More accurate feeding can cut pollution and boost margins

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Dairy cows feeding

With the Clean Air Strategy (CAS) targeting a 46% reduction in atmospheric particulates by 2030, and ammonia emissions – including those from agriculture – a major contributor to that pollution, nitrogen use within the dairy sector is coming under increasing scrutiny.

Seen as yet another potential restriction by many, it can also be viewed as an opportunity for herds to reduce variable costs – including feed costs – by improving nitrogen retention and utilisation, claims KW nutritionist Dr Anna Sutcliffe.

Controlling ammonia emissions

“Agriculture is currently responsible for 88% of the UK’s total ammonia emissions, so the CAS already includes proposals for tighter controls over fertiliser applications, manure storage and livestock housing design,” she explains. “However, improved feeding also has a part to play.

“Around 75% of the nitrogen fed to dairy cows is excreted in urine or faeces. Not only is that a problem in terms of pollution, it’s also a massive waste of costly nutrients.”

Although some of this excreted nitrogen is recycled as fertiliser through slurry or manure, it’s another relatively inefficient process. Much of the nitrogen is lost into the atmosphere as ammonia or nitrous oxide (a potent greenhouse gas), or as nitrate that leaches into waterways.

And when ammonia and nitrous oxide combine in the atmosphere, the result is the exact type of particulate pollution the CAS is designed to tackle.

“…it’s possible to reduce the amount of nitrogen that’s excreted…”

“Although the CAS will focus heavily on manure and slurry management, by improving efficiency of nitrogen use within the cow, it’s possible to reduce the amount of nitrogen that’s excreted in the first place,” Dr Sutcliffe continues. “And the key is to better match fermentable energy (FME) supply in the rumen to the release of nitrogen from rumen degradable protein (RDP).

“This maximises nitrogen uptake by the rumen microbes, and reduces the amount of excess nitrogen absorbed into the bloodstream and excreted. In fact, research at CEDAR has shown that this approach can allow ration crude protein levels – and feed costs – to be reduced without negatively affecting milk production.”

Reduced protein rations

The trial followed 215 first lactation heifers Holstein heifers during their first three years in milk. The maize silage-based total mixed rations (TMR) were formulated to supply metabolisable protein (MP) levels that were either 10% below, matched to, or 10% above cow requirements (14, 16 or 18% crude protein).

“Although the efficiency of nitrogen use was highest on the 14% CP ration, average daily milk yields were around 1.5 litres/cow lower, with milk protein levels also reduced slightly,” Dr Sutcliffe outlines. “Pregnancy rates were numerically poorer, and over the entire study there were more abortions, embryo losses and lameness.”

The same issues weren’t recorded for the other two groups. More importantly, those fed the 18% CP ration had the worst nitrogen use efficiency of all three groups, producing no more milk than those fed to MP requirement (16% CP) despite higher intakes and greater weight gain (figure 1).

Graphs of CEDAR trial results

Figure 1 – Milk yield and bodyweight at different ration crude protein levels (Source: CEDAR) – Click to enlarge

Improving nitrogen efficiency

“This reduction in crude protein levels is possible because the ration supplied a better balance of energy and RDP release in the rumen, and more accurately matched cow protein requirements,” Dr Sutcliffe states.

“The challenge is that much of the RDP in a typical dairy ration comes from grass silage, which is both very rapidly released in the rumen and hugely variable – from as low as 9% CP to 18% or more.”

Regular analysis of silage samples is therefore essential if ration formulations are to remain accurate, aiming for no more than 20% oversupply of RDP in the rumen compared to energy supply. Monitor milk urea concentrations to track progress, with around 0.020-0.025% (20-25mg/l) ideal and levels above 0.030% (30mg/l) indicative that ration RDP and energy levels are out of balance.

“It’s particularly important to match the rapid release of grass silage RDP with at least 5% sugars to drive early microbial growth,” Dr Sutcliffe adds. “Combined with low protein sources of starch and digestible fibre, the result is a sustained release of energy in the rumen that maximises nitrogen capture.

“Best value sources of sugars include the molasses-based or high-lactose liquid feeds such as Molale or Ecomol, whilst sugar beet feed and soya hulls are both good choices for digestible fibre. For starch, maize silage and sodawheat are great options, as are the various confectionery blends like SweetStarch and SugaRich Dairy.”

The additional protein required by the cow over and above that supplied by the rumen is then made up using feeds high in rumen-bypass protein (table 1). ProtoTec heat-treated rapemeal or SoyPass rumen-protected soyabean meal are typically the most cost-effective options.

Table 1 – Comparison of feeds supplying high levels of rumen-bypass protein


Energy content

(MJ ME/kg DM)

Crude protein
(% DM)

Rumen-bypass protein
(g DUP/kg)

Hi-pro soyabean meal








SoyPass (rumen-protected soyabean meal)




ProtoTec (heat-treated rapemeal)




“Such high quality protein isn’t cheap, but the overall effect of cutting back on excess RDP and more accurately matching cow requirements can reduce feed costs overall,” Dr Sutcliffe concludes. “The fact that it also reduces nitrogen pollution is a bonus that’s good for everyone.”

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