Feed efficiency of dairy cows is an important performance indicator for a dairy farm. It can be used to improve sustainability and profitability of dairy farming. It is most commonly defined as the amount of protein and fat corrected milk that a dairy cow produces divided by the amount of dry matter she consumes. It is however also possible to express feed efficiency as the amount of fat and protein corrected milk produced per hectare of pastureland, per unit of nitrogen fed, per unit of energy fed or per unit of methane produced.

More efficient dairy cows may generate the same amount of milk as the rest of the herd while consuming significantly less feed. Feed efficiency in lactating cows can vary from <1.3 to >2.0 (See table 1). When comparing feed efficiency of dairy herds, it is however important to compare "like for like". In the US, fat and protein corrected milk yield is ususally calculated towards 3.5% fat, whereas in Europe it is more common to calculate to 4.0% fat.

According to the FAO, livestock production is responsible for 14.5% of all global greenhouse gas emissions. Within that, the dairy sector accounts for about 2.2% of the greenhouse gas emissions. A high correlation has been found between feed efficiency and methane production (see Figure 1). If dairy cows are more productive, they produce less methane or greenhouse gas per kg of milk produced.

Factors that can influence feed efficiency of dairy cows include milk production and component yield, feed intake, forage quality and quantity, cow age, ration protein levels and composition of the diet, body weight, environmental stress, exercise, pregnancy, poor udder health, subacute rumen acidosis, problems with hindgut health and "leaky gut", other health issues and feed additive use. Out of those, important factors are:

• Milk yield and days in milk, highly productive dairy cows are more efficient
• Feed digestibility, which relates to feed quality
• Somatic cell count because poor udder health of dairy cows can result in lower milk yields
• Subacute rumen acidosis and hindgut acodosis which affect digestion, impede feed passage and reduce energy partition of dairy cows.

The amount of energy required for maintenance is not depending on the milk production level of a dairy cow. As a result, highly productive dairy cows are more efficient (see table 2). This also means that days in milk has an impact, with cows becoming less efficient during the 2nd half of lactation.

If more digestible feeds that release more energy and protein are offered, feed efficiency of dairy cows will go up (see Figure 2).

The newly revised NASEM 2021 guidelines for dairy cows have different dietary protein ration requirements compared to the NRC guidelines for dairy cows. An improved understanding of the role that metabolizable protein (MP) plays for lactating dairy cows has established new relationships between protein in the diet and milk yield. Ruminal microbes need amino acids and peptides, but ammonia can also be an important nutrient for ruminal microbes. Ammonia can be provided by a range of sources including grass silage and urea.

Sources of amino acids include rumen degraded microbial protein, rumen undegraded protein (RUP) and rumen protected amino acids. Among amino acids, several are considered essential but are not provided at required levels within most feed ingredients. Many feed ingredients for dairy cows have enough of one, but not all amino acids. In those cases, the use of rumen-protected amino acids can result in an increase in milk production with higher protein and fat levels.

The amount of milk urea nitrogen (MUN) is indicative of the ability of the rumen microflora to convert ammonia into microbial protein. Increased MUN levels are indicative of poor rumen function and will result in a reduction of feed efficiency and fertility.

Somatic cell counts of dairy cows are indicative of subclinical udder infections. Inflammation of the udder results in loss of milk production. A Dutch study114 showed that a case of subclinical mastitis in heifers results in loss of production of 0.28-0.31 kg of milk/day. A case of subclinical mastitis in multiparous cows results in a loss of 0.50-0.58 kg of milk/day. A Danish study115 showed that milk loss in animals suffering from subclinical mastitis in an entire lactation of 305 days was 155 kg of milk in primiparous dairy cows and 445 kg of milk in multiparous dairy cows.

Subacute ruminal acidosis (SARA) and hindgut acidosis are a common problems in lactating dairy cows that cause chronic health problems, impair feed efficiency, and increase the environmental impact of milk production. Low ruminal pH appears to be the main instigator of the pathophysiology of SARA, although other metabolites produced in the rumen may be involved. Inflammatory responses to SARA are variable but important determinants of a cow's response to SARA. The same factors that increase the risk for dairy cows to develop SARA (e.g. low forage diets with high levels of starch) also put dairy cows at risk to develop hindgut acidosis and “leaky gut”. Prevention of SARA and hindgut acidosis in dairy cattle requires excellent feeding management and proper diet formulation.