UC Davis

The environmental impact of cattle includes enteric and manure management derived greenhouse gas (GHG) emissions and reactive nitrogen (Nr) production. In the United States, beef and dairy cattle are responsible for 27.4% and 5.2% of total methane (CH4) emissions through enteric fermentation and manure management, respectively (EPA, 2024). Concurrently, beef and dairy cattle manure management accounts for 8.4% of total GHG emissions from agriculture, while agricultural soil management (including manure fertilizer application) accounts for 49% of total agricultural GHG emissions (EPA, 2024). Feed additives are a well-studied technique with potential to reduce enteric GHG emissions. With respect to additives, containing plant secondary metabolites (PSM), or constituent molecules derived from PSM, it remains an open question as to whether these molecules retain bioactivity throughout the gastrointestinal tract. If so, these feed additives could then continue affecting GHG and Nr production in manure. Additionally, improved diet formulation can aid in increasing digestibility, delivering more nutrients to the animal and less in manure (feces and urine). In Chapter 2 of the present dissertation, the objective was to determine the effect of plant secondary metabolite-based feed additives on fecal chemistry, and how these chemical properties would lead to differences in GHG emissions and Nr when feces were amended to different soils and at different water content levels. This was achieved by recovering feces from a live-animal trial where 24 Angus and crossbred Angus steers were divided as a randomized incomplete block design to determine the effects of the feed additive treatments Agolin (AG) or Mootral (MT) as compared to cattle on an un-supplemented diet (UN) as controls. The AG treatment was fed at 1 g/day dry matter (DM) and MT was fed at 23.5 g/day DM. Fresh fecal samples were collected from all animals per pen after cattle had been on treatment for 4 weeks, and were amended to two different soil types: sandy loam and clay, at two different water holding capacity (WHC) levels: 50% and 90%, in a completely randomized factorial design laboratory incubation. Soil-only control (CON) samples were prepared alongside feces-amended samples. The GHGs and Nr were recorded for 30 days. The AG (-73.2 mg kg-1) versus MT (-60.5 mg kg-1) treatment had lower cumulative mineralized N (Nmin) in the clay, 90% WHC category ( p < 0.01). The AG (98.2 mg kg-1) versus MT (144.5 mg kg-1) treatment had lower carbon dioxide (CO2) emissions in the clay, 90% WHC category (p < 0.01), and higher CO2 emissions in the sandy loam, 90% WHC category (p < 0.01). Cumulative nitrous oxide (N2O) emissions were not different among feces-amended soils and soil-only controls and were only affected by soil type, with clay having higher emissions than sandy loam (p = 0.01). Cumulative CH4 emissions were only affected by the three-way interaction of treatment x soil type x WHC. We conclude that the C and N dynamics as influenced by AG and MT inclusion in the diet are transient and highly dependent on soil characteristics. Chapter 3 of the present dissertation entails a similarly designed laboratory incubation with a focus on how crude protein content in the diet affects GHGs and Nr in feces-amended soils. Three levels dietary crude protein were achieved by mixing corn dried distillers grains with solubles (DDGS) into diets at the University of California, Davis feedlot which were fed to three groups of cattle randomized to each treatment diet. Levels of DDGS inclusion in treatment diets were: deficient (DEF; 19.63% DM), balanced (BAL; 27.28% DM), and excess (EXS; 28.82% DM). Feces were collected from Angus steers on each treatment and amended to a sandy clay loam and a clay loam at 50% and 90% WHC. In a 32-day incubation experiment, EXS (961 µg kg-1) had greater cumulative N2O than DEF (686 µg kg-1) and BAL (392 µg kg-1) at 90% WHC in both the sandy clay loam and clay loam (p < 0.01). BAL also had lower N2O than DEF in the clay loam at 90% WHC (p < 0.01). The soil type and WHC variables impacted emissions, leading to increased variability among treatments. Reduced DDGS in the diet provided sufficient dietary N for animal performance while minimizing excreta N emissions. In Chapter 4 of the present dissertation, a feed additive blend of quebracho and chestnut extracts along with a carrier from cereals rich in saponins was fed to dairy cattle at 0.15% DM for 64 days. A group of 24 multiparous dairy cows were blocked by parity, dry matter intake, baseline CH4, and days in milk, and randomized to receive the quebracho treatment (BPX) or an un-supplemented diet (CON). In this study, fecal chemistry and fecal microbiota were analyzed over the course of the animal trial by collecting samples from each animal on days 0-, 16-, 32-, and 64 of the study. Using 16S rRNA gene amplicon sequencing, a total of 1,538 amplicon sequence variants were identified. Phylogenetic diversity of microbiota exhibited a treatment  day effect (p < 0.01). Fecal C and N exhibited a corresponding treatment  day effect (p < 0.01); however, differences were only seen between BPX and CON on day 16, indicating a potential transient effect of the feed additive, where the microbiota and N partitioning returned to baseline after a slight perturbation. Differences in fecal chemical properties were also observed, with BPX having greater levels of indole-3-lactate than CON (p = 0.01). This corroborates known effects of tannins on microbial protein degradation. Greenhouse gas emissions were also measured for feces in a 14-day laboratory incubation, no differences were observed between BPX and CON for N2O (p = 0.45), CH4 (p = 0.69), or CO2 (p = 0.97). While feed additives like those tested here, have been implemented successfully, our results suggest that the microbiota can tolerate low doses of a tannin-based product, leading to no long-term changes. Higher doses may be more efficacious, but cost prohibitive. Our results stand alongside a paucity of currently published results investigating the effects of a PSM-based feed additive on feces amended to soil. Further research on the biotransformation of the active ingredients in the gastrointestinal tract, and minimum inhibitory concentrations toward microbes in manure and soil is needed to best align dosing with animal health and performance and intended effects on manure and soil chemistry.