194 Grassland Science in Europe, Vol. 21 – The multiple roles of grassland in the European bioeconomy
The potential of α-linolenic acid to predict herbage quality
Bracher A. and Mosimann E.
Agroscope, Institute for Livestock Sciences ILS, 1725 Posieux and 1260 Nyon, Switzerland;
annelies.bracher@agroscope.admin.ch
Abstract
The calculation of the energy value of herbage relies on the digestibility of the organic matter which in turn is estimated by regression from proximate analysis with crude protein and crude fibre or ADF being the most important predictors. It has been shown in pure swards that particularly α-linolenic acid evolves with the phenological stage which conveys a predictive potential to this fatty acid. That assumption was tested for mixed swards. Fatty acid profiles were analysed in 76 herbage samples from 14 dairy farms situated in contrasting climatic zones in Switzerland to document changes in the nutrient and fatty acid contents over the spring growth period. Fibre content increased from 120 to 300 g kg-1 DM and protein content decreased from 220 to below 100 g kg-1 DM. The α-linolenic acid proved to be the most dominant fatty acid accounting on average for 50% in the extracted fat. High correlations were found between α-linolenic acid (g g-1 extracted fat) and crude fibre content (g kg-1 DM) (r=-0.71), ADF (r=- 0.69), crude protein (r=0.79) and extracted fat (r=0.79). These relations confirm that α-linolenic acid evolves with the nutritive value and has a predictive potential for mixed swards.
Keywords: α-linolenic acid, proximate analysis, herbage quality, spring growth
Introduction
The calculation of the energy value of herbage relies on the digestibility of the organic matter (dOM) which is either determined in vivo, in vitro or estimated by regression from proximate analysis. The latter approach is widely used with crude protein (CP) and crude fibre (CF) or ADF being the most important predictors. The regressions applied in Switzerland for mixed swards differentiate between 7 botanical types and include correction factors for cycle number and growth stage. The prediction accuracy is limited in cases of unknown botanical composition and growth stage. In addition to the classical feed nutrients, more specific predictors relating to functional plant traits that vary with plant maturity have the potential to improve the prediction accuracy in a generalized manner. In this context it has been shown that particularly α-linolenic acid (C18:3n-3) evolves with the phenological stages and varies with plant species and plant parts (Boufaïed et al., 2003; Wyss and Collomb, 2010; Wyss, 2012). C18:3n-3 is present in high proportions in the thylakoid membranes of chloroplasts (Hawke, 1973) where it regulates the membrane fluidity of the photosynthetic tissues, i.e. the leaves. Thus, variations in C18:3n-3 reflect variations in leaf/stem ratio, leaf age and temperature and relate C18:3n-3 to herbage quality. In our study, that assumption was tested for mixed swards. This perspective completes most studies investigating first of all links between fatty acids in herbage and milk (Khiaosa-ard et al., 2015).
Materials and methods
During the spring period of 2014, herbage growth and quality were monitored from mid-march to mid- June in experimental plots of 14 farms situated between 450 and 1200 m a.s.l. in contrasting climate zones of western Switzerland. Experimental plots represented the typical wide range of meadow and pasture types used in dairy cow feeding ranging from grass-legume leys to evolved old leys and permanent grassland with varying productivity level and botanical composition. Proximate analysis was carried out throughout the spring period while amino acid and fatty acid profiles were analysed in a reduced sample set at intervals corresponding to early, mid, and late growth stages. Fatty acid profiles were obtained by an improved fat extraction method based on gas chromatography (GC) with in situ transesterification and
Grassland Science in Europe, Vol. 21 – The multiple roles of grassland in the European bioeconomy 195 solid-phase-extraction (Ampuero Kragten et al., 2014). FatGC refers to the sum of fatty acids calculated as triglycerides. Results of the most important fatty acids are given in absolute and relative quantities.
Data analysis emphasized on evolution patterns and correlations.
