The effect of harvest time of forage on carbohydrate digestion in horses quantified by in vitro and mobile bag techniques

Abstract

Carbohydrates in forages constitute an important part of the feed ration for all horses. The aim of the present study was to investigate the effect of harvest time on carbohydrate composition and digestion of various grass species. The experiment was divided into three parts 1) characterization of the chemical composition of experimental feeds (6 grass species: meadow fescue (MF), cocksfoot (CF), perennial ryegrass (PR), smooth bromegrass (SB), tall fescue (TF) and timothy (TI), and 3 harvest times: early, medium and late first cut), 2) measurements of the in vitro digestion of selected experimental feeds (the 6 grass species, and 2 harvest times (early and late)) measured by in vitro gas production, and 3) in vivo digestion of selected experimental feeds (2 grass species: CF and PR, 2 harvest times (early and late)) measured by the mobile bag technique using caecum cannulated horses. An experimental field was established with plots containing each of the grass species in three replicate blocks. Grass samples were cut between 1200 and 1400 h at 4 th of June (early first cut), 17 th of June (medium first cut) and 1 st of July (late first cut) and analysed for crude protein (CP), neutral detergent fibre with heat stable amylase and free of residual ash (aNDFom) and water-soluble carbohydrates (WSC). The in vitro fermentation was investigated using the ANKOM RF gas production technique, where feeds were incubated for 48 h using horse caecal fluid as an inoculum. Gas production was modelled, and maximum gas production (MGP) was used to evaluate the potential digestibility of the feeds. Based on the chemical analyses and the in vitro experiment, early and late harvested CF and PR were selected for the in vivo experiment, which was conducted as a randomized 4x4 Latin square design including four periods, four horses and four feeds. In general, the CP content decreased whereas the aNDFom content increased as the grasses matured. The content of WSC increased in SB and TI, but decreased in CF, and fructans increased in SB, TI, PR, and TF as they matured. The in vitro MGP showed a clearer difference between harvest times than between grass species. Harvest time had larger effect on digestibility than grass species, and a high pre-caecal disappearance of the WSC fraction was measured by the mobile bag technique. Cocksfoot was identified as a grass species with potentially low digestibility and low WSC content and could potentially be used more for horses.

Comparison of forages’ digestion levels for different in vitro digestion techniques in horses

“…Similar to the present study results, Stang et al. ( 2022 ) reported that the in vitro T‐DMD value for six grass species (meadow fescue, cocksfoot, perennial ryegrass, smooth bromegrass, tall fescue and timothy for three harvest times: early, medium and late first cut) was between 60% and 64%. The same researcher reported that the in vitro T‐DMD value reduced from about 70% to 54% with a decrease in the level of WSC (glucose and fructose), an increase in the NDF and a decrease in the WSC (glucose and fructose) levels in forage during harvest time.…”

“…The indigestible structural carbohydrates which reach the colon and the feedstuffs after being pre‐digested/processed for fermentation and wait for fermentation are significantly different for forage fermentation. In the present study, it was understood that the methods using the rumen content (Menke et al., 1979 ; Theodorou et al., 1994 ), which were converted to a modified horse digestion method with inoculum including horse faeces (Gandarillas et al., 2021 ; Kujawa et al., 2020 ; Kholif et al., 2015 ; Stang et al., 2022 ), may be inaccurate for forage digestion. According to the present study results, it is wrong to think that the digestion of forage materials is completed only in colon fermentation.…”

“…The indigestible structural carbohydrates which reach the colon and the feedstuffs after being pre-digested/processed for fermentation and wait for fermentation are significantly different for forage fermentation. In the present study, it was understood that the methods using the rumen content (Menke et al, 1979;Theodorou et al, 1994), which were converted to a modified horse digestion method with inoculum including horse faeces (Gandarillas et al, 2021;Kujawa et al, 2020;Kholif et al, 2015;Stang et al, 2022), may be inaccurate for forage diges- for alfalfa herbage, which had the highest CP and NSC values among the forages studied, were 40%-50% higher than the in vitro SDT and in vitro MDT values. However, the lack of a high level of difference in wheat straw compared to alfalfa herbage was thought to be due to the fact that the content of this forage largely consists of NDF.…”

“…The in vitro digestion levels of the analysed forage differed in methods and among themselves. In general, ranking the forages as alfalfa herbage > Italian ryegrass > meadow hay = wheat straw, according to the in vitro T‐NDFD levels in the three methods, was thought to be related to the water‐soluble carbohydrates (WSC) and lignocellulose complex contents of these forages (Kara & Baytok, 2017 ; Murray et al., 2006 ; Stang et al., 2022 ). The starch content in the hay or straw of legumes and grasses, which is widely used in horses, is between 1% and 3% in DM (NRC, 2007 ).…”

“…Stang et al. ( 2022 ) stated that the in vitro T‐DMD value for six grass species was between 60% and 64%. Therefore, Ersahince and Kara ( 2017 ), using the in vitro SDT, reported that the in vitro T‐DMD of Jerusalem artichoke in different phenological periods had approximately 43%–58% of in vitro T‐DMD and was 48%–65% of the in vitro T‐OMD.…”