Cow-calf systems for the US beef industry, representing 86% of beef operations and 84% of the beef cattle population in the country. Reproductive failure is the key factor limiting productivity in cow-calf systems, and pregnancy loss has been recognized as one of the main reproductive challenges in cattle. Although ~90% of fertile beef cows conceive after a single service, nearly 50% remain pregnant 30 days after service and even less females give birth to a live calf. The majority of pregnancy losses in cattle occur during the early embryonic period (conception to day 28 of gestation) and are classified as early embryonic loss. Hence, a long-term goal of our research program is to develop management strategies that mitigate early embryonic loss in cow-calf systems. The overall objective of this proposal, which is the next step toward our long-term goal, is to examine nutritional interventions that promote pregnancy establishment and maintenance in beef females.Our research group has focused on supplementing beef cows during the early embryonic period with a lipid source rich in omega-6 fatty acids (FA) - soybean oil in the form of Ca salts (CSSO) to minimize ruminal biohydrogenation. Supplementing CSSO effectively increased the incorporation of omega-6 FA into maternal and conceptus tissues, promoted interferon-tau (IFN-τ) synthesis by the conceptus, and increased pregnancy rates to artificial insemination (AI) by 25%. These results diverged from the traditional concept that omega-6 FA are detrimental to pregnancy rates by serving as precursor to prostaglandin (PG) F2α and favoring luteolysis. Therefore, the mechanisms by which supplementing a lipid source rich in omega-6 FA improves pregnancy establishment and maintenance warrant full clarification, which will provide biological support to a nutritional intervention targeted to minimize early embryonic loss. Omega-6 FA are also precursors of PGE2 and PGI2 that favor pregnancy success and promote other mechanisms that modulate early pregnancy development, such as proinflammatory cytokines and peroxisome proliferator-activated receptors (PPAR). Hence, our central hypothesis is that omega-6 FA create an intra-uterine physiological environment favorable to pregnancy establishment and maintenance. We plan to test this hypothesis and attain our overall objective by pursing the following aims:1) Examine the effects of increasing omega-6 FA supply during the early embryonic period. Supplementing beef cows with 100 g/day of CSSO post-AI promoted IFN-τ synthesis by the conceptus and subsequent pregnancy rates. In turn, an excess of omega-6 FA (i.e. 250 g/cow daily of CSSO) may overstimulate the PG 2-series and other physiological cascades, resulting in a disruptive environment to the pregnancy. Both approaches warrant investigation to unravel the effects of supplementing a lipid source rich in omega-6 FA during the early embryonic period, and validate a nutritional intervention to mitigate reproductive failure in beef cows. We hypothesize that omega-6 FA improve or disrupt, in a dose-dependent manner, maternal and conceptus responses required for pregnancy establishment and maintenance.2) Contrast the effects of omega-6 and omega-3 FA on pregnancy establishment and maintenance. Differing from omega-6, the omega-3 FA have been traditionally associated with reduced pregnancy losses in dairy cows. Omega-6 and omega-3 FA are precursors of signaling molecules with opposing effects, as omega-3 FA downregulate synthesis of PG from the 2-series and proinflammatory cytokines. The potential benefits of omega-3 FA in mitigating early embryonic loss in beef cows are still unclear and require investigation, which will also help discerning the specific roles of omega-6 FA during the early embryonic period. We hypothesize that supplying lipid sources rich in omega-3 or omega-6 FA yield divergent physiological effects that regulate pregnancy establishment and maintenance in beef cows.The proposed aims and hypotheses are novel and are supported by compelling evidence, both from our research group and published literature. Our preliminary data were generated across several trials using > 5,000 beef cows from different subspecies (Bos taurus and B. indicus) managed in temperate or tropical environments. We are well-prepared to conduct the proposed research because we have assembled a highly collaborative team with extensive experience in studying reproductive and nutritional interactions in cattle with a specific focus on embryonic and uterine responses. Aim #1 will compare different levels of CSSO supplementation using physiological analyses and transcriptome sequencing to thoroughly investigate the effects of omega-6 FA during the early embryonic period. Aim #2 will contrast the effects of supplemental omega-6 and omega-3 FA on pregnancy establishment and maintenance, including blood collection from the uterine-ovarian drainage to directly assess intrauterine physiology (Aim #2a), as well as reproductive and productive responses critical to cow-calf systems (Aim #2b). This proposal is directly aligned with the following AFRI priority area: embryonic and fetal development including interaction between the conceptus and its uterine environment (Animal Reproduction; A1211). This knowledge will support management strategies that mitigate early embryonic loss, which is a major cause of reproductive and economic losses to the US cow-calf industry.