Selection for growth rate at pasture in Angus cattle results in heavier cattle that eat more in the feedlot

Context

Selection for growth rate has received considerable attention in beef cattle but the evidence for an improvement in the efficiency of feed conversion is equivocal.

Aim

To examine whether feed efficiency by beef cattle finished in a feedlot had been changed in response to divergence selection for growth rate.

Methods

The Angus cattle used came from three lines of cattle selected for over five generations for fast growth rate to yearling age (High-line), slow growth (Low-line), or from an unselected Control-line. Over sequential years, a cohort of steers, then of heifers and then of steers, representative of the lines, were measured for feedlot performance, and carcase- and meat-quality traits. The animals were fed a high-energy feedlot ration and after an adjustment period they underwent a performance test of at least 70 days of duration. After slaughter, muscle samples were taken for subsequent measurement of the components of the endogenous calpain proteolytic enzyme system. Their carcasses underwent a standard chiller assessment and meat samples were taken after 1 day and 14 days (steers) or 17 days (heifers) for objective measurement of tenderness.

Key results

Cattle from the High-line grew 48% faster (P < 0.05), and ate 48% more feed (P < 0.05) than did those from the Low-line, but had similar (P > 0.05) feed conversion ratio and residual feed intake. There were no differences between the High-line and Low-line in the visual meat-quality attributes of meat colour, fat colour and marbling, and no differences in the objective measurements of tenderness and connective-tissue toughness. There was no evidence of a selection response in the circulating concentrations of the metabolites and hormones measured, nor in the endogenous calpain proteolytic enzyme system in muscle.

Conclusions

The superior growth demonstrated by the High-line cattle over the feedlot test was accompanied by a higher feed intake, with no evidence for an improvement in feed efficiency.

Implications

Selection for growth rate is a powerful tool to alter animal performance but the beef industry needs to be cognisant of the proportional increase in feed requirement from breeding bigger animals.