A demand for goat meat has increased in both developing and developed countries, resulting in a rising number of livestock producers raising goats. However, there has been little research on goats and goat products compared to other ruminants such as cattle, and sheep, even though goats have been significantly rolled into food and in economic security over the world [1]. Goats have been traditionally raised for meat production in Korea and this has increased over the last decade due to a rising demand for goat meat, being a healthier option as well as the migration of Chinese people in Korea. Korean native goat (KNG, Capra hircus coreanae) is the main breed for goat meat production in Korea and most of them are predominantly black [2]. However, this species is low in productivity as well as in average daily gain (ADG) with about 50 to 70 g/d [3] compared to other breeds such a boar (about ADG of 124 g) for meat production [4]. This is mainly due to a lack of breed improvement and improper feeding management. Therefore, it is necessary to improve the productivity of KNG for the sustainability of the goat industry.

Currently with the widespread use of total mixed ration (TMR) in ruminants, including beef cattle, deer, and goat as well as in dairy cows, it is important to provide proper fiber content and its physical traits in TMR [5]. Generally, ruminants require adequate amounts of coarse textured feeds to avoid metabolic disorders [6]. To attain better performance without any metabolic disorders, animals need adequate dry matter intake (DMI) and proper fiber content in their rations. Thus, it is essential to accurately estimate the portion of dietary fiber required by ruminants as forages and roughage are lower in internal availability and available energy than grains. It is also desirable to reduce fiber to a minimum level in the diet for high-performance [7]. However, there is limited information on the physical traits of dietary fiber and the appropriate amount required in goats, especially in KNG.

Hence, this experiment was aimed to determine the proper physical traits in the diet of goats by investigating the effects of physical effective neutral detergent fiber content on DMI, digestibility, and chewing activity in black goats fed with TMR.

In general, the physical effect of diets for ruminants is one of the most important factors that makes growth performance or digestibility. Several researchers have taken the time to note that diets particle size or length had an impact on the physiology of digestion in ruminants [7]. Basically, ruminants will select a diet of better quality [13]. In the current study, DMI was not affected by dietary peNDF. This result is consistent with previous studies [14–17]. DMI were 21.1, 21.0, 20.0 kg/d (high, medium, and low) which were no differences [15] and reducing dietary particle size resulted in a small increase (19.4 [high concentrate short chop] to 20.1 [high concentrate long chop] kg/d) in feed intake [14]. In all of these studies, distribution divided two screens into the top (19 mm) and middle (8 mm). In contrast, it has been reported that steers are preferred to feed the low peNDF content and thereby increase intake (8.7 kg/d to 11.1 kg/d) [5]. The DMI decreased (7.4 kg/d [fed the short barley silage] to 6.3 kg/d [fed the long barley silage]) with increasing dietary peNDF [18]. Studies in heifers [5] and steers [18] were conducted further detail distribution which was divided into 1.18 mm. Although the effects of peNDF on DMI were different according to an experimental animal in the previous studies, DMI was similar between groups in this study. This was assumed that restricted feed supply by 2% of BW for all wethers was to ensure similar DMI. This procedure used in the present study was based on the experiment of Li et al [17], who studied the effect of peNDF on rumen fermentation in the dairy goat. Li et al [17] suggests that this procedure can eliminate the confounding effects of dissimilar nutrient intake among treatments. However, peNDF of diets have not been as effective as expected in this study. In the current study, the reduced dietary peNDF had no impact on chewing activity and digestibility. The peNDF values (16.2% to 23.8%) of the diets offered in the present study were lower than those (range of 18.7% to 24.3%) reported by Li et al [17] because a higher proportion of alfalfa hay (43.7%) was used in that study. Furthermore, Yang and Beauchemin [19] summarized that feed intake is influenced by digestion rate and passage rate of feeds through the rumen. It is possible to assume that although there were significant differences in peNDF contents among the groups, passing rate has no difference with same DMI of low level (2% of BW). When low-peNDF diets are fed to lactating dairy cows, there may be a relationship between retention time and digestion efficacy [19]. In contrast, retention time has no difference when goats eat the same quantity of good quality forage [20]. Also, the retention time of feed particles tends to be shorter for goats (28-h) than other ruminants (cattle 36-h, sheep 34-h) [21]. Also, goats can eat diets of the higher surface area of absorptive mucosa than in grass and roughage eaters [20]. In addition, the level of intake and the characteristics of digestion depend closely on the feeding behavior of goats [20]. Dairy cows may select long feed particles to maintain ruminal pH in the normal range [15]. Goats are able to control the chewing activity of ruminal frequency to maintain in the ruminal pH normal range [22]. In the current study, however, dietary peNDF did not affect chewing activity. This result is a contrast to other ruminants [16,5,17]. For example, reducing peNDF increased rumen total volatile fatty acids and decreased chewing time [23]. In fact, Li et al [23] used a proportion of particles on the 19 mm (range of 3.5% to 17.8%) and 8 mm (range of 21.1% to 39.7%) Penn State Particle Separator (PSPS) screens. In the present study, a lack of the effectiveness of dietary peNDF on chewing activity was likely due to the proportion of 19 mm (range of 0% to 1.6%) and 8 mm (range of 0.5% to 10.0%). Goats are likely to be better digesting diets regardless of particle size differences. Morand-Fehr [20] suggests that the total tract of goats is very similar to those of other ruminants. However, it is possible that BW or total tract size of goats varies greatly from other ruminants. Therefore, this might be due to relatively small ruminant digestion characteristics [20]. Although it was not significantly dietary of physical characteristics in the current study, peNDF of diets are important when the goats fed to TMR.

The feeding of peNDF-based TMR shows no impact on daily gain and chewing activity of black goats. Furthermore, the effect of peNDF did not affect the apparent nutrient digestibility. These results indicate that goats have the ability to digest diets regardless of particle size differences. There are no studies that investigate the effect of dietary peNDF measured using the PSPS for goats. Also, there is limited information for expected particle size distribution for goats. In conclusion, further studies are required with a wider range of dietary peNDF level and particle size to better identify the effect of dietary peNDF and particle size on chewing activity and performance in goat.