Microalgae: Suitable protein for cows

Microalgae are a diverse group of microscopic organisms growing in very different environments from water to land, from hot to cold climates, and fresh to salt water. This is also reflected in the nutritive composition of microalgae that can vary greatly between species, but also within species depending on e.g. strain, cultivation conditions, harvesting method and growth stage. This plasticity of microalgae composition enables multiple different ways of utilising microalgae in animal nutrition and optimising the composition for specific targets. For example, species rich in protein (such as Spirulina platensis and Chlorella vulgaris) can be used as protein feeds, whereas species rich in lipids or carbohydrates are suitable energy feeds, and species rich in e.g. polyunsaturated fatty acids or antioxidants can serve as dietary supplements. In a doctoral dissertation published May 2019 at University of Helsinki, the potential of various microalgae as protein feeds for dairy cows was assessed.

Four feeding experiments were conducted with lactating dairy cows to investigate the impact of different protein feeds on feed consumption, milk production, energy metabolism, nitrogen utilisation and amino acid metabolism. In the experiments, non-defatted Spirulina platensis, Chlorella vulgaris and Nannochloropsis gaditana microalgae substituted for the protein found in conventional protein feeds (rapeseed meal, soybean meal and faba beans) either wholly or partially. Furthermore, the effect of protein feeding on milk yield and metabolism was investigated in one of the experiments. The protein concentration of the Spirulina platensis and Chlorella vulgaris microalgae was notably higher compared to the crude protein of conventional protein feeds, approximately 600–700 g/kg in dry matter, while the protein concentration of Nannochloropsis gaditana, 385 g/kg in dry matter, corresponded to that of conventional protein feeds.

The palatability is a challenge

The palatability of microalgae was clearly poorer than that of conventional protein feeds. When roughage and concentrate were fed to the cows separately, the intake of concentrates containing microalgae decreased, but the cows compensated for this by eating more roughage. As a result, the total intake of feed did not change when microalgae were used as a substitute for conventional protein feeds. When offered as total mixed ration (TMR), the cows ate less feed overall, as they were unable to avoid microalgae. However, it is unclear whether the poorer palatability of microalgae was caused by their sensory properties (i.e taste, odour and texture) being different from conventional protein feeds, physiological responses to microalgae ingestion, or both. Further research is also needed on the impacts of cultivation, harvesting and processing practices on microalgae composition and palatability, because for example high feed sodium concentration has the potential to decrease feed intake in ruminants. There is also indication that protein quality might be influenced by microalgae production methods.

Minor effects on milk production

With the Spirulina platensis microalgae partially substituting for the protein found in rapeseed or faba beans, milk output decreased in conjunction with rapeseed feeding, but increased in conjunction with faba bean feeding. Similar effects were observed in the production of milk protein. Milk and milk protein yields achieved through microalgae feeding were comparable to those achieved through soybean feeding. Substituting rapeseed and soybeans with Spirulina platensis increased milk fat yields, possibly due to differences in rumen fermentation, a decrease in the body weight of the cows, or an increased intake of methionine, an amino acid. Overall, despite of their poorer palatability, microalgae performed surprisingly well in comparison to conventional protein feeds. The superiority of rapeseed meal was expected, because this protein feed has been proven to be well suited to dairy cow diets based on grass silage and cereals. Improving the palatability of microalgae to dairy cows has potential to further increase the milk production responses because feed intake is the main factor affecting milk yield.

Nitrogen utilisation

The effect of microalgae on nitrogen utilisation depended on the substituted conventional protein feed. When Spirulina platensis substituted for rapeseed, the effect was negative, as a smaller share of the nitrogen and human-edible protein in the feed ended up in the milk. With rapeseed feeding, the utilisation rate of protein suitable for humans was 150% at its highest, which means that cows fed with rapeseed produced more human-edible protein in their milk than what they consumed in the feed. Therefore, rapeseed feeding promoted food security due to the fact that rapeseed meal is a by-product of food production with a very good milk yield response. With microalgae feeding, the corresponding ratio was approximately 100%, making the impact of microalgae on food security neither positive nor negative.

When microalgae substituted for faba beans or soybeans, the effects were more positive compared to rapeseed feeding, as the proportion of nitrogen ending up in the milk increased or remained unchanged. The utilisation rate of human-edible protein was approximately 80% with soy and faba bean feeding. In other words, cows fed with these feeds consumed more human-edible protein in their feed than they produced in their milk. Thus, these feeding types may impair food security compared to rapeseed and microalgae. Nitrogen excretion into urine also decreased, as microalgae substituted for soybeans in feeding dairy cows. The opposite effect was seen when microalgae substituted for faba beans, as the total excretion of nitrogen into the environment increased.

Further research needed

Of the microalgae amino acids, the concentration of histidine was lower and that of methionine higher compared to conventional protein feeds. The availability of these amino acids is often the first factor limiting the milk yield of dairy cows. In all of the experiments, microalgae decreased the cows’ histidine intake, while increasing the intake of methionine. However, these affected plasma concentrations of histidine or histidine metabolites only in half of the experiments. The potential reason for this might be related to the short duration of the experiments, when animals were able to cope with temporary dietary histidine shortages by utilising body histidine reserves. It is also possible that despite of decreased histidine intake, the supply from microalgae remained sufficient enough in comparison to control feeds in order to maintain milk production level.

The difference on responses of plasma methionine and methionine metabolites was very distinctive between rapeseed meal and microalgae. Both of these feeds increased methionine intake, but only rapeseed meal was able to boost also plasma methionine and methionine metabolites indicating improved metabolisable protein status of dairy cows. The lack of these responses on microalgae diets might be caused by the protein characteristics of microalgae, such as high ruminal protein degradability, or low intestinal protein digestibility. Both of these factors may limit the intestinal availability of amino acids utilisable for production purposes. Currently, the ruminal protein degradability and intestinal digestibility of microalgae, and the effect of microalgal cultivation and harvesting methods on these factors are poorly understood, even though they are very important factors affecting protein quality in addition to protein concentration and amino acid composition.

Going for large-scale utilisation

The results of these experiments demonstrated that microalgae can be used as a protein source for lactating dairy cows in intensive milk production systems. Microalgae seemed to be very well suited to substitute especially grain legumes soya bean and faba bean in dairy cow diets. Nevertheless, the large-scale feed utilisation of microalgae is still hindered by the high production cost of microalgae relative to conventional protein feeds. However, the competitive position of microalgae may change in the future due to e.g. rapid technological development in microalgae production, and different policy interventions targeted to unsustainable feed production practices. Moreover, the utilisation of microalgae as animal feed can also contribute to decreasing production costs of microalgae, if protein-rich feed material is generated as a by-product of biofuel production, and because drying of microalgae biomass, one of the most energy-intensive steps of microalgae production, is not necessary in some animal feeding techniques such as using TMR in ruminant or liquid feeding in pig production systems.