본 연구는 비섬유탄수화물(NFC)과 반추위분 해단백질(RDP)을 젖소사료에 다양하게 배합하여 NFC : RDP의 비율을 3.5, 3.0 및 2.5로 다양하게 조정하였을 때, in vitro 발효성상 및 젖소의 산유성적에 미치는 영향을 조사하여 적정 NFC : RDP비율을 결정하고자 실시하였다. In vitro 발효시험은 반추위 cannulae가 장착된 Holstein 젖소로부터 반추위액을 얻어 3, 6, 9, 12시간 동 안 배양하였다. 반추위내 pH는 NFC비율이 증가함에 따라 유의한 영향을 받지 않았고, 암모 니아질소농도는 배양 6시간까지 처리군간에 유의한 차이가 나타나지 않았으나, 9시간과 12 시간에서는 NFC : RDP비율이 증가함에 따라 유의하게 감소하였다(P < 0.05). 휘발성지방산생산 량은 acetate 및 propionate농도가 발효 3시간에 NFC함량이 증가함에 따라 유의하게 증가하였 으나(P < 0.05), 나머지 시간대에서는 유의차가 나타나지 않았다. Valerate 및 A : P비율은 NFC : RDP비율에 의하여 영향을 받지 않았고, iso- acids 및 총 휘발성지방산은 전반적으로 NFC함 량이 증가함에 따라 유의하게 증가하였으며(P < 0.05), 3.0비율구가 가장 높은 값을 나타내었다(P < 0.01). 한편, 사양시험은 18 두의 착유우를 공시하여 처리구당 6 두씩 배치하여 총 24주간 실시하였 다. 건물 및 에너지섭취량은 NFC : RDP비율이 증가함에 따라 유의하게 증가하였고(P < 0.01), 섬 유소섭취량은 NFC : RDP비율이 감소함에 따라 직선적으로 증가하였다(P < 0.01). 산유량은 3.5(32.7 kg)비율구가 다른 처리구보다 유의하게 높았다(P < 0.05). 유지방율은 3.0(3.79 %) 및 2.5(3.79 %)비율구가 3.5(3.48 %)비율구보다 유의하 게 높았고(P < 0.05), 유지방생산량은 처리구간 에 차이가 나타나지 않았다. 유단백질과 무지 고형분은 NFC : RDP비율이 증가함에 따라 직선 적으로 증가하였다(P < 0.01). 그러나 우유중 요소태질소농도는 NFC함량이 증가함에 따라 유 의하게 감소하였다(P < 0.05). 이상의 결과로부터 사료내 증가하는 수준의 NFC는 반추위발효, 질소이용효율 및 산유량을 향상시켰고, 사료중 NFC : RDP비율이 3.0이상일 때 반추위발효뿐만 아니라 산유성적이 최대화되는 것으로 나타났다. This study was conducted to determine effects of different ratios (3.5, 3.0 and 2.5) of nonfibrous carbohydrate (NFC) to ruminally degradable protein (RDP) on in vitro fermentation and lactation performance of dairy cows and optimum ratio of NFC to RDP in dairy rations. In vitro trial was conducted up to 12 hr with ruminal fluidtaken from ruminally cannulated Holstein cows. The level of dietary NFC did not affect ruminal pH. The ammonia-N concentration was not significantly different among treatments until 6 hr incubation, however, it was significantly (P < 0.05) decreased as the ratio of dietary NFC to RDP increased on 9 and 12 hr incubation. For volatile fatty acids, concentrations of both acetate and propionate were significantly (P < 0.05) increased on 3 hr incubation as dietary NFC contents of treatments increased, in other incubation times, they had no significant differences among treatments. Valerate and A:P ratio were not affected by the ratio of NFC to RDP. Isoacids and total VFAs were significantly (P < 0.05) increased with increasing dietary NFC contents and their values were highest in the treatment of 3.0 ratio. Meanwhile, for in vivo trial, 18 Holstein lactating cows were allotted to treatments in three groups of 6 cows. They were employed for 24 weeks to investigate nutrient intakes, and milk yield and composition according to different ratios of dietary NFC to RDP. Intakes of dry matter and energy were significantly (P < 0.01) increased, but NDF intake was significantly (P < 0.01) decreased as the ratio of dietary NFC to RDP increased. Milk yield for the ratio of 3.5 (32.7 kg) was significantly (P < 0.05) higher than those of other treatments. Milk fat (%) was significantly (P < 0.05) higher for the treatments of 3.0 (3.79 %) and 2.5 (3.79 %) than that (3.48 %) for the ratio of 3.5, but milk fat yield was not different among treatments. Contents and yields for milk protein and solids-not fat were linearly (P < 0.01) increased as the ratio of dietary NFC to RDP increased. However, milk urea nitrogen concentration was significantly (P < 0.05) decreased with increasing dietary NFC levels. Our results showed that the increasing level of NFC in the diet of dairy cows enhanced ruminal fermentation, N utilization and milk production and suggested that maximal fermentation and lactation performance were achieved when the dietary ratio of NFC to RDP was more than 3.0 in dairy rations.