Essential oils improved weight gain, growth and feed efficiency of young dairy calves fed 18 or 20% crude protein starter diets
Limited consumption of solid feed is an obstacle to meet the nutrient requirements for growth and for rumen development in young dairy calves (Van Amburgh & Drackley, 2005). In addition, their efficiency of nitrogen (N) utilization decreases as they grow (Van den Borne, Verdonk, Schrama, & Gerrits, 2006). Optimal protein content of starter diet in dairy calves remains controversial. In a study that evaluated effects of protein concentration in starter diets (range, 15.0–224 g/kg of DM) on performance of dairy calves, growth was maximum at 196 g/kg CP (Akayezu, Linn, Otterby, & Hansen, 1994). Similarly, in another study (Luchini, Lane, & Combs, 1991), there were no benefits to exceeding NRC (2001) recommendations for protein content in milk‐fed calves. In addition, greater escaping protein from ruminal degradation to the small intestine did not benefit pre‐weaned dairy calves (Kazemi‐Bonchenari et al., 2016). However, improving ruminal nitrogen metabolism may be an alternative strategy. In that regard, essential oils (EOs) modify rumen microbes and may enhance N utilization efficiency (Calsamiglia, Ferret, Reynolds, Kristensen, & van Vuuren, 2010), as some EOs inhibit hyper‐ammonia‐producing bacteria and improve N efficiency both in vitro (Busquet, Calsamiglia, Ferret, & Kamel, 2005) and in vivo (Bach, Calsamiglia, & Stern, 2005). Previously, supplementary EO (Apex) improved starter intake and feed efficiency in dairy calves (Hill, Aldrich, Schlotterbeck, & Bateman, 2007). Newbold, McIntosh, Williams, Losa, and Wallace (2004) reported improvements in N use efficiency in response to a mixture of essential oil (MEO) added to soybean meal; that was attributed to altered ruminal protein degradation, including suppression of amino acid deamination by rumen micro‐organisms. A commercial mixture of essential oils (CRINA®, containing thymol, eugenol, vanillin, limonene and guaiacol in an organic carrier; McIntosh et al., 2003) improved performance of dairy cows (Benchaar et al., 2007; Kung, Williams, Schmidt, & Hu, 2008; Tassoul & Shaver, 2009) and beef cattle (Tomkins et al., 2015). Although a beneficial effect of EO on calf performance has been reported (Jeshari, Riasi, Mahdavi, Khorvash, & Ahmadi, 2016), interactive effects of EO with starter protein content in dairy calves have apparently not been studied. The objective was to examine effects of dietary supplementation of mixed essential oil containing thymol, eugenol, vanillin, limonene and guaiacol (i.e., CRINA®) with two levels of dietary protein (i.e., 180 and 200 g/kg, dry matter basis) on performance, structural growth, blood metabolites and rumen fermentation in young dairy calves.