Author ORCID Identifier

https://orcid.org/ 0009-0009-2550-0283

Date Available

12-12-2025

Year of Publication

2025

Document Type

Master's Thesis

Degree Name

Master of Science (MS)

College

Agriculture, Food and Environment

Department/School/Program

Animal and Food Sciences

Faculty

Donald G. Ely

Faculty

David Harmon

Abstract

Sheep and cattle are often used interchangeably due to having the same gastrointestinal anatomy; however literature directly comparing the two species is minimal. Previous in vitro studies have pointed towards potential species differences in rumen carbohydrate utilization and acidosis susceptibility. The objective of this study was to evaluate in vivo microbiological and physiological responses of wethers and steers when abruptly transitioned from a high-forage to a high-concentrate diet. We hypothesized there are differences between species in response to abrupt dietary transition with respect to intake, rumen microbiology, and the rumen environment. Individually housed, rumen cannulated purebred Polypay wethers (n = 12; initial BW: 60.96 ± 6.11 kg) and Holstein x Angus steers (n = 12; initial BW: 485.41 ± 34.93 kg) were blocked by BW and randomly assigned to one of two diets: control (CON; 10:90 concentrate to forage) formulated with chopped orchardgrass (OG) hay and a cracked corn-based concentrate or high-concentrate (HC; 90:10 concentrate to forage) with chopped OG hay and a ground corn-based concentrate. All animals were first adapted to CON for 14 d, restricted at 2% of BW. On day 1, HC animals were abruptly transitioned to their diet with both diets offered ad libitum with feed delivered daily at 0730. Feed refusals were collected daily, beginning on day 1 and through day 15. On days 1, 2, 3, and 15, rumen samples were collected for subsequent short-chain fatty acid (SCFA) analysis and rumen pH was recorded using a benchtop pH meter at hours 0, 0.5, 1, 2, 4, 6, 8, 12, 16, 20, and 24. Rumen bacterial functional guilds were enumerated using serial dilution on the previously mentioned sampling days at hours 0, 2, 4, 8, and 24 post-feeding. All enumeration data were log transformed prior to statistical analysis. All data were pre-processed using Python (v. 3.11.11) and analyzed using the PROC MIXED procedure of SAS with repeated measures (v. 9.4, SAS Inst. Inc., Cary, NC). Dry matter intake (DMI; g kg BW-1) differed by species, with wethers consuming more than steers throughout the study (P = 0.03). When animals were abruptly transitioned to a HC diet, regardless of species, lactate-utilizing and amylolytic bacteria proliferated, including amylolytic sub-populations, gram-positive cocci (GPC; Streptococcus bovis and Enterococcus spp.) and Lactobacillus spp. (P < 0.01). Consequently, rumen pH declined (P < 0.01) and cellulolytic bacteria were inhibited (P < 0.01). Regardless of diet, Lactobacillus spp. were greater in steers the first day (P < 0.01), but GPC were more prevalent in wethers on day 1 (P = 0.02). Total lactate-utilizing bacteria tended to be greater in wethers than steers (P = 0.06) on day 1. Diet was the primary driver of differences in molar proportions of acetate, propionate, butyrate, and acetate:propionate (P < 0.01 in all cases) throughout the study. However, initial acetate and propionate molar proportions were greater for wethers (P = 0.04; P< 0.01 respectively). Total VFA concentrations were higher for steers than wethers for the first 72 hours after abrupt dietary transition (day 1: P < 0.01; day 2: P = 0.06; day 3: P < 0.01). Steers had greater rumen fill and percent dry matter (P < 0.01) than wethers. Liquid dilution rate (% h-1) was faster for steers, but wethers had 64% higher rates of VFA absorption (P < 0.01). These results indicate that there are key differences in the rumen bacterial communities and digestion dynamics of sheep and cattle under a dietary challenge that could influence acidosis susceptibility. Further research is necessary to evaluate these comparisons under a more severe high-concentrate challenge, as well as more detailed approaches to microbiological and physiological differences between species.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2025.598

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