The relationship of residual feed intake and visceral organ size in growing lambs fed a concentrate- or forage-based diet

We hypothesized that a portion of the individual differences observed for feed efficiency in ruminants can be attributed to gastrointestinal tract (GIT) size, which would vary based on diet type. The objective of this study was to determine GIT and visceral organ size in high and low efficiency growing lambs fed either a concentrate or forage-based diet. Growing wethers (n=82; 51.3±1.2 kg body weight [BW]) were fed a concentrate (CONC; 12.1% crude protein [CP], 17.6% neutral detergent fiber [NDF], 2.98 Mcal/kg metabolizable energy [ME]) or forage-based pelleted diet (FOR; 16.2% CP, 36.3% NDF, 2.31 Mcal/kg ME) for 49 d. Individual intake was measured with the GrowSafe System to determine residual feed intake (RFI). The 20% highest efficiency (low RFI, n=8) and 20% lowest efficiency (high RFI, n=8) lambs from each diet were slaughtered (66.6±2.3 kg BW; n=32 total), and the viscera was dissected and weighed. Data were analyzed as a 2×2 factorial with RFI class (high efficiency vs. low efficiency), diet type (FOR vs. CONC), and their interaction in the model. Organ masses were not affected (P>0.10) by the RFI class x diet type interaction. High efficiency lambs tended to have greater (P=0.09) pancreas and spleen actual mass than low efficiency lambs, although RFI class did not affect (P>0.15) other organ actual (g) or relative (g/kg BW) mass. Lambs fed FOR had greater (P≤0.01) actual and relative reticulum, omasum, large intestinal, and kidney mass and tended to have greater (P≤0.09) actual and relative small intestinal masses compared with lambs fed CONC. However, lambs fed CONC had greater (P≤0.05) actual rumen, heart, liver, and relative rumen mass than lambs fed FOR. All other visceral organ masses were unaffected (P>0.11) by diet type. Diet type, RFI class, and their interaction did not affect small intestinal length (P>0.10). Results of this study suggest that visceral organ size in growing lambs is more affected by diet type than individual feed efficiency. Based on this data, pancreas and spleen sizes may play a role in efficiency of feed utilization in lambs, however.