Abstract
Animals suffering from fescue toxicosis have impaired capacity for thermoregulation, experience increased heat stress susceptibility, and reduced welfare and performance. The objective of this study was to determine if endophyte-infected tall fescue (E+TF) naive heifers (FN) would have different responses to environmental heat stress over the first two months of exposure in comparison to E+TF adapted heifers (FA). The hypothesis was FN heifers grazing E+ TF (FN-E+) would have greater respiration rates (RR) and consequently lower average daily gain (ADG) when compared to FN heifers grazing endophyte-free TF (FN-E-) and FA heifers grazing E+TF (FA-E+). Yearling beef heifers from Montana (n = 31; fescue naïve, FN) and Kentucky heifers (n = 18; fescue adapted, FA) were used. Heifers were stratified by body weight and hair coat score (HCS) into nine pastures that were assigned to one of three treatments: FA-E+ (n = 18), FN-E+ (n = 16), and FN-E- (n=15). All pastures had shade structures, water, and loose mineral provided ad libitium. Heifers were weighed at the start and end of the grazing period for determination of ADG. Monthly HCS was recorded during the grazing period. Respiration rates were counted by observing flank movements for 15-sec., 3× a week in the afternoon (1200 – 1400 h) in May and June. Weather data was collected from an onsite NOAA weather station for calculation of THI. Data was analyzed using the PROC MIXED procedure in SAS with significance set at P≤0.05. Average afternoon THI in May was 67.6 (65.7-69.3) and June was 72.5 (70.8-74.2) and were positively correlated with RR in both months (r=0.64-0.87). Overall, FN-E+ had higher RR (May:120.0±2.2 breaths·min(-1); June:111.2±2.2 breaths·min(-1)) than FA-E+ (May:106.1±2.1 breaths·min(-1); June:102.8±2.1 breaths·min(-1)) and FN-E- treatments (May:103.4±2.3 breaths·min(-1); June:103.3±2.3 breaths·min(-1); P ≤ 0.05). In contrast, FA-E+ and FN-E- RR were similar in both months (P>0.10). At the start of the grazing period, FN heifers (4.6±0.2) had higher HCS than FA heifers (3.6±0.2; P< 0.05). Over May and June, FN-E- heifers had the greatest change in HCS (-2.9±0.3) in comparison to both FN-E+ (-2.1±0.3) and FA-E+ (-1.9±0.3) heifers (P< 0.01). However, at the end of June, FN-E+ (2.6±0.2) still had a higher HCS than FA-E+ (P< 0.05; 1.7±0.2), but FN-E- and FA-E+ were similar (P > 0.10). Over the grazing period, FA-E+ (0.36±0.05 kg·head·d( -1)) and FN-E- (0.45±0.05 kg·head·d( -1)) had greater ADG than FN-E+ (0.14±0.05 kg·head·d( -1);P< 0.01). The results confirm that FN-E+ heifers experienced greater heat stress when first exposed to E+TF as evidenced by increased respiration rates and lower ADG which could be partly attributed to HCS. Furthermore, these results suggest that adaptation for FN heifers to E+TF pastures takes longer than two months.