Effects of a Positive Dietary Cation–anion Difference Diet on Growth Performance and Physiological Responses of Feedlot Steers During Heat Stress

Forty-eight Black Angus yearling steers (458 ± 31 kg) were used in a 90-day (June-August) trial evaluating effects of feeding a positive dietary cation-anion difference (DCAD) diet during heat events on growth performance and blood metabolites. Steers were stratified by weight and housed into partially shaded pens of 6 steers, each with a GrowSafe bunk (Vytelle, Lenexa, KS). The common finishing diet contained inorganic trace minerals (CON; 8 meq/100 g DM). During heat events, steers assigned to DCAD received a positive DCAD diet (38 meq/100 g DM; additional sodium bicarbonate and potassium carbonate). Heat events (HE) were defined as at least two consecutive days where the maximum temperature-humidity index (THI) was ≥ 80 and the minimum THI was ≥ 60 and DCAD was fed one day prior to through one day after the event. Average minimum/maximum THI during non-heat events (NH) and HE were 60/78 and 67/84, respectively. Across the 90-day period, the DCAD diet was fed for 33 days across six different HE. All DCAD steers and half (n = 12) of the CON steers were fitted with tags (CowManager, Harmelen, Netherlands) to continuously monitor ear surface temperature. Two-day BW were collected on d 0, 56 and 91, and blood (i-STAT blood gas analyzer; Abbott, North Chicago, IL) and respiration rates were collected on d 53. Data were analyzed in SAS using Proc Mixed with fixed effects of dietary treatment and event (NH or HE) and the interaction. Ear surface temperature was not affected by the interaction between dietary treatment and event, or by diet alone (P ≥ 0.33). However, average minimum and maximum ear temperatures were greater during HE compared to NH events (P < 0.01). Blood pH tended (P = 0.09) to be greater in DCAD vs. CON, and blood K was lesser (P = 0.005) in DCAD vs. CON. There were no differences in blood PCO2, PO2, HCO3, Na, or glucose or respiration rate on this day (P ≥ 0.22). Dry matter intake was affected by dietary treatment x event (P = 0.015), where DCAD ate more during HE than NH events, while CON did not differ between events. Overall, 90-d period average daily gain, feed efficiency, and final body weight were not different between dietary treatments (P ≥ 0.18). Supplementing a positive DCAD mineral blend affected acid-base balance and dry matter intake during periods of high heat but did not affect growth performance in yearling steers.