Understanding the Protein Needs of Ruminants

Technical articles - Productivity, Nutritional quality, Protein self-sufficiency

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Holsteins ingesting a total mixed ration at the trough

Feeding ruminants is done using forages and concentrates. Within the framework of protein autonomy, it is essential to calculate rations for the herd in order to determine the area needed to produce the feed. We will focus on the protein part of the ration.

Estimating needs and supplies

Needs

In ruminants, most digestion takes place with the help of microorganisms in the rumen. The bacteria and protozoa of this rumen degrade feed into nutrients, and synthesize proteins (in the form of amino acids) which can then be assimilated by the animals via their intestine. Therefore, the protein needs of ruminants are expressed in PDI (Digestible Proteins in the Intestine). Below is a generic estimate of the average needs of some ruminants. However, the calculation of needs must be done case by case.

Average protein needs of ruminants estimated from the INRA tables (2007) BW: Body Weight; CP: milk protein content; MP: milk production

Species	Characteristics	Average PDI needs (g/day)
Beef cattle	Charolais, BW = 650 kg, early lactation	800
Dairy cattle	Holstein, BW = 650 kg, CP = 32 g/L, MP = 30 kg/day	1920
Dairy sheep	Lacaune, BW = 70 kg, CP = 56 g/L, MP = 3 kg/day	247
Dairy goats	Saanen, BW = 60 kg, CP = 32 g/L, MP = 3 kg/day	185

Supplies

Once needs are calculated, it is necessary to determine the characteristics of the feeds, in particular their protein content in order to provide a coherent ration for the animals. Although the commonly used unit to quantify this content is MAT (Total Nitrogen Matter), it is not entirely suitable for ruminant farming. Indeed, the microbial activity in the rumen that degrades these proteins requires energy and nitrogen to be sufficiently effective. For this reason, proteins in feeds consumed by ruminants are distinguished into two categories:

• PDIN or Digestible Proteins in the Intestine small intestine (PDI) allowed by the nitrogen (N) supplied by the feed.
• PDIE or Digestible Proteins in the Intestine small intestine (PDI) allowed by the energy (E) supplied by the feed.

A ration is largely composed of forages, inexpensive and fiber-rich feeds, and may be supplemented or not with concentrates. Their feed value is calculated using various characteristics summarized in the table below.

Type	Characteristics influencing feed value
Forage green	Species or origin for permanent grasslands, cycle (number of cuttings+1), stage.
Conserved forage	Species or origin for permanent grasslands, cycle, stage. Method of conservation, harvesting conditions, dry matter content.
Concentrate feed	Nature (protein concentration, crude fiber, etc.).

The PDIN and PDIE values of these feeds can be found, as averages, in the feeding tables developed by INRA. Some examples are given in the table below:

	PDIE	Gross PDIN
Hay	82	69
Soybean meal	220	320
Maize silage	98	61
Dehydrated Alfalfa	100	119

Limiting amino acids in the dairy cow

In dairy cows, the two amino acids most frequently considered limiting protein synthesis are lysine and methionine. Regardless of lactation stage, needs are 2.5% of PDIE for methionine and 7.3% of PDIE for lysine. The contents of feeds in methionine (MetDi) and lysine (LysDi) are available in the INRA tables. If needs are not met, a decrease in milk protein content will be observed. A decrease in milk production is also possible.

Example: Barley has a content of 102 g PDIE/kg DM. Within this PDIE, there is a LysDi content of 6.83% and MetDi of 1.88%. We can therefore predict the quantities of these two amino acids supplied according to the amount of barley included in the ration. If needs are not met, the response of the milk protein content (Protein Rate) can also be predicted.

Formulating a ration

There are 4 types of rations:

• Complete ration: This technique offers the farmer a considerable time saving, as it consists of mixing forages and concentrates beforehand and then distributing this mixture to the animals. There is no individual concentrate supply. While it also allows good rumen function, it takes into account an average production goal. In the case of dairy cattle, for example, high-producing cows will be underfed and low-producing cows will be overfed.
• Semi-complete ration: To avoid this drawback of the complete ration, the farmer can choose to reduce the energy portion of the ration and provide high-producing cows with a concentrate supplement. Low-producing cows will thus not be overfed. However, this requires a significant time investment from the farmer.
• Individualized supplementation: This time, feeding is fully individualized. Concentrates are administered animal by animal. This allows precise adjustment to the needs of each individual. However, it requires considerable time.
• Group ration: This feeding method consists of separating the herd into groups and creating a ration for each group based on production and/or lactation stage in the case of dairy cows.

In all cases, a well-balanced ration will have a supply of PDIE equal to the ruminant's PDI needs, and a supply of PDIN equal to or possibly greater than the supply of PDIE. Indeed, if PDIN<PDIE, there will be a lack of degradable nitrogen for the rumen microbial flora. A slight deficit, defined using the Rmic (Microbial ratio), can however be accepted for each species and production.

This Rmic ratio allows checking the proper functioning of the rumen. It is defined by the formula (PDIN – PDIE)/UF*, and must be greater than a threshold value, the Rmin (Minimum microbial ratio), to ensure good rumen function.

*UF is the forage bulk, a value also available in the INRA tables.

