Date of Award

Spring 2026

Abstract

Dairy cows contribute significantly to agricultural greenhouse gas emissions through enteric methane (CH4) emissions and manure nitrogen (N) losses. Enteric CH4 represents a loss of 2-12% of gross dietary energy, and elevated ruminal NH3-N reduces microbial N capture efficiency and increases N losses from the animal. Hyper-ammonia-producing bacteria (HAB) are the primary bacteria responsible for the rapid degradation of dietary protein in the rumen, thereby increasing NH3-N accumulation. Strategies that suppress methanogenesis and HAB activity while improving N utilization are therefore important for enhancing the environmental sustainability of dairy systems. Phytogenic feed additives, including essential oils (EO) and isoflavones (phytoestrogen) can modulate rumen microbial activity and may improve feed efficiency while reducing nutrient losses. The overall objective of this dissertation was to evaluate the effects of supplementing EO and biochanin A (BCA; isoflavone compound) on production performance, rumen fermentation, enteric CH4 emissions, and N utilization in dairy cows. Furthermore, this research evaluated the ruminal degradation kinetics of isoflavones to better understand their stability in the rumen under different feeding conditions. An experiment was conducted to investigate the effects of supplementing a blend of EO containing eugenol, geranyl acetate, and coriander EO on production performance, ruminal fermentation, N utilization, and enteric CH4 emissions in organically managed lactating Jersey cows during the transition from confinement to pasture-based grazing. Twenty-two lactating cows were blocked in pairs by parity, DIM, and milk yield, and within pair, randomly assigned to 1of 2 treatments in a randomized complete block design with repeated measures over time. Treatments were: 1) basal diet without EO supplementation (control = CON) or 2) 1 g/cow/d a blend of EO (Agolin NATURU; AGO). Cows in confinement received (DM basis) a diet with a 60:40 forage:concentrate ratio, with 35% of herbage, as formulated, replacing the alfalfa-grass mixture baleage during grazing. Supplementation of AGO increased yields of milk, 4% FCM, ECM, milk fat, and milk TS, whereas milk true protein and lactose tended to increase in AGO versus CON cows, with similar total DMI and milk component concentrations. A treatment × period interaction was observed for feed efficiency (ECM yield/DMI), which was greater in AGO versus CON cows during grazing. Treatments did not affect the concentrations of milk fat, milk true protein, milk TS, and MUN, whereas milk SCC tended to decrease in AGO versus CON cows. Enteric CH4 production and CH4 yield were unaffected by diets, with lower CH4 intensity observed in cows fed AGO versus CON. Nitrogen intake and urinary and fecal N excretion were not affected by treatment; however, urinary purine derivative excretion tended to increase with AGO supplementation. Overall, AGO supplementation (1 g/d) improved production performance and reduced CH4 intensity in Jersey cows, with effects largely independent of dietary systems (confinement vs. grazing; Chapter 2). A second experiment was conducted to evaluate the in situ ruminal degradation kinetics of isoflavones from grass-legume (GR-LG) baleage composed by a mix of orchardgrass, white clover, and red clover in ruminally cannulated lactating Holstein dairy cows fed diets with different forage:concentrate (F:C) ratios. Cows were fed 2 experimental diets (DM basis): 1) low-forage (LF = 50:50 F:C ratio; 24.9% starch and 28.1% ash-free NDF), and 2) high-forage (HF = 70:30 F:C ratio; 20.8% starch and 33.4% ash-free NDF). Effective degradability (ED) of total (93.3 vs. 92.5%) and individual isoflavones was greater in HF than LF diet. However, soluble fraction a (52.7%), potentially degradable b (46.6%), and the rate of degradation Kd (30.4%/h) of total isoflavones were not affected by diets. Except for the ruminal disappearance of genistein, which was greater in cows fed HF (85.5%) than LF (81.1%), that of total isoflavones (87.1%), formononetin (87.4%), daidzein (80.6%), and BCA (86.5%) was not affected by treatments. Overall, ED of isoflavones from GR-LG baleage was greater with feeding HF versus LF, but treatment differences were small and potentially not biologically meaningful (Chapter 3). A third experiment was conducted to evaluate the dose-response effects of synthetic BCA on intake, milk yield, and composition, ruminal fermentation, apparent total-tract digestibility, and N utilization in lactating Holstein dairy cows. Cows were used in a replicated 3 × 3 Latin square and fed a TMR (62:38 forage:concentrate) and assigned to three treatments: (1) control diet without BCA (CON), (2) CON plus 14 g/d of synthetic BCA (L-BCA), and (3) CON plus 28 g/d of synthetic BCA (H-BCA). The BCA doses were selected to represent the levels of this isoflavone that would be supplied in diets containing (DM basis) 30% and 60% red clover, respectively. Supplementation of BCA did not linearly or quadratically affect DMI, milk fat and milk true protein concentrations, milk fat yield, and feed efficiency. However, yields of milk, 4%FCM, ECM, and milk true protein all tended to decrease linearly with increasing dietary BCA levels. Furthermore, milk lactose concentration and yield also decreased linearly in cows fed incremental amounts of BCA. Dietary BCA supplementation did not affect the concentrations of MUN and serum urea-N linearly or quadratically, which aligns with the absence of treatment effects on ruminal NH3-N concentration. Apparent total-tract digestibility of OM and CP tended to increase, whereas DM, NDF, and ADF increased linearly with increasing dietary BCA levels; however, differences were small and likely not biologically meaningful. Similarly, ruminal pH, total VFA concentration, and the molar proportions of most individual VFA did not change linearly or quadratically with feeding BCA. Urinary excretions of total N and urea-N, and total purine derivatives were not affected linearly or quadratically by BCA supplementation. Overall, supplementation of BCA at 14 g/d (LBCA) and 28 g/d (HBCA) had minor negative effects on production performance and did not improve N utilization in lactating dairy cows (Chapter 4).

Document Type

Dissertation

First Advisor

Andre F. AFB Brito

Second Advisor

Peter S. PSE Erickson

Third Advisor

Nancy L. NLW Whitehouse

Department or Program

Agricultural Sciences

Degree Name

Doctor of Philosophy

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