Dormant season grazing: effect of supplementation strategies on heifer resource utilization and vegetation use

Authors: WYFFELS, SAMUEL - Montana State University; Petersen, Mark; BOSS, DARRIN - Montana State University; SOWELL, BOK - Montana State University; BOWMAN, JANICE - Montana State University; MCNEW, LANCE - Montana State University

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/25/2019
Publication Date: 8/8/2019
Citation: Wyffels, S.A., Petersen, M.K., Boss, D.L., Sowell, B.F., Bowman, J.G., Mcnew, L.B. 2019. Dormant season grazing: effect of supplementation strategies on heifer resource utilization and vegetation use. Rangeland Ecology and Management. https://doi.org/10.1016/j.rama.2019.06.006.
DOI: https://doi.org/10.1016/j.rama.2019.06.006

Interpretive Summary: In Montana, economic efficiency of cattle production is threatened by high feed and input costs. Dormant season livestock grazing reduces reliance on harvested feeds. Grazing dormant forage under low-input heifer development strategies typically exposes cattle to low quality forage. Protein supplementation while grazing dormant range can enhance heifer growth and reproductive performance. However, information relating supplementation strategies to individual grazing behavior and resource utilization on dormant forage is lacking. Thus, the intent of this research was to examine heifer grazing behavior and vegetation conditions on rangelands grazed during the dormant season under two supplementation management strategies. Approximately 100 weaned composite heifer calves were randomly selected and placed into one of two supplementation treatments in each of 2 years; one receiving a free access 62% crude protein self-fed concentrate, and the other receiving a daily hand-fed 20% crude protein cake. Vegetation conditions were monitored pre- and post-grazing. Daily space use and behavior was evaluated for twenty-one individuals within each treatment using GPS collars. Heifers supplemented with self-fed concentrated protein spent more time grazing per day than heifers supplemented with cake. Cake supplemented heifers selected grazing locations close to supplement delivery sites. Heifers in both supplementation strategies selected grazing locations relative to standing biomass and crude protein of the forage. Supplementation strategy had little effect on vegetation use, however, the time at which grazing occurred did impact the effects on residual vegetation conditions. Daily space use had high levels of variability by heifers, suggesting future research should incorporate individual animal measurements to account for individual animal variability.

Technical Abstract: Grazing dormant forage under low-input heifer development strategies typically exposes cattle to low quality forage. Protein supplementation while grazing dormant range can enhance heifer growth and reproductive performance. We examined resource utilization of heifers and the effects of dormant season grazing on residual vegetation characteristics under two supplementation management strategies. Approximately 100 weaned composite heifer calves were randomly selected and placed into one of two supplementation treatments in each of 2 years; one receiving a free access 62% crude protein self-fed concentrate, and the other receiving a daily hand-fed 20% crude protein cake. Grazing occurred from December (2015 & 2016) through March (2016 & 2017). Thirty transects were randomly located within each pasture for measuring vegetation quality, and structure pre- and post-grazing. Daily space use and behavior was evaluated for twenty-one individuals within each treatment using GPS collars and resource utilization functions. Heifers supplemented with concentrated protein spent more time grazing per day than heifers supplemented with cake (6.92 ± 0.18, 6.24 ± 0.17 h). Relative use by heifers in the cake treatment were negatively related to horizontal distance from the supplement delivery site early- to mid-winter ( = -0.41 ± 0.16, -0.53 ± 0.17). Both treatments selected grazing locations relative to standing biomass of perennial grasses ( = 0.0005 ± 0.00004) and crude protein ( = 0.12 ± 0.007). However, resource selection was highly variable among individuals for both supplementation treatments. We found no treatment effects on pre-post grazing differences in residual cover of litter, grass, forbs and shrubs (P > 0.24). However, the time period when grazing occurred had an effect on residual vegetation conditions (P < 0.01). Our results indicate high levels of variability in grazing site selection by heifers, suggesting future research should incorporate individual animal measurements in an attempt to account for individual animal variability.