Repeatability of residual feed intake across dietary forage concentration

Abstract Text: Residual feed intake (RFI) has received considerable attention as a possible breeding goal in the near future. For RFI to be useful as a breeding goal, it should be repeatable for cows under different types of diets. Our objective for this study was to determine the repeatability of RFI across two levels of dietary forage NDF.  Holstein cows in mid-lactation were studied in winter (n=32) and summer (n=32) . The study followed a crossover design with 2 treatment periods of 31(winter) and 28 (summer) days. Cows were milked twice daily and fed treatment diets once daily. Treatments were a high forage-low starch diet (HF; 36%NDF, 19% starch) and a low forage-high starch diet (LF; 26%NDF, 32% starch). Forage composed 70% of the DM in HF and 47% in LF. Dry matter intake (DMI) and milk yield (MY) were recorded daily.  Body weight (BW) was measured 3x weekly and milk composition was measured for 4 consecutive milkings each week. Body condition score (BCS) was measured at the beginning and end of each experimental period. Statistical analysis was performed using the GLM procedure (SAS 9.4). An RFI value was obtained for each cow under each treatment, cows were ranked using standard deviation (SD) of the RFI value as HRFI (>+0.5 SD), MRFI (±0.5 SD) or LRFI <-0.5 SD). A group rank was established for all cows under each treatment diet. The HF diet decreased DMI by 2.5 kg/d and milk yield by 3 kg/d when compared to the LF diet.BW changed by 0.4 Kg/d on LF but 0.2 kg/d on HF.  Fat yield, BW and BCS were not altered by treatments. The decrease in DMI as well as the difference in energetic density of the diet could explain the differences observed in production performance as there was no significant difference in energy partitioned towards milk production. The RFI ranking was relatively repeatable (r=0.44).  Of all animals, 48% maintained their group ranking across treatments while 47% changed ranking by 1 group.  Only 5% moved in the ranking from the HRFI to the LRFI group or vice versa. In conclusion, although intake, production and energy partitioning were significantly altered by dietary treatments, RFI was relatively repeatable across these diets. Thus, genomic breeding values of RFI estimated from cows fed a high starch diet should still be useful when animals are fed more forage and less starch.

Keywords: dairy cow, residual feed intake