Enteric methane (CH₄) production in ruminants is recognized as one of the major sources of greenhouse gas emissions associated with agriculture [1]. Although a large number of goats and Sika deer inhabit East Asia [2], few studies have estimated enteric CH₄ production of goats and Sika deer. Enteric CH₄ production by ruminants is generally influenced by various dietary factors such as the forage-to-concentrate ratio [3], carbohydrate type [2,4], forage processing [5], fat addition [6], and ionophore addition [7]. Fundamentally, because enteric methane production is a part of the energy utilization process of ruminants, the amount of feed intake has been suggested as the major factor driving enteric CH₄ production. Changes of feed intake level affect passage rate and rumen fermentation characteristics and, thereby, can alter nutrient digestibility and CH₄ production of the animal [8]. Therefore, the aims of the present study were i) to determine the effects of feeding level on nutrient digestibility and enteric CH₄ production and ii) to estimate the for the respective enteric CH₄ production prediction equations for growing goats and Sika deer.

Experiments were conducted to determine the nutrient digestibility and enteric CH₄ and CO₂ emissions in goats (Capra hircus hircus) and Sika deer (Cervus nippon hortulorum) in accordance with the Institutional Animal Care and Use Committee of Konkuk University.

Dry matter intake (DMI) of both goats and Sika deer increased linearly (p<0.05) as feeding level increased (Table 2). The DM, OM, CP, and neutral detergent fiber digestibility (NDFD) of goats decreased linearly (p<0.05) as the feeding level increased. However, the DM, OM, CP, and NDFD of Sika deer did not differ significantly among feeding level treatment groups.

In goats, enteric emission of CH₄ expressed as g/d and g/kg BW^0.75 increased linearly (p<0.05) with increasing feeding level (Table 3); however, when expressed as g/kg DMI, g/kg organic matter intake (OMI), and % of gross energy intake (GEI), it decreased linearly (p<0.05) with increasing of feeding level. No difference in enteric CH₄ production when expressed as g/kg digested dry matter intake (DDMI) and g/kg digested organic matter intake (DOMI) was observed among feeding levels. In Sika deer, enteric emission of CH₄ expressed as g/d and g/kg BW^0.75 increased linearly (p<0.05) with increasing feeding level, while it was not affected by feeding level when expressed as g/kg DMI, g/kg OMI, or % of GEI. In goats, CO₂ production expressed as g/d and g/kg BW^0.75 increased linearly (p<0.05) with increasing feeding level, and in Sika deer, enteric CO₂ production expressed as g/kg was not affected by feeding level. However, CO₂ production expressed as g/kg BW^0.75 increased linearly (p<0.01) with increasing feeding level.

For goats, the model that used the DMI, DM digestibility (DMD), and NDFD as independent variables (equation 1), was most accurate (Table 4; R² = 0.69, RMSE = 1.05, and p = 0.096), whereas for Sika deer, the model that used the DMI, DMD, and NDFD as independent variables, was most accurate (R² = 0.95, RMSE = 0.98, and p = 0.001).

The amount of feed intake is fundamentally important for animals because it directly affects animal productivity. In general, feeding level and nutrient digestibility in ruminants are negatively correlated [14,15] because a change in feeding level can alter the passage rate and fermentation pattern of the reticulo-rumen [16]. In the current experiment, the nutrient digestibility of goats decreased linearly (p<0.05) as feeding level increased. This result was consistent with the findings of previous studies using other ruminants [14,15,17]. However, interestingly, nutrient digestibility of Sika deer was not affected by increasing the feeding level. Although a few studies have determined both nutrient digestibility and the DMI of white-tailed and roe deer [18,19], to our knowledge, the effect of feeding level on digestibility has not been previously determined for Sika deer. Nutrient digestibility varies for different species of ruminants [20,21]. Galbraith et al [22] reported that OM, NDF, and ADF digestibility of white-tailed deer tend to be lower than those of Wapiti. [2,23]. According to the morphophysiological classification of Hofmann [24], Sika deer are classified as concentrate eaters. Therefore, it seems that the nutrient digestibility of Sika deer is different from that of goats and cattle because rumen capacity and retention time of Sika deer are lower than those of both intermediate-type and grass/roughage eaters. In addition, since the experimental diet of Sika deer in this study was comprised of 50% roughage sources, it may have been difficult for the deer to digest a large amount of fiber contents due to being concentrate eaters rather than to the feeding levels.

In both goats and Sika deer, enteric emissions of CH₄ expressed as g/d and g/kg BW^0.75 increased linearly (p<0.05) with increasing feeding level, and in goats, enteric production of CH₄ expressed as g/kg DMI, g/kg OMI, and % of GEI decreased linearly (p<0.05) with increasing of feeding level. These results agreed with those of studies that have found that CH₄ production (g/kg DMI) decreased with increased feeding level in cows [25,26] and sheep [27]. Because the residence time of rumen contents decreases with increased feed intake, consuming larger amounts of feed can result in shorter exposure time to rumen microbes [28,29]. However, enteric production of CH₄ expressed as g/kg DMI, g/kg OMI, and % of GEI were not affected by increasing feeding levels in Sika deer, and, as mentioned above, the morphophysiological differences of the gastro-intestinal tracts of Sika deer can affect rumen capacity and retention time [24]. Consequently, in comparison with both intermediate-type and grass/roughage eaters, it seems that the overall effect of feeding level on enteric CH₄ production can be diminished in Sika deer. In both goats and Sika deer, enteric CH₄ production expressed as g/kg DDMI and g/kg DOMI did not differ statistically. Although digestibility is difficult to measure and requires complex equipment, Na et al [3] reported that digested nutrient intake is a critical factor that explains CH₄ production in goats and Sika deer. Therefore, digested nutrient intake can be used as a CH₄ prediction factor when the experimental condition allows. Mineral salt supplementation can influence the enteric methane emission and methanogenic archaea population in rumen [30]. In the current study, mineral block intake did not measure, however, it appears that the effect of mineral block was slight because the animals can freely access to the mineral block.

The models that used nutrient digestibility as a factor exhibited the greatest accuracy in this as well as other experiments [3,31]. However, equations 3 and 4, which used the DMI as the only factor, can also be used to predict enteric CH₄ because the model without digestibility is more practical. To our knowledge, only one other study has predicted CH₄ production in Sika deer [3]. Therefore, the models evaluated in the current study contribute to our understanding of enteric CH₄ emissions in Sika deer.

Nutrient digestibility of goats decreased linearly as the feeding level increased, whereas the digestibility of Sika deer was not affected by feeding level. The enteric production of CH₄ expressed as g/kg DMI, g/kg OMI, and % of GEI decreased linearly with increasing feeding levels in goats. However, enteric production of CH₄ expressed as g/kg DMI, g/kg OMI, and % of GEI of Sika deer were not affected by feeding level.