In vitro fermentation and in situ rumen degradation kinetics of summer forage brassica plants Juan P. Keim A,F , Jaime Cabanilla B,E , Oscar A. Balocchi A , Rubén G. Pulido C and Annick Bertrand D A Animal Production Institute, Faculty of Agricultural Sciences, Universidad Austral de Chile, PO Box 567, Valdivia, Chile. B Graduate School, Faculty of Agricultural Sciences, Universidad Austral de Chile, PO Box 567, Valdivia, Chile. C Animal Science Institute, Faculty of Veterinary Sciences, Universidad Austral de Chile, PO Box 567, Valdivia, Chile. D Soils and Crops Research and Development Centre, Agriculture and Agri-Food Canada, Québec City, QC, G1V 2J3, Canada. E Present address: Department of Animal Production, Faculty of Veterinary Medicine and Zootechnics, Universidad Agraria del Ecuador. F Corresponding author. Email: juan.keim@uach.cl Abstract. The aim of the present study was to assess and compare the nutrient concentration, the in vitro fermentation and the in situ rumen degradation characteristics of Brassica rapa ssp. rapa L. (turnips) and Brassica napus ssp. biennis L. (forage rape). Five varieties of each species were established in three field replicates and were organised in a randomised complete-block nested design. All varieties were harvested and further analysed for chemical composition, in vitro gas-production kinetics, volatile fatty acid (VFA) production and in situ degradation kinetics of dry matter (DM) and crude protein. Turnips showed higher ash, total sugars, raffinose, sucrose, glucose and fructose concentrations (P < 0.001) than did forage rape. Turnip varieties differed in their sucrose, glucose, fructose and total soluble sugar concentration (P < 0.001), whereas rape varieties differed in their neutral detergent fibre concentration (P= 0.004) and digestible organic matter on a DM basis (P < 0.01). Regarding DM-degradation parameters, turnips had a higher soluble fraction ‘a’ (P < 0.01) and a lower insoluble, but potentially degradable fraction ‘b’ (P < 0.01) than did rape, but the fractional degradation rate ‘c’ (0.18/h) was similar to that of rape. Rates of gas production were slightly higher (P = 0.018) for turnip than for rape. No effects for brassica species nor for varieties within species were detected (P > 0.05) for total in vitro VFA production, as well as for the relative proportions of acetate, propionate, butyrate, branch chained VFA and the actetate : propionate ratio. Our study showed that most of the differences that were observed in terms of chemical composition and degradation kinetics did not result in differences in in vitro fermentation products. Additional keywords: forage rape, in situ degradation kinetics, summer turnip, volatile fatty acids. Received 3 August 2017, accepted 4 July 2018, published online 27 August 2018 Introduction Brassica crops are used to supply feed in times of seasonal shortage, or when pasture quality is low, particularly during winter and summer (Barry 2013). They can produce high levels of DM in a short period of time, are highly digestible (Valentine and Kemp 2007) and are characterised by their low neutral detergent fibre (NDF) content (Westwood and Mulcock 2012). It has been observed that brassica forages can improve animal performance (Moate et al. 1999) and reduce the environmental impact due to a lower methane production (Sun et al. 2012, 2016) than from ruminants grazing ryegrass pasture-based diets. Aditionally, summer brassicas contain high concentrations of water-soluble carbohydrates (WSC) and pectins. However, there are differences in the nutrient content of the different brassica species (White et al. 1999) and their nutrient content varies from year to year, among locations (Jacobs et al. 2002) and among varieties within species (Westwood and Mulcock 2012). However, the high nutrient content of brassicas is not necessarily linked with high animal performance. Barry (2013) suggested that the poor animal-performance response to highly digestible brassicas might be due to detrimental effects of anti-nutritional factors (secondary metabolites such as S-methyl-cysteine sulphoxide, glucosinolates and nitrates) that are present or in higher concentrations than in other forages. Most of the nutritional characterisation of brassicas focused mainly on their digestibility and nutrient (crude protein (CP), CSIRO PUBLISHING Animal Production Science https://doi.org/10.1071/AN17534 Journal compilation Ó CSIRO 2018 www.publish.csiro.au/journals/an