Time to consider legumes as a protein source in low protein diets?

Recent studies led by Liam Sinclair, Professor of Animal Science, Harper Adams University and funded by AHDB have been exploring how forage legumes, including red clover and lucerne, can replace conventional sources of protein such as soyabean and rapeseed meal in low protein diets while maintaining milk yield and composition.

Low protein diets are not a new concept. However, Liam and his team set out to understand how the right balance of protein can be achieved on a low protein diet based on high protein forage legumes to ensure farmers are protected from either a drop in yield or change in composition.

“There’s been a high volatility in the price of purchased protein feeds such as soyabean and rapeseed meal and they have become more expensive,” says Liam. “Lowering protein or using forage legumes reduces your sensitivity to market prices. There is also major pressure coming from environmental aspects.

“A lot of dairy farmers are in nitrate vulnerable zones which aim to reduce the amount of organic manure that is put back on to the land. Obviously, the more protein you feed, the more nitrogen (N) you are going to be putting back on to the land. Lowering protein levels means your cows will be excreting less nitrogen; if you can maintain performance.

“Feeding less protein can also reduce N loss in urine and subsequent ammonia release. Government targets in the Clean Air Strategy 2019 identify dairy farming as one of the major contributors to ammonia release and farmers need to consider ways to reduce this.”

Legumes, including the red clover and lucerne used in these trials, fix nitrogen in the soil in their nodules. They can reduce your need for artificial fertiliser and because they are higher in protein (typically 18-20%) they reduce the need to buy in protein feeds.

However, farmers have previously faced challenges when reducing protein levels in the diet, seeing a reduction in intake, and therefore milk yield, which can offset any cost benefit. These trials look to ensure the right balance of protein is fed to meet the cow’s requirements without a loss in yield.

“If you reduce protein levels too far you reduce intake and you can reduce milk yield. Legumes, although they are higher in protein, they are not as high as feeds such as soyabean meal. And the problem we have is it’s the wrong balance of protein in forage legumes.”

A cow’s protein supply is partly microbial from the rumen and partly undegraded protein from her feed. Some of the protein consumed by the cow is broken down in the rumen, feeding the microbes in the cow’s stomach. These are then washed through the cow, providing her with a source of protein. The rest by-passes this process as un-degradable protein. Together the microbial protein and the by-pass protein form what is called metabolisable protein.

“Rumen microbes can supply all of the cow’s protein requirements, in theory up to about 15/20 litres of milk. But once you get above that you need more un-degradable protein to meet the cow’s protein requirements.”

A cow yielding around 32 kg of milk per day cow consumes roughly four kilos of protein a day, coming from about 23 kg of dry matter at about 17/17.5% protein. About 75% of this is excreted, mainly in the urine, and 25% goes into milk. For high yielding cows a higher level of this bypass protein is needed.

“The challenge when feeding lower protein diets is that if we reduce the protein level too far and don’t supply enough for the rumen microbes, the cow will digest less fibre and eat less and therefore, she’ll produce less milk.”

Red clover

The red clover trials examined the effects of different levels of protein in a cow’s diet, including at 17.5% (that of a typical dairy cow), 16.5% and 15% protein, with red clover included in the silage at a ratio of 50:50 to grass silage.

“Although we lower the total protein content, we kept the metabolisable protein supply pretty much the same across all three diets,” explains Liam. “That means that we are feeding less purchased feeds, but the sources that we fed were more expensive as they had been treated to reduce the amount of rumen degradable protein and increase the amount of bypass protein. This is because red clover is too high in rumen degradable protein.

“What we found in this study is the cows produced the same amount of milk with the same composition. They ate about a kilo of dry matter less on the lowest protein level. We didn’t get any differences in liveweight or body condition but were aware that this could have occurred over a longer period of time,” says Liam.

A further, longer term, study concluded that reducing the dietary crude protein concentration from 17.5 to 15% in a red clover/grass silage-based diet can be achieved without affecting intake, animal performance or weight change if the diet is balanced for metabolisable protein.

Lucerne

For the lucerne trials, the team looked at the difference between 17.5% and 15% protein, initially with a 50:50 ratio of lucerne to maize silage, before increasing this to 60% lucerne and 40% maize silage. Liam explains, “Dry matter intake was about one to two kilos less on the low protein diets, but milk yield and composition, liveweight and body condition score were all similar to cows on the higher protein diet.”

Nitrogen use efficiency

In the average dairy cow, about 25% of protein will go into milk production. “When we reduced the protein in the red clover diet, we managed to increase this to over 30%,” says Liam. “And in the lucerne study we went up from about 26% to 32%.”

Similar results were seen in the longer red clover trial. “We didn’t have any differences in milk yield or composition across any of the different trials, but the nitrogen use efficiency went up from 28% to 35% and we had much less nitrogen being excreted in the urine.

“We calculated that there would be somewhere in the region of 20 kilograms of nitrogen per cow per year less being excreted in urine. And if that goes on to land, that's about 50 kilograms of nitrogen per hectare. So, if you have 200 cows, that cuts 4,000 tons of nitrogen from being excreted or lost to the atmosphere as ammonia.”

Cost benefit

Investing in more expensive proteins to ensure the correct protein balance impacted the potential savings to be found from growing their own legumes. When added methionine was included, there was little difference in cost of production. However, in “the low protein diet with starch we were getting up to 1 pence per litre savings in purchase feed costs,” adds Liam.

So why should dairy farmers in Britain consider moving to a low protein ration with forage legumes? “Potentially there are purchased feed cost savings; between half a pence and a pence per litre. Economically you're not going to lose money on it, in fact, you may make money on it.

Secondly, there is environmental aspects; we have NVZ regulations, we have the clean air strategy, which we need to be considering. But we also need to be aware of some of the hidden issues such as the carbon cost of using soyabean meal or artificial fertilizer.”