Grazed bite item diversity of beef cows grazing semi-natural grassland in relation to herbage nutritive value and the actually available diversity of bite items.

Abstract

Heterogeneous extensive grassland offers herbivores a variation of potential bite items (BI) from which they can select. But there is a lack of knowledge on the relationship between the actually available (BI_a) and the grazed (BI_g) BI diversity albeit this information is essential to understand grass sward composition. We assessed the BI_a in the sward and the BI_g to evaluate the BI diversity under two grazing intensities in two seasons in the replicated long-term grazing experiment Forbioben. The BI_a diversity was measured pre-grazing using a modified sward stick (200 measurement points per 1-ha paddock) whereby at each point the botanical composition, phenological stage, color and height were recorded and consequently structured into BI_a. Herbage samples were taken for each BI and analyzed via near-infrared-reflectance spectroscopy for the concentration of crude protein (CP), acid-detergent fiber in the organic matter (ADF), in vitro digestibility of the organic matter (ivdOM) and metabolizable energy (ME) and labeled as grazed and non-grazed. For the BI_g diversity each cow was observed in the morning and afternoon four times, 2 min each on 1 d each season using an app for counting the bites and following video analysis to determine the composition of the BI_g. A reduction in growth rates during the hot and dry summer might have caused the decline of BI_a diversity of 25% under moderate and 22% under lenient grazing towards autumn (p < 0.0001). This also influenced the BI_g as in spring 50% and 53% of the BI_a diversity were grazed under moderate and lenient grazing, respectively. In autumn only 36% and 28% were grazed, respectively corresponding well to the generally lower herbage nutritive value of BI in autumn as the offered grassland resource changed over time and provided greater ivdOM and ME in spring compared to autumn (p < 0.0001). This shows a greater selectivity from the limited choice in autumn. All cows were able to maintain a good quality diet irrespective of grazing intensity which is underlined by the fact that the digestibility of ingested herbage was the same among the grazing intensities but greater in spring than in autumn.