Feeding efficiency and behavior of young Simmental bulls selected for high growth capacity: Comparison of bulls with high vs. low residual feed intake

•Simmental bulls growing 1.57 kg/d have a RFI standard deviation of ±0.79 kg/d.•Feeding behavior traits explain little of DMI variability.•Efficient bulls eat slow and access the trough less. Thex objectives of this study were to evaluate the variability of residual feed intake (RFI) in young Simmental bulls and to study the potential use of certain feeding behavioral traits as indicators of feed efficiency. At an average age of 9 months, multiple groups of 6 young bulls were moved from the same pen to a pen equipped for monitoring of intake and feeding behaviors. For the first part of the study, 272 young bulls subjected to performance tests at the Genetic Center of Italian Simmental were considered. The overall ADG was 1.57 kg/day and the overall dry matter intake (DMI) was 10.35 kg/day. The RFI values ranged from −2.55 to +1.86 kg DM/day. Bulls visited the trough 49.5 times/day and the total feeding time (FT) was 113.1 min/day. There were 8.65 missing events (MEv) per day for a total time of 5.98 min/day. The base model, which considered metabolic body weight and ADG, explained 50.5% of the variability in DMI. Addition of feeding behaviors to the analysis explained an additional 5% of the variation in DMI (P < 0.05). In a dataset restricted to bulls with an average daily gain (ADG) greater than the overall average (1.57 kg/day), we selected two groups of bulls, a Low RFI group (n = 66) and a High RFI group (n = 79). As expected, the High RFI and Low RFI groups had the same ADG (1.77 kg/day) and significantly different DMI (P < 0.01). The different feed efficiency resulted in a significant reduction in excretions (P < 0.01), which were approximately 11% lower for methane, 9% lower for fecal N, and 13% lower for urinary N in the high efficiency group. The High RFI group had a higher feeding events number than the Low RFI group (P = 0.02), but the groups had similar daily FT (overall average: 115.3 min/day). The Low RFI group had a feeding rate that was 9.8 g DM/min lower than the High RFI group (P = 0.01). Most likely, the longer time of mastication and the slower feed intake allowed more efficient digestion in the Low RFI group. The two groups had no significant differences in MEv, in number or total time (both P > 0.05). This study is the first to evaluate feeding efficiency in an Italian Simmental breed. Although some behavioral traits differed significantly between the Low and High RFI groups, their inclusion in the DMI prediction model only had a sma