Effects of crude protein and undegradable intake protein on growth performance, nutrient utilization, and rumen fermentation in growing Thai-indigenous beef cattle

The objective of this study was to investigate growth performance, nutrients apparent digestibility, nitrogen utilization, rumen fermentation, and rumen microorganism of growing Thai-indigenous beef cattle receiving different levels of crude protein (CP) and undegradable intake protein (UIP) diets. Eighteen healthy growing Thai-indigenous beef cattle were used in a 2 × 3 factorial randomized complete block design (RCBD). There were six treatments: two levels of CP (10% and 12% of dry matter (DM)) and three levels of UIP (15%, 25%, and 35% of CP). The results indicated that dry matter intake (DMI) and average daily gain (ADG) increased linearly (P < 0.01) with increasing UIP level. The apparent digestibility of DM, organic matter (OM), acid detergent fiber (ADF), and neutral detergent fiber (NDF); rumen fermentation parameters; microbes counts; and microbial nitrogen synthesis (MNS) were unaffected (P > 0.05) by CP and UIP levels. The nitrogen (N) intake and N retained increased linearly (P < 0.01) with increasing UIP level. Specifically, the metabolizable protein (MP) requirement of 1 g/kg BW^0.75 gain was 0.34 g MP/kg BW^0.75; the 10% dietary CP of DM was able to meet animals' normal nutrition requirements and 6.5% DIP of DM can provide adequate N source for the requirement of rumen microbe growth in. Collectively, it was indicated that the supplemental level of 10% CP and the ratio of UIP to DIP was 35:65 in diets shown the best growth performance for growing Thai-indigenous beef cattle under the conditions in the current study.