Relationships between long-term fertilization management and forage nutritive value in grasslands

Abstract

Effects of fertilization on grassland have been widely investigated in terms of botanical composition, forage yield and quality, and their agricultural and environmental consequences. However, there are no published studies of the contribution of fertilization to variability of forage quality involving systematic investigation of this effect. Therefore, our objectives were to: (i) evaluate the impact of long-term fertilization treatments on Arrhenatherion elatioris-type meadow vegetation, (ii) investigate effects of interaction among year, cut and fertilization on forage quality in association with forage yield and functional groups proportion with quantification of fertilization effect, and (iii) compare the predictive ability of visually estimated functional groups coverage (FGC) vs. weight proportion ratio of functional groups (FGW) for variability in forage quality in the first cut. Six mineral fertilization treatments (control, N0P40K100, and P40K100 in combination with 50, 100, 150 and 200 kg N ha−1) were evaluated over a three-year period (2014–2016) under a three-cut regime. Intensive NPK fertilization doubled forage yield and increased the grass proportion with a corresponding decrease of forbs and legumes. Grass proportion in harvested forage was positively correlated with fibre content in the first cut and with organic matter digestibility and net-energy for lactation in subsequent cuts. In all cuts there was a consistent correlation in ash and crude protein content with legumes and forbs. Fertilization had less influence on forage quality, compared with inter-year effects or seasonal variability. For each cut, fertilization contributed around 20% of the variability in forage quality, where half of its contribution was realized through increased yield and changes in proportion of functional groups. The explanation power of functional groups in the first cut was slightly higher for the FGC method than the FGW proportion method, which suggests both methods are at least as effective in relation to grassland nutritive value. In summary, if fertilization results in only small changes in grassland yield or botanical composition, its effect on forage quality is greatly reduced. Plant developmental stage or nutritive value of particular species are potential explanations for the rest of the variability.