Rmin according to the INRA tables (2007)

Species	Characteristics	Rmin
Beef cattle	In lactation	-17
Dairy cattle	MP = 30 kg/day	-4
Dairy sheep	MP = 2 kg/day	-6
Dairy goats	MP = 3 kg/day	-7

A more detailed table is available in the INRA tables (2007).

• If the Rmic ratio is above the threshold value, the ration is acceptable. However, if it is much higher, there will be more nitrogen in the urine and thus excessive nitrogen emissions which can have economic and environmental consequences.

• If the Rmic ratio is below Rmin, the ration must be revised. Three possibilities are then envisaged:
  • Add to the ration a feed rich in fermentable nitrogen (example: urea as a feed supplement)
  • Change the concentrates for a feed with a higher PDIN value (example: replace rapeseed meal with sunflower meal).
  • Introduce a forage rich in PDIN into the ration (example: grass silage as a supplement to Maize silage).

Other needs (energy, minerals, trace elements and vitamins) must also be checked to avoid deficiencies.

Feeding strategies: case of dairy cows

Feeding strategies for dairy cows are most often based on the forage available on the farm supplemented by concentrates produced or not on the farm, to cover the animals' needs. If the energy density needed increases with milk production, the PDIE/UFL (Milk Forage Units) ratio varies little and a ratio of 100 g PDIE/UFL is considered ideal for a balanced ration .

A compromise must then be found between very different individual needs within the herd and a simplification of feeding while optimizing the use of concentrate feeds. For this, different strategies exist. Here are some examples of ration strategies studied from the protein feeding perspective.

Ration strategy for a dairy cow in mid-lactation

For this example, we choose a multiparous dairy cow of 700 kg, in mid-lactation, with a maximum potential production of 41 L, producing 34 L per day in the 16th week at 40 g/kg fat (Fat Content) and 32 g/kg protein (Protein Content). Her nitrogen needs are 2146 g PDI.

Her ration is detailed in the following table:

Feed		PDIN (in ration)	PDIE (in ration)	Quantity ingested	PDIN supply	PDIE supply
Forage	First-cut orchardgrass silage, 1 week before heading, short stems, with preservatives	112g/kgDM	75g/kgDM	17kgDM	1904 g	1275 g
Concentrate	Triticale grain	72g/kgDM	96g/kgDM	4.8kgDM	345.6 g	460.8 g
		Total			2250 g	1735 g

A deficit of 410g PDIE is observed. In this ration, energy is the limiting factor for microbial synthesis. Therefore, part of the triticale must be replaced by a concentrate richer in nitrogen. Soybean meal is chosen:

Feed		PDIN (in ration)	PDIE (in ration)	Quantity ingested	PDIN supply	PDIE supply
Forage	First-cut orchardgrass silage, 1 week before heading, short stems, with preservatives	112g/kgDM	75g/kgDM	17kgDM	1904 g	1275 g
Concentrate	Triticale grain	72g/kgDM	96g/kgDM	2.3kgDM	165.6 g	220.8 g
Concentrate	Soybean meal	377g/kgDM	261g/kgDM	2.5kgDM	942.5 g	652 g
		Total			3012 g	2147 g

PDIE supplies now match PDI needs and PDIN supplies are higher than PDIE. This ration is therefore balanced from a protein point of view.

To check proper rumen function, we calculate Rmic (in our example it is a cattle with milk production so we use UFL):

(PDIN-PDIE)/UFL = (3012-2147)/23.5 = 36.8 > 0. Rumen function is ensured.

Feeding strategy for a dairy cow at pasture

For this example, a multiparous dairy cow of 650 kg BW, with a potential milk production of 32 kg, and nitrogen needs of 2040 g PDI. The grazed grassland is a permanent lowland grassland, grass height at entry is 12 cm and 5 cm at exit. Maize silage is provided as a supplement as well as barley grain.

Feed		PDIN	PDIE	Quantity ingested	PDIN supply	PDIE supply
Grazed pasture	Permanent lowland grassland	114g/kgDM	98g/kgDM	13.1kgDM	1493.4g	1283.8g
Forage	Maize silage	61g/kgDM	98g/kgDM	3kgDM	183g	294g
Concentrate	Barley grain	79g/kgDM	101g/kgDM	4.6kgDM	363.4g	464.6g
		Total			2039.8g	2042.4g

PDIE supplies cover nitrogen needs well. However, PDIN supplies are slightly lower than PDIE, but this difference is acceptable.

To check proper rumen function, we calculate Rmic:

(PDIN-PDIE)/UFL = (2039.8-2042.4)/21 = -0.1 > -4

Thus, Rmic is well above the Rmin for dairy cows (-4). Rumen function is therefore ensured.

Limitations to consider

Digestibility of protein crops

Some forages or protein crop seeds are known to be flatulent and rather poorly digestible by ruminants. This is a factor to consider when choosing species to include in the ration. However, technical solutions exist to address this problem, notably through thermal treatment technologies such as "toasting", or "extrusion", which aim to increase degradability in the rumen and digestibility in the intestines of these feeds . However, it should be noted that, outside the theoretical framework, the technical benefits of these technologies are sometimes debated , especially due to variability of observed results and their Economic viability.

Protein efficiency

In recent years, the concept of "feed versus