Estimation of Direct, Residual, and Total Effects of Canopy Treatments Affecting Intake by Grazing Cattle

Abstract

AbstractEffects of transient treatments imposed on grazing cattle diminish over time but many experimental designs and statistical analyses only estimate integrated responses. We describe a change‐over design that estimates direct, first residual, second residual, and overall effects of three treatments (water, saline solution, or syrup solution) applied to alfalfa (Medicago sativa L.) canopies on the ingestive behavior of Angus cows (Bos taurus L.). Syrup‐treated canopies evoked a direct effect. Animals grazed water‐treated canopies at 32 bites min−1 and ingested herbage dry matter (DM) at 0.55 kg 100 kg−1 body mass h−1, while others grazed syrup‐treated canopies at 48 bites min−1 (P < 0.01) and ingested herbage DM at 0.64 kg 100 kg−1 h−1 (P < 0.01). As residual effects were not significant (P > 0.05), the animal responses to syrup‐treated canopies were considered preingestive responses and were attributed to enhanced sweetness. Salt‐treated alfalfa canopies, conversely, had no direct (immediate) effect (P > 0.05) on herbage intake, but summed first and second residual effects, expressed the day after application of NaCl, revealed that animals grazing untreated and syrup‐treated alfalfa ingested DM at 0.47 kg 100 kg−1 h−1, while those grazing salt‐treated alfalfa ingested herbage DM at 0.55 kg 100 kg−1 h−1 (P < 0.05). The residual effects of salt were considered postingestive responses and were attributed to Na deficiency. Change‐over designs that estimate direct and residual effects of treatments may be used to determine preingestive and postingestive components of intake.