Influence of grazing season, residual herbage, and precipitation on rumen extrusa diet quality

Authors: Waterman, Richard; Vermeire, Lance; Reinhart, Kurt; Rinella, Matthew - Matt

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/9/2021
Publication Date: 7/21/2021
Citation: Waterman, R.C., Vermeire, L.T., Reinhart, K.O., Rinella, M.J. 2021. Influence of grazing season, residual herbage, and precipitation on rumen extrusa diet quality. Rangeland Ecology and Management. 78:117-126. https://doi.org/10.1016/j.rama.2021.06.004.
DOI: https://doi.org/10.1016/j.rama.2021.06.004

Interpretive Summary: To better understand the effects of grazing management and precipitation on indicators of forage quality and animal productivity, we conducted a 6-yr field experiment (2013-2018). The experiment tested the effects of stocking rate (1 [moderate] or 1.5 [heavy] AUM/ha) and season (June [summer], October/November [fall]) on rumen extrusa nutrient composition (defined as diet quality at the time of turn out with five treatment replications). We hypothesized that diet quality would be greater in summer than fall and greater in wet compared to dry yrs. Heavy stocking is predicted to increase grazing-tolerant plant species and possibly increase the abundance of forages that are less susceptible to defoliation by ruminants. In conclusion, the present research over 6-yrs of differing ambient precipitation nutritional composition of diets were altered by season of grazing and grazing pressure imposed on paddocks utilized in the study. Authors caution that findings may not directly correlate to larger pasture setting as these were 60 × 30 m paddocks under high intensity short duration grazing management. This study focused on longer-term stocking rate effects along with repeated seasonal grazing on diet extrusa quality. Rumen cannulated animals were collected on the first day that grazing occurred and likely consumed the most favorable diet available in each paddock. Effects of stocking rate are likely more strongly expressed at the end of grazing periods. Results from this study imply that precipitation can influence plant characteristics such as energy, protein, and mineral values of rangeland forages and that season of use and grazing pressure also contribute to forage nutritive quality. Although, studies have shown the N depletion effect in forages when above average precipitation occurs, this study identifies that supplementation of protein to meet animal requirements is still necessary when there is an abundance of grazable forage with a lot of shoot mass. Furthermore, mineral incorporation into plant biomass is greatly influenced by precipitation. This was especially observed for microminerals such as Cu and Mn that have an important role in metabolism of range livestock and are important in reproductive performance. Results from this study imply that precipitation can influence plant characteristics such as energy, protein, and mineral values of rangeland forages and that season of use and grazing pressure also contribute to forage nutritive quality. Although, studies have shown the N depletion effect in forages when above average precipitation occurs, this study identifies that supplementation of protein to meet animal requirements is still necessary when there is an abundance of grazable forage with a lot of shoot mass. Furthermore, mineral incorporation into plant biomass is greatly influenced by precipitation. This was especially observed for microminerals such as Cu and Mn that have an important role in metabolism of range livestock and are important in reproductive performance.

Technical Abstract: While considerable research has quantified effects of grazing management and precipitation on plant communities, less is known about how seasonal effects influence diet quality selected by grazing ruminants. We tested effects of two cattle stocking rates (1 [moderate] or 1.5 [heavy] AUM/ha) and two grazing seasons (summer and fall) on 20 indicators of ruminant diet quality over a six-yr period. We found no effect of stocking rate on 13 indicators of diet quality (P = 0.13) and no effect of season on 4 indicators of extrusa diet quality (P = 0.0773). We did, however, detect effects of yr on diet quality metrics and nutrients, likely due to variation in annual precipitation, which ranged from 63.9 to 138.6% of the long-term average. Most noticeably, the mineral Mn in rumen extrusa was substantially lower in yrs with higher precipitation (P < 0.0001). While grazing intensity had divergent effects on forage quality in the summer versus fall, annual precipitation was often important. Crude protein was lowest in fall of 2013 (6.5%) and 2018 (7.78%), two high precipitation yrs, but it was also low in fall of 2015 (7.30%), a low precipitation yr (P < 0.0001). Forage extrusa Cu, an important catalyst in ruminant metabolism, exhibited an yr × season interaction. Copper in fall of 2018 was greater compared to all other yrs and seasons in the study (P < 0.0001), 35.7% greater than summer of 2018, a high precipitation yr, and 82.9 and 43.5% greater than summer and fall of 2013, another high precipitation yr. Seasonal variation in diet nutrients and quality indicators were complex and complicated by yr effects which sometimes related to precipitation amounts.