Evaluation of nutritive value, in vitro fermentation, and antimethanogenic potential of native South African macroalgae species

Abstract

Macroalgae are, as of yet, an untapped source of nutrients as well as a potential means by which to improve the sustainability of ruminant production. In this study the chemical composition and in vitro digestibility of four South African macroalgae, Gelidium pristoides (Rhodophyta), Porphyra sp. (Rhodophyta), Ulva sp. (Chlorophyta), and Ecklonia maxima (Ochrophyta), which were whole, consisting of both the blade and stipe, were determined. The analyses were repeated for four E. maxima samples which were comprised of the blade, the stipe, the whole macroalgae, and an industry by-product. The effect of including the macroalgae samples in two different rations, a total mixed ration (TMR) and Rhodes grass, at inclusion rates of 5%, 10%, 15%, and 20% on a dry matter (DM) basis on in vitro digestibility was determined. The effect of including the macroalgae spamples to the TMR diet at the same inclusion rates on in vitro total gas and methane production was determined at 3, 6, 9, 12, 24, and 48 hours (hrs) of incubation. The E. maxima samples were also assessed for in vitro microbial protein synthesis. The Rhodophyta and Chlorophyta contained the highest concentrations of crude protein (CP), of which Porphyra sp. had the highest concentration at 191.82g Kg-1 DM. Porphyra sp. also had a significantly (P<0.05) lower concentration of total minerals, 180.49g Kg-1 DM, compared to the other species, 263.74-360.45g Kg-1 DM. The most limiting minerals for including macroalgae into animal feeds in this study were sulphur (S) and potassium (K). The latter was the most limiting for Ulva sp., limiting inclusion to 5.68%, the lowest maximum inclusion rate of all species assessed in this study. Gelidium pristoides had the lowest organic matter (OM) digestibility, 39.95%, significantly (P<0.05) lower by at least 44% compared to any other species. The E. maxima blade and stipe samples had significantly (P<0.05) lower digestibilities compared to the whole and by-product samples by approximately 30%. Only G. pristoides significantly (P<0.05) affected the OM digestibility when included with either the TMR or Rhodes grass compared to either control. Gelidium pristoides reduced the OM digestibility of the TMR diet at inclusion rates of 15% and 20% compared to the TMR. The E. maxima blade showed a trend (0.10<P≤ 0.05) to reduce the digestibility of the diet when included with the TMR at an inclusion rate of 15% compared to the control. All of the samples reduced the total gas production. There was a strong negative relationship (R2>0.70) between inclusion rate and total gas production of the Rhodophyta and Chlorophyta. Ulva sp. significantly (P<0.05) reduced in vitro methane production at a 20% inclusion rate by 25%. With the exception of G. pristoides, the South African macroalgae assessed in this study could hereby potentially serve as valuable sources of nutrients for ruminants. Of the macroalgae species assessed in this study only Ulva sp. significantly (P<0.05) reduced methane production, however due to the high concentrations at which an effect was observed either extraction of active compounds or ash removal would be required to prevent mineral toxicity. Identification of active compounds in Ulva sp. is necessary for a better understanding of its antimethanogenic effects.