## 1 Introduction  
In recent years, feed production costs have increased by 70-80% in the livestock industry largely due to dietary demands for raw materials. Particularly, Korea imports 60% of feed grains for livestock animals from other countries. This rising feed cost directly affects domestic livestock farms. It is well-recognized that feed plays a key role in modulating fatty acid compositions of livestock, goats, and sheep. Feed is considered the most important factor in livestock production. Its cost is often higher than half of the production cost. To overcome this issue, the Korean Government, the livestock industry, and farmers have paid more attention to the production of new low-cost alternative feedstuffs for livestock [1]. The Korean native beef cattle Hanwoo are a crossbreed of _Bos Taurus \(\times\) Bos Taurus_. It is the major producer of cattle meat, yielding extremely marbled beef. It accounted for roughly 85% of all slaughtered cattle in 2017 [2]. Hanwoo steers are typically fattened until nearly 30 months of age. They are fed high-density concentrate diets during the fattening period to obtain a high level of marbling. [PERSON] et al. [3] have reported that the marbling of Hanwoo steers can be substantially enhanced between the ages of 12 and 27 months. Marbling is one of the key factors deciding Hanwoo quality grade. It can improve by feeding high-density concentrate diets [4]. However, feeding high-density concentrate diets during the final fattening cycle might have negative impacts on digestive metabolism, feed quality, and feed intake [5]. In addition, the demand for highly marbled beef has increased. Thus, beef producers have recently increased their use of grain-based feeds during the fattening season. Concentrate feed contains an excessive amount of fermentable carbohydrate, causing acidosis in cattle. During the fattening period, replacing animal-based materials with plant-based materials or microbially fermented feed can prevent acidosis and preserve the integrity and health of ruminal papillae and nutrient absorption [6]. Nowadays, large amounts of concentrate are formulated with grass or whole-grain cereals, among other items, to provide energy, protein, and essential nutrients to animals. They have been used to fatten cattle in Korea despite the fact that such feed can cause excessive drops in ruminal pH during the late fattening period [7]. Grain crops, such as maize, are increasingly being moved to the energy market from agricultural markets for food and feed. As a result, there is competition for biomass as bioenergy raises forecasts of international grain prices [8]. Therefore, the use of other types of feeds, such as grass, to replace grain-based feeds must be investigated. Grass and grass-based feeds are inexpensive and low-cost natural sources of high-quality roughage for ruminants. Furthermore, grass-fed cattle have a wide range of polyunsaturated fatty acids (PUFAs), saturated fatty acids (SFAs), and conjugated linoleic acid. Animal nutrition has a significant effect on fatty acid (FA) compositions and animal proteins. Beef quality is primarily dependent on fat content and FA compositions [9; 10]. Owing to the strong market demand in South Korea, Hanwoo steers have a fattening period before slaughter. In general, beef skeletal muscles have large amounts of polyunsaturated fatty acids, such as alpha-linolenic acid, linoleic acid, and oleic acid, all of which have health benefits for consumers. Meat production processes should attempt to comprise grass-enriched beef production systems while meeting consumer quality standards in terms of food quality at the same time [2] to satisfy retailer and customer demands. However, fat from grass-finished beef has been confirmed to have a yellowish appearance due to an increased level of carotenoid, suggesting that grass-rich feeds can adversely affect the quality of the meat [11]. A higher concentrate in forage-based feeds for fattening ruminants can improve the quality of meat production.