The nutritive value of seaweeds for ruminants varies widely. It depends on the species, on the composition of the algae (protein, minerals, polysaccharides, tannins) and also on the adaptation of the animal to this particular feed.
Digestibility, degradability and energy values
There are limited data on in vivo and in vitro digestibility, degradability and energy values of seaweeds for ruminants. The in vivo energy digestiblity of Ulva lactuca measured with young rams was 60% (Arieli et al., 1993). The in vitro OM digestibility of brown and red seaweeds, when measured with rumen fluid obtained from seaweed-fed sheep, was found to be very high for brown algae Laminaria digitata, Saccharina latissima and Alaria esculenta species (94, 97 and 81% respectively), and for the red algae Palmaria palmata (81%). However, it was low for other brown seaweeds such as Ascophyllum nodosum, Fucus serratus and Fucus vesiculosus (33, 15 and 26% respectively) (Greenwood et al., 1983a). In a comparison of the brown algae Macrocystis pyrifera and Sargassum spp., the in situ DM degradability of Macrocystis pyrifera was found to be low (50%) but higher than that of Sargassum (29%), due to a better composition of the former. Crude protein was found to be rumen-undegradable in situ, but to have a high in vitro trypsic digestibility, which could make brown algae a good source of protein for ruminants in spite of their low protein content (Gojon-Baez et al., 1998). Degradability values for the green alga Ulva lactuca were also relatively low (54% and 41% for OM and protein degradability respectively) (Arieli et al., 1993). However, the high mineral content of seaweeds limits their gross energy content, and, therefore, their digestible, metabolizable and net energy value: even a protein-rich (25% DM) alga like Ulva lactuca was found to have a digestible energy value of 9.1 MJ/kg DM, similar to that of a low-quality hay (Arieli et al., 1993).
Adaptation of the rumen flora
Seaweeds are typically low in cellulose (about 4%) and rich in specific polysaccharides (alginate, laminarin and fucoidan) and in mannitol. Sheep that are usually fed with seaweeds, such as the Orkney sheep, have a rumen flora that does not include phycomycete fungi or cellulolytic bacteria, but cilia species are present, including Dasytricha ruminantium and Entodinium spp., together with lactate-utilizing bacteria, such as Streptococcus bovis, Selenomonas ruminantium and Butyrivibrio fibrisolvens (Orpin et al., 1985; Greenwood et al., 1983a). As a consequence, the in vitro OM digestibility of seaweeds measured with rumen fluid obtained from grass-fed sheep (Orkney or another breed) was generally lower, particularly for Laminaria digitata and Saccharina latissima (Greenwood et al., 1983a).
Kelp meal and extracts from Ascophyllum nodosum have been used to feed ruminants for decades. For example kelp meal is a common feed additive in dairy farms in the USA, notably in organically-managed farms. It is usually fed in low concentrations, typically less than 5% of the DM. As noted in Nutritional attributes above, kelp has a low protein and a high mineral contents, and, therefore, a low energy value. Also, kelp meal is not very palatable: kelp meal fed to calves at up to 60 g/d was found to depress DM intake, and it was suggested that the presence of free glutamic acid, which enhances palatability in human food, may have had an inverse effect in calves (Erickson et al., 2012).
Kelp meal is, therefore, used in ruminants as an additive rather than in order to provide protein and energy. Early research found that it was a valuable source of minerals and could correct mineral-deficient diets for milk production, for example by providing copper (Dunlop, 1953). More recently, kelp meal and kelp extracts has been shown to enhance immunity and antioxidant properties in cattle, sheep and goats (Allen et al., 2001a; Allen et al., 2001b; Fike et al., 2001). Kelp meal increased the activity of superoxide dismutase, which is an antioxidant for ruminants raised on forage or pasture (Fike et al., 2001). In lambs and kids, kelp meal enhanced immunity, improved the overall health of the animals and protected them against prolonged heat or transport-induced oxidative stress (Kannan et al., 2007a; Saker et al., 2004). This latter effect may be due to the presence in kelp of sodium chloride and potassium gluconate (Archer et al., 2008). However, beneficial effects have not always been observed with transport-stressed animals and did not last more than 3 or 4 days in cattle (Pompeu et al., 2011; Williams et al., 2009; Galipalli et al., 2004b; Carter et al., 2000). No alleviating effects on body weight losses or blood metabolite levels were shown in goats (Kannan et al., 2007b).
