Sources of Error for Ultrasonic Measurements of Steer Compositional Traits in Grazing Studies

Sources of Error for Ultrasonic Measurements of Steer Compositional Traits in Grazing Studies

Real-time ultrasound technology has potential use for measuring compositional traits in grazing studies. Sources of error with this application must be identified for designing experiments with adequate precision to determine treatment effects. A study was conducted to evaluate breed type (Angus x Brahman, AB; Simmental x [Angus x Brahman], SAB; Simmental x [Brahman x Hereford], SBH), initial body weight, and initial ultrasonic measures as sources of experimental error in the analyses of external fat (EF) depths and longissimus dorsi (ribeye) areas (LDA). Steers were ultrasonically scanned at the initiation and conclusion of a grazing study that examined stocking rate effects on average daily gain (ADG) and body condition of steers grazing a mixture of rye (Secale cereale L.) and ryegrass (Lolium multiflorum L.). Breed type did not affect (P > 0.10) final or increases in EF, and curvilinear trends of increased EF with increasing ADG (P > 0.10) among the breed types over the range of ADG (0.03 kg d^-1 to 1.62 kg d^-1). The three breed types showed curvilinear increases in LDA as ADG increased, but the trends were different (P < 0.05) for SBH than for the other two types. Final LDA and magnitude of increases in EF and LDA over the experimental period were influenced (P < 0.05) by initial body weight. Results indicated that allotting cattle to pastures based on a combination of breed type and body weight should improve precision in detecting treatment effects.