Proposal and validation of new indexes to evaluate maize silage fermentative quality in lab-scale ensiling conditions through the use of a receiver operating characteristic analysis

Abstract

In the context of dairy cow feeding, it is increasingly important to know the quality of the maize silage used in the ration and therefore, it appears to be crucial optimizing the techniques necessary to assess it. The aim of this study was to evaluate whether the Flieg-Zimmer's score (FZS), could properly estimate the quality of fermentations of maize silage made in a lab-scale ensiling system, and to calculate and validate new quality indexes suitable for lab-scale fermentations. The experimental dataset was obtained by analysing through near-infrared spectroscopy 522 samples of whole maize crop ensiled immediately after the harvest, using the vacuum-packing technique. The five (I1-I5) new indexes were calculated on the basis of seven parameters chosen among pH, lactic, acetic, propionic and butyric acids, ethanol, mannitol and ammonia. All the indexes were tested for normality with the Shapiro-Wilk test. In order to define the accuracy with which the new indexes ranked the maize silage on the basis of its fermentation quality, a ROC analysis was performed, using the FZS as gold standard test and dichotomizing the FZS in two levels according to a cutoff (FZS < 2 80, non-excellent vs. FZS ≥ 80, excellent). Accuracy was determined through the value of the area under the curve (AUC). Finally, a one-way ANOVA model was used to compare the quality of maize silage with low (< 320 g/kg), medium (320-360 g/kg) and high (> 360 g/kg) dry matter (DM). In the lab-scale silages the new indexes were normally distributed, whereas the FZS was not. The new indexes showed values of AUC ranging between 0.78 and 0.89, with the I5 index showing the best combination of sensitivity (0.87) and specificity (0.77) in discriminating between good and poor quality silage. The cutoff of the new indexes ranged between 45.5 and 57.4 points. The lab-scale silages were all excellent, no matter the category of DM. However, while the FZS did not differ among the 3 categories (mean FZS = 98.7), all the other indexes were significantly higher in silages with low DM (P < 0.001). Silages with low DM had the highest concentrations of lactic acid (56.4 g/kg DM, P < 0.001), ammonia (61.4 g/kg DM, P < 0.001) and butyric acid (0.62 g/kg DM, P < 0.001) as well. Data confirmed that the new proposed indexes are promising in describing the fermentation quality of maize silage in both field and lab-scale conditions.

Effect of maturity stage at harvest on the ensilability of maize hybrids in the early and late FAO classes, grown in areas differing in yield potential

“…In this study, all the hybrids showed an FQI value higher than 48.2, which, according to Andrighetto et al (), represents the cut‐off between excellent and not excellent maize silage from a fermentative point of view, confirming the goodness of the ensiling method used. In this regard, the early hybrids have generally shown a slightly lower production of ammonia, acetic and propionic acid, as also reported by Gerlach et al (), although both compounds have a concentration well below the suggested critical limit (Kung et al, ).…”

“…Each whole maize sample was immediately subjected to determination of DM and WSC content by means of a portable near‐infrared (NIR) system (poliSPEC NIR ; ITPhotonics), using the calibration curve reported in Table . After the analyses, for each of the 522 whole maize samples, two subsamples ( n = 1,044) were immediately ensiled in vacuum‐packed bags (Orved 2633040, Orved SpA), as described by Andrighetto et al ().…”

“…All fermentation products were used to assess the quality of fermentation, as reported by Kung et al (). In addition, the concentrations of lactate, ammonia, ethanol, acetate and butyrate were used to calculate an index of maize silage fermentation quality (FQI), described by Andrighetto et al () as index I5. Briefly, FQI is an index with values ranging from 1 to 100 and is calculated on the basis of the concentrations of lactic acid, ammonia, ethanol, butyric and acetic acids measured in each sample.…”

“…These parameters are listed here in order of importance: with one being the most important, to the parameter with the least weight within the index. The FQI value increases when the lactic acid level increases to a maximum of 60 g/kg DM, whereas it decreases when the other parameters increase over a specific threshold, which is considered physiological (Andrighetto et al, ).…”

“…Among these compounds, acetic acid, at moderate concentrations, seems to be beneficial, as it improves silage aerobic stability owing to its antifungal activity (Nishino, Wada, Yoshida, & Shiota, 2004). The analysis of these compounds, along with the measure of pH, still represents the main way of assessing the fermentation quality of maize silage (Andrighetto et al, 2018;Kung et al, 2018).…”