Estimated pre-caecal digestible crude protein for horses in forage from species-rich grasslands

Abstract

The methodological procedure of the new German protein evaluation system for horses estimates protein supply at the duodenum on the level of pre-caecal digestible crude protein (pcdCP). Limited information is available on the pcdCP content of forage from species-rich grasslands, commonly used as forage for horses. A characteristic of forage from species-rich grasslands is elevated contents of specific phenolic compounds. The contents and variation of pcdCP were determined in forage samples from three long-term fertilisation field experiments located in the Swiss mountains. The samples were characterised by a large variation in the botanical composition (16–36 species) and were hence analysed by contrasting crude nutrient composition and contents of total extractable phenols (TEP), total tannins (TT) and condensed tannins (CT). The contents of neutral detergent soluble CP (NDSCP) and pcdCP were calculated and the potential influences of varying TEP contents on the pcdCP analysed. Forage (N = 64) crude protein (CP) contents ranged from 67.3–144 g/kg dry matter (DM). On average, the contents of pcdCP varied from 35.5–70.7 g/kg DM and from 260 to 634 g/kg CP. The contents of pcdCP, NDSCP and neutral detergent insoluble crude protein (NDICP) followed an unspecific pattern across all investigated swards when related to either fertilisation or growth cycle. There was a positive correlation (r = 0.4030, P < 0.05) between pcdCP (g/kg DM) and CP (g/kg DM) and between forage NDICP contents and CP (r = 0.7090, P < 0.05). Negative correlations were determined between NDICP and TEP (r = -0.2490, P < 0.05) and TT (r = -0.3550, P < 0.05). Forage NDICP and CT were negatively correlated in both forages from the generative (rg = -0.4010) or vegetative harvest (rv = -0.5570) or as a sum of both growth cycles (r = -0.3710) (P < 0.05). Therefore, the new German protein evaluation system for horses requires further evaluation in vivo to examine whether the contents of pcdCP are predicted adequately in forage with elevated contents of phenolic compounds.