Meta-analysis of endophyte-infected tall fescue effects on cattle growth rates

Abstract

The objective of this study was to quantitatively summarize literature reporting endophyte-infected (Neotyphodium coenophialum) tall fescue (Festuca arundinacea) effects on cattle ADG. This meta-analysis evaluated endophyte infection level, climate, and forage yield using a literature dataset of 138 treatments from 20 articles. Three infection level measurements were tested: endophyte infection as a percentage of infected tillers (E%); ergovaline concentration in ppb ([E]); and total ergot alkaloid concentration ([TEA]). Three types of climate variables were used: base values (temperature, humidity, and relative humidity), climate indices (heat index and temperature-heat index [THI]), and novel climate variables accounting for duration of climate effects. Mixed effect models, weighted by 1/SEM, including a random effect of study were built for each factorial combination of measurement method and climate variable group. Because many studies were missing SEM, two datasets were used: one containing only data with SEM reported and one that also included missing-SEM data. For the complete-SEM dataset (CSD), models were weighted by 1/SEM. In the missing-SEM dataset (MSD) the mean reported 1/SEM was assigned as the weight for all missing SEM treatments. Although 18 initial models were created (2 × 3 × 3 factorial approach), the backward stepwise derivation resulted in models that included only endophyte infection level, suggesting a negative relationship between infection level and ADG. The CSD models predicted ADG to decrease 39 and 33 g/d with each increase of 100 ppb of [TEA] and [E], and by 39 g/d for each increase of 10% E%. In the MSD dataset, predicted ADG decreased by 39 and 33 g/d with each increase of 100 ppb of [TEA] and [E], and by 47 g/d for each increase of 10% E%. All relationships reported had P < 0.05. After visual inspection of the data, piecewise regression was used to identify an infection threshold (IT) of 60 ppb [E] and 11 E%, where the effect of infection level was constant on either side of the IT. The ADG was 40% and 49% greater for infection levels below the IT for [E] and E%, respectively. Across THI values in the analysis, ADG decreases ranged from 11.2% to 45.0% for cattle grazing endophyte-infected tall fescue compared to non-ergot alkaloid endophyte infected tall fescue. Pasture E%, [E], and [TEA] have a negative relationship with ADG in growing cattle, and increasing temperature decreases ADG when infection level is greater than the IT.

Figure 1.

Boxplot of relationship between THI and ADG in all Georgia data. Dataset was comprised of all studies done in Georgia within the dataset to determine a relationship between THI and ADG not observed in the models derived for ADG.

Figure 2.

(A,B) AICc values associated with pairwise regression models built at an infection threshold (IT). The optimal IT was selected as the first point at which the AICc value reached its minimum. (C,D) Plots of infection measurement method, illustrating the sparsity of intermediate level infection data.

Figure 3.

(A) Cumulative BW gain over 1 yr by maximum infection level of a given pasture. Infection level was corrected using Ju et. al (2006) and temperature-humidity index (THI) data for each month was derived from the weather data collected for each study. ADG was calculated using equation 7 with inputs for THI and infection level based on 60 ppb [E] threshold. (B) Cumulative BW gain over a theoretical fall growing season. (C) Cumulative BW gain over a theoretical spring growing season.