Meta-Analysis of Endophyte-Infected Tall Fescue Effects on Cattle Growth Rates.

The objective of this study was to quantitatively summarize the literature describing endophyte-infected fescue effects on cattle ADG. The effects of three different infection-level measurement methods were tested using 138 treatments from 20 articles. The dataset was split into two subsets based on whether SEM were reported for the ADG measurements: only complete-SEM data (CSD) and all data including missing-SEM data (MSD). Weighting factors were calculated as 1/SEM in both datasets when SEM was reported. In the MSD group, values with missing SEM were weighted according to the average of the reported SEM. Three endophyte-infected fescue measurement methods were employed in the literature including: endophyte infection as a percentage of infected tillers (E%); ergovaline concentration in ppb ([E]); and total ergot alkaloid concentration in ppb ([TEA]). Three types of weather variables were used to explain climate differences: base measurements (temperature, humidity, and relative humidity), climate indices (heat index and temperature-heat index), and a novel parameter accounting for the average infection- level change based on date and duration of study (AEL). Models for ADG were derived in a 2x3x3 factorial approach using backwards stepwise elimination multiple regression to determine significant parameters. For each 100 ppb increase in [E] and [TEA], ADG decreased by 39 and 33 g/d, in both the CSD and MSD. For each 10% increase in E%, ADG decreased by 39 and 47 g/d in the CSD and MSD. All endophyte infection level relationships were significant (P<0.05) but no other significant variables were identified. Visual data inspection suggested an infection threshold (IT) was needed because intermediate measurements of infection-level were scarce. Significant ITs were identified at 60 ppb [E] and 11 E%, with no IT identified for [TEA] due to insufficient sample size. The ADG was 40 and 49% higher below the IT, for [E] and E% in the CSD. The use of an IT resulted in significant effects of climate variables in all three categories (P<0.05). Infection levels above the IT combined with increasing temperature, temperature-humidity index, or AEL resulted in ADG decreases greater than would be expected from infection level alone. The results of this meta-analysis show a significant negative relationship between endophyte-infection level and ADG in growing cattle. An IT for measuring the endophyte infection level increased accuracy of models and shows an additional negative relationship between various climate variables and ADG. [ABSTRACT FROM AUTHOR]