PSLBII-5 - Impacts of residual feed intake measured as a heifer in drylot on mature cows and calves grazing native pasture.

Abstract: Selection for feed efficiency is crucial for improving economic and environmental sustainability in beef cattle production. Residual feed intake (RFI_fat) is a commonly used measure of feed efficiency and is determined in a drylot setting by calculating the difference between observed and expected dry matter intake (DMI), where expected feed intake is adjusted for metabolic body weight, average daily gain (ADG) and body fatness. Lower RFI_fat cattle are more efficient animals as they eat less than predicted for the same growth, body size and body fatness. Limited studies have been conducted evaluating the role of RFI_fat on the subsequent performance of cow-calf pairs on native range. Kinsella Composite cows and calves (n = 120) grazed native pasture from July 11 to Sept 12 (summer) and from Sept 13 to Nov 2 (fall) of 2023 were included in this study. All dams had previously been evaluated for RFI_fat as a heifer (8-12 months of age) using the GrowSafe Feed Intake System® (Vytelle, Canada). Cow and calf ADG were subjected to an analysis of covariance using lm function from stats R core package (R Core Team, 2024), with cow age (2 to 8 yr of age) as the fixed effect and RFI_fat as the covariate, analyzing separately by season. An unbalanced Tukey HDS post-hoc were applied for age and p-values < 0.05 were considered significant. Weight gain was assessed on cows with an initial body weight on pasture of 598.4 ± 68.4 kg, and 2 to 8 years of age, and on calves having an initial on pasture body weight of 106 ±21.5 kg, and 65 ±14 days of age. Cow age affected (P < 0.01) cow ADG during summer, with the youngest dams (2 yr old) gaining more weight than older animals (7 yr old) (Figure 1A). No effect of RFI_fat was found on cow weight gain during summer grazing. However, during fall grazing, RFI_fat affected (P < 0.01) cow ADG (Figure 1B). Low-RFI_fat cows (efficient) had increased weight gain during fall grazing as compared to high-RFI_fat cows (inefficient: Figure 1B). Calf ADG was not affected (P > 0.1) by cow age, cow RFI_fat and cow ADG during summer and fall. We conclude that cows with low RFI_fat exhibited superior body weight gain, during fall grazing on native range when forage nutrient quality and quantity become limiting.