**1. Introduction**

Cattle production is one of the important industries in North America, accounting for \$78.2 billion (US) and \$10.5 billion (Canada) in cash receipts during 2015. A substantial part of this economic benefit is derived from beef sector, where 10.6 and 2.5 million head of cattle and calves in the USA and Canada, respectively, were slaughtered for the beef market in 2015 [1, 2]. The number of beef cattle in the North American farms and ranches will continue to increase over the next decade

due to the growing red meat demand by an increasing world population, with an estimated increase from 7.6 billion people in 2017 to 8.6 billion people in 2030 [3].

Despite advances in veterinary medicine, animal husbandry and animal welfare, economic impacts of cattle disease on the beef cattle industry still remain significant, with BRD being the most significant health problem in modern feedlot industry in North America. Bovine respiratory disease is commonly associated with pneumonia in nursing beef calves and recently weaned feedlot cattle. Cattle are mostly affected by BRD within the first 45 days after feedlot placement [4, 5]. According to the Feedlot 2011 study conducted by the National Animal Health Monitoring System (NAHMS), 97% of feedlots across the USA reported having cattle with BRD, and 16.2% of the cattle in a feedlot were affected by BRD during the feeding period [6]. The annual financial loss attributed to BRD, including mortality, reduced feed efficiency and performance and treatment costs, has been estimated to be more than \$4 billion to the US beef industry [7]. This loss surpasses the economic losses incurred by all other cattle diseases combined [8], given that BRD accounts for 70–80% of all morbidity and 40–50% of all mortality in the US feedlots [9].

Bovine respiratory disease is a complex disease with a multitude of stressors that predispose cattle to viral and bacterial infection. *Mannheimia haemolytica*, *Pasteurella multocida*, *Histophilus somni* and *Mycoplasma bovis* are the main bacterial pathogens involved in BRD and are, therefore, the main targets of antimicrobial treatments to control BRD [10]. In the North American feedlots, cattle considered at high risk for the development of clinical BRD signs are often given antimicrobial metaphylaxis upon feedlot arrival to prevent BRD [11]. However, recent studies have shown the emergence of BRD bacterial pathogens that are resistant to several classes of antibiotics used to both control and treat BRD [12–14]. The multidrug resistance in BRD pathogens towards particularly tilmicosin, tulathromycin and oxytetracycline has been increased in feedlot cattle in the last decade [15], and such increase may partially due to the increased use of these antibiotics as metaphylaxis. The continued rise in AMR in BRD bacterial pathogens necessitates the developing antimicrobial alternative approaches to mitigate bacterial pathogens associated with BRD in feedlot cattle. Recent research results suggest the potential use of probiotic and essential oil (EO) as antibiotic alternative approaches to mitigate bovine respiratory pathogens [16, 17]. The objective of this chapter is to provide a brief overview of pathogenesis of BRD, to review the current status of antimicrobial resistance in bacterial pathogens associated with BRD and to discuss the potential antimicrobial alternative strategies, including probiotic and EO approaches, to mitigate bovine respiratory pathogens in feedlot cattle.
