Effects of dietary rumen-degradable protein on the growth performance, energy, and nitrogen metabolism of dairy buffalo heifers.

Abstract

The goals of ruminant protein nutrition are to provide sufficient rumen-degradable protein (RDP) for optimizing nitrogen utilization efficiency and achieve the desired level of animal productivity, all while minimizing dietary crude protein (CP) intake. The present study aimed to examine the effects of decreasing dietary protein by reducing RDP on the growth performance, energy, and nitrogen metabolism of dairy buffalo heifers. Three diets were formulated: high RDP (H-RDP, RDP = 80.36 g/kg, CP = 15.42%), medium RDP (M-RDP, RDP = 72.79 g/kg, CP = 14.03%), and low RDP (L-RDP, RDP = 66.48 g/kg, CP = 12.87%), yielding digestible crude protein (DCP) balances of 166.07, 114.89, and 66.59 g/d, respectively. The diets were fed to 36 heifers (259.19 ± 3.93 kg) divided into three groups (12 heifers per treatment) over a 51-day trial. The results showed that when dietary RDP decreased from 80.36 g/kg to 72.79 g/kg, the average daily gain (ADG) and dry matter intake (DMI) remained stable; however, a further decrease to 66.48 g/kg resulted in a 19.12% decrease in ADG and an 6.80% decrease in DMI (P < 0.05). The feed-to-gain (F/G) ratio increased linearly with decreasing dietary RDP (P < 0.01) and was significantly lower in the M-RDP and H-RDP groups than that in the L-RDP group (P < 0.05). The apparent digestibility of CP decreased linearly (P = 0.01) and was significantly higher (P < 0.05) in the H-RDP group than in the L-RDP group. There was a linear increase in fecal nitrogen efficiency (FN)/IN, which was significantly lower (P < 0.05) in the H-RDP group than in the L-RDP group. Reducing dietary RDP linearly decreased urinary nitrogen excretion (UN) (P < 0.01) without affecting the efficiency of the retained nitrogen (RN), intake nitrogen (IN), and energy profiles. Overall, this study revealed that reducing dietary RDP (from 80.36 to 72.79 g/kg) and CP (from 15.42% to 14.03%) effectively reduced UN excretion while maintaining dairy buffalo heifer performance. Collectively, these findings suggest an optimal dietary RDP range of 72.79-80.36 g/kg for 13- to 15-month-old dairy buffalo heifers.