A kelp extract containing large concentrations of phlorotannins (up to 500 µg/ml) linearly reduced in vitro fermentation, of both mixed forage and barley grain. Inhibition of in vitro rumen fermentation was greater with cellulolytic bacteria than with amylolytic bacteria, which suggests that the influence of kelp on animal performance depends on the diet (Wang et al., 2008). In steers, kelp meal increased the slowly degraded protein fraction and total protein degradability, and was more beneficial to forage digestibility when supplementing low-quality forage diets (Leupp et al., 2005). In feedlot cattle and lambs, kelp meal supplemented at 2% of the diet for 2 weeks at the beginning of the feedlot period maximized carcass weight and shelf life, with no detrimental effect on performance in the feedlot (Anderson et al., 2006; Tavasoli et al., 2009). In cattle and lambs, feeding kelp meal at 2% during the 2 last weeks before slaughter reduced animal shedding of Escherichia coli O157:H7 (Bach et al., 2008; Braden et al., 2004; Braden et al., 2007). In meat goats, the addition of kelp extract for 8 weeks before slaughter increased colour stability of loin/rib chops, even though there was no effect on lipid oxidation (Galipalli et al., 2004a).
Feeding kelp extract sprayed on the pasture or mixed in the diet decreased the body temperature in steers suffering from a fever caused by fescue toxicosis (due to the infection of fescue with the fungus Neotyphodium coenophialum) (Saker et al., 2001; Spiers et al., 2004). Steers that had grazed the kelp-treated pastures had higher marbling scores and a better colour stability of the steaks (Allen et al., 2001a; Montgomery et al., 2001).
Laminaria and Saccharina
The Orkney sheep in the North Ronaldsay Island feed almost exclusively on seaweeds for most of the year. Their preferred species are the brown algae Laminaria digitata, Laminaria hyperborea and Saccharina latissima (formerly known as Laminaria saccharina): these species may account for up to 90% of the diet in summer, depending on availability. They also eat other brown algae such as Alaria esculenta, Ascophyllum nodosum and Fucus species as well as red algae (Palmaria palmata) and green algae (Hansen et al., 2003). Their activity pattern depends on the tidal cycle, as the sheep eat the algae at night when the tide is low (Paterson et al., 1982). The sheep consume seaweeds in amounts high enough to sustain maintenance requirements, but they suffer from mineral overload (see Potential constraints above) (Hansen et al., 2003).
Macrocystis pyrifera was used at up to 30% of the diet as a supplement for goats without affecting in vivo and in situ digestibility, and parameters of rumen fermentation, such as pH and ammonia nitrogen (Mora-Castro et al., 2009). In situ DM digestibility was high (77 to 85%). Including Macrocystis pyrifera in the diet increased rumen pH, water intake and urinary excretion (Gojon-Baez et al., 1998; Mora-Castro et al., 2009).
Sargassum spp. were introduced at up to 30% in the diets of growing sheep and goats without depressing intake, growth performance and diet digestibility (Casas-Valdez et al., 2006; Marin et al., 2003; Marin et al., 2009). Feeding Sargassum increased water consumption, probably due to the high concentration of minerals, notably Na and K, which could make Sargassum less suitable for feeding during dry periods. Sargassum meal could be used to limit the decrease in rumen pH resulting from acidogenic diets. It also tended to decrease the concentration of volatile fatty acids (Marin et al., 2009).
Palmaria palmata has a potentially high nutritive value in ruminants (Greenwood et al., 1983b). It was observed in the 19th and early 20th century to be very palatable to sheep, goats and cattle living on the shores of Northern and Western Europe (Brittany, Scotland, Scandinavia) (Sauvageau, 1920). However, neither Palmaria palmata or other red algae have received much attention in modern scientific literature concerning their use in animal feeding.
There have been several experiments about calcareous seaweed extracts: for example Lithothamnium calcareum extract (containing 2.8% of Ca) fed at a ratio of 0.5 g/kg DM to steers receiving a diet with 70% concentrate had a buffering effect on rumen pH, but did not improve fibre digestion or modify rumen fermentation (Montanez-Valdez et al., 2012).
In dairy cows, a by-product of agar production from Phyllophora added at the rate of 100 g/d to a diet deficient in copper, zinc and cobalt increased milk yield by 4.42% and milk fat content by 0.24% (Tolokonnikov et al., 1992).
Information on green algae for ruminants is limited. Sea lettuce (Ulva lactuca) was fed to male lambs at up to 20% (DM basis) in a vetch hay/concentrate based diet without hampering the palatability of the diet. It had a low protein degradability (40%) but also a moderate energy digestibility (60%). Ulva was considered to be comparable to a medium to low quality forage and suitable for use with feeds that have a high energy/low protein content such as cereal grains (Arieli et al., 1993).
In Tunisia, air-dried Chaetomorpha linum included at 20% in the diet of growing lambs (partially replacing barley) had a slightly depressing effect on growth and feed conversion ratio, possibly due to the higher ash content (Ktita et al., 2010). In the United Arab Emirates, an unspecified green seaweed (possibly Enteromorpha), harvested from fish ponds and dried, was included in the diet of growing lambs at 1%. The seaweed meal slightly depressed the feed conversion ratio but showed a tendency to decrease meat fat and digestive tract fill (Al-Shorepy et al., 2001).