Digestibility of starch, ruminal protozoa count, and blood parameters of Dorper lambs with differences in residual feed intake.

• Efficiency in the animal production is related to its ability to transform the feed in products. • Residual feed intake (RFI) is an tool for selecting more efficient production animals. • For sheep there are few studies on feed efficiency from the perspective of RFI. • RFI in sheep changes the population and species of protozoa. • RFI in sheep does not modify the use of starch in the total digestive tract. This study aimed to evaluate the digestibility of starch, ruminal protozoa count and blood parameters of sheep with high or low residual feed intake (RFI) fed different levels of concentrate in the diet. Sixty male Dorper lambs with a mean initial body weight of 17.8 kg ± 2.43 were confined for 40 days for the determination of RFI, receiving a complete diet in a 70:30 forage: concentrate ratio, on a dry matter basis. RFI was estimated considering the differences in observed and predicted values of dry matter intake, based on average daily gain and metabolic body weight. Sixteen animals with the highest values of RFI (high RFI) and 15 animals with the lowest values of RFI (low RFI) were selected. The experimental design was a randomized block design (2 × 3), with two groups of animals (high and low RFI) and 3 treatments with diets containing different levels of concentrate (30, 45 and 60 %). The lambs remained under this confinement regime for 60 days. The different levels of concentrate promoted differences (P < 0.05) between hemoglobin and hematocrit counts. There was an interaction between RFI and diet for hemoglobin and hematocrit counts (P < 0.05). Animals with high RFI presented larger volume of urine excreted in relation to animals with low RFI (P = 0.006), with mean values of 1.59 l/day and 1.09 L/day, respectively. Diets containing 30 % concentrate resulted in lower starch intake (P < 0.0001) and lower digestibility of starch in the total digestive tract (P < 0.0001). Animals with low RFI had lower counts of protozoa in the ruminal fluid (P = 0.04). Diets containing 60 % concentrate in their composition provided lower counts of total protozoa (P = 0.0003) in the ruminal fluid. This diet also promoted a smaller population of protozoa of the genus Dasytricha (P = 0.04) for high RFI animals. Blood parameters and utilization of starch in the total digestive tract are not affected, while ruminal fauna is affected by RFI in lambs. [ABSTRACT FROM AUTHOR]