the underlying principles.
Other research by Dr. Fred Provenza suggests that heavy stocking for short periods encourages diet mixing. Animal behavior and culture is often the force driving diet selection. We are unsure exactly why animals in this study selected a more nutritious diet under short duration rotational grazing; the research was unable to capture these underlying factors.