Results and discussion
Phenological changes occurring during spring growth are associated with decreasing protein and fat content and increasing cell wall constituents (CF, ADF, NDF) which reduce the digestibility of the organic matter (dOM) as observed in the present study (Table 1). Fibre content increased from 120 to 300 g kg-1 DM and protein content decreased from 220 to below 100 g kg-1 DM. As expected, α-linolenic acid proved to be the most dominant and most variable fatty acid, on average accounting for 50% of the extracted fat (Table 1). Figure 1 illustrates the transient reduction in absolute and relative C18:3n-3 content over the spring growth period dropping from 25 to 4 g kg-1 DM (Figure 1A) and from 60 to 40%
of extracted fat (Figure 1B), respectively. This declining pattern was observed across all sites giving strong evidence for the relationship that exists between C18:3n3 and plant phenology (maturity) and herbage quality at harvest. The level of C18:3n-3 content gives an additional indication for sward use intensity.
The two extensively used sites ‘Moudon ext’ and ‘Puidoux ext’, both having high proportion of herbs and non-ryegrass gramineae, figure at the low end of the range. Figure 1 also reveals the altitudinal gradient with a temporal shift (sites >900 m: St-George, La St-George and La Frêtaz).
Relating C18:3n-3 content and proportion with nutrient contents of herbage resulted in high positive correlations to CP and fat content and negative correlations to fibre fractions and dOM (Table 1) which confirms previous results of investigations with grass and maize silages (Khan et al., 2012). Another interesting outcome of the present study is the high positive correlation between C18:3n-3 and the Table 1. Proximate analysis, calculated digestibility, fatty acids and correlations between C18:3n-3 and nutrients of herbage from 14 sites sampled over the spring period.
Nutrients Overall Early growth stage Late growth stage Correlation C18:3n-3 g kg-1 DM
Correlation C18:3n-3 g g-1 fat (GC)
Complete data set (76) avg avg avg r r
Ash g kg-1 DM 90.6 100.7 73.8
CP g kg-1 DM 168.1 198.9 115.8 0.933 0.786
Cfat g kg-1 DM 35.8 41.0 26.7 0.672
CF g kg-1 DM 190.0 151.3 259.8 -0.880 -0.709
ADF g kg-1 DM 220.6 179.9 294.1 -0.859 -0.693
NDF g kg-1 DM 385.2 327.8 487.5 -0.823 -0.659
dOM % 77.9 80.7 71.4 0.774 0.647
Reduced data set (41)
FatGC g kg-1 DM 29.4 37.3 21.6 0.786
C16:0 g kg-1 DM 4.11 4.90 3.27
C18:2n-6 g kg-1 DM 4.97 5.85 4.02
C18:3n-3 g kg-1 DM 14.78 19.82 9.98
C16:0 g g-1 fatGC 0.14 0.13 0.15
C18:2n-6 g g-1 fatGC 0.17 0.16 0.19
C18:3n-3 g g-1 fatGC 0.49 0.53 0.46
Tyr g 100 g-1 CP 3.19 3.43 2.94 0.857 0.741
196 Grassland Science in Europe, Vol. 21 – The multiple roles of grassland in the European bioeconomy amino acid tyrosine (Table 1). Given the function of tyrosine as an electron carrier in the photosystem II, that finding is not surprising after all.
Conclusions
As underlined by our results, C18:3n-3 content and proportion decline with advancing maturity of mixed swards and correlate with protein content, cell wall components and calculated digestibility.
The involvement of C18:3n-3 in the photosynthetic process of the highly digestible leaves conveys to α-linolenic acid the potential to improve the accuracy of prediction equations that estimate the nutritive value of herbage. It can be assumed that the same holds true for tyrosine. Next steps should analyse the behaviour of C18:3n-3 in herbage of regrowth cycles and evaluate in a series of digestibility trials the relative contribution of α-linolenic acid to the prediction accuracy of multiple regressions. In case of a relevant contribution, a NIR calibration of the expensive fatty acid analysis may be worth the effort.
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Figure 1. Evolution of C18:3n-3 in herbage from 14 sites over the spring period: (A) in g kg-1 DM; (B) g g-1 fat (GC).
