PSII-17 Sheep Foraging Relative to Stock Density, Time of Day, and Sheep Type in a Poisonous Plant Context

• Published in: Journal of animal science 2023-11, Vol.101 (Supplement_3), p.480-481
• Main Authors: Kersh, Aaron, Scasta, John D, Derner, Justin D, Fraley, Hannah, Stewart, Whit C
• Format: Article
• Language: English
• Subjects: Consumption; Delphinium; Density; Diet; Diterpenes; Foraging behavior; Generalized linear models; Grasses; Grazing; Lamb; Morning; Photosystem II; Plant species; Rangelands; Satiety; Sheep; Statistical models; Time of use; Toxicology
• ISSN: 0021-8812; 1525-3163
• DOI: 10.1093/jas/skad281.570

Abstract Plains larkspur (Delphinium geyeri) is a poisonous plant species that can be highly toxic at certain growth stages on western Great Plains rangelands. Targeted sheep grazing has been proposed as a mitigation strategy to reduce larkspur abundance and poisoning risk for cattle due to their greater toxicological resilience to the diterpenoid alkaloids. We incorporated targeted-sheep grazing in a northern mixed-grass prairie with our objective to determine the influence of stocking density, time of day, and sheep type on foraging behavior. During May 2022, Dorper (< 1-year-old lambs, n = 123, BW 25 kg ± 0.4; and 3 to 6-year-old ewes, n = 14, BW 47 kg ± 1.8) and Rambouillet (3 to 6-year-old ewes, n = 13, BW 76 kg ± 2.9) sheep were used in three stocking density treatments 1) 120 AU/day (1 day in a 0.25 ha paddock), 2) 60 AU/day (2 days in a 0.5 ha paddock), and 3) 40 AU/day (1 day in a 0.75 ha paddock) that were replicated four times. Sheep were housed overnight in a common area. We observed bite counts daily for nine randomly chosen sheep (three from each category) over five-minute intervals, in the morning and afternoon. We used a generalized linear model to assess the fixed effects of sheep type (Rambouillet ewe vs. Dorper ewe; Dorper ewe vs. Dorper lamb) and stock density on bite counts. Least squared means were analyzed for stock density using Tukey test, and for breed and age using student’s T test. We observed contrasts between morning and afternoon sheep foraging and diet selection. Mechanistically, this is likely due to the introduction of sheep into a new paddock in the morning after confinement and satiation diluting differences in the afternoon. Sheep breed influenced the proportion of graminoids consumed (96.9% Rambouillet vs 99.3% Dorper, P = 0.02), but differences were not observed between Dorper ewes and lambs (P = 0.57). The proportion of forb bite counts did not differ between Dorper ewes and lambs (0.6% vs. 0.4%, respectively, P = 0.6), nor between the Dorper ewes and Rambouillet ewes, despite the latter having a 3.5-fold greater selectivity (2.1%, P = 0.08). Larkspur consumption was no different between Dorper ewes and lambs (0.04% ewes vs. 0.01% lambs, P = 0.48), nor between Dorper ewes and Rambouillet ewes (0.28%, P = 0.09). Stock density influenced forb consumption; the proportion of forb bite counts were 12 times greater in the 0.5 ha paddock (3%) compared with the 0.25 ha paddock (0.3%, P = 0.005). Despite all consuming > 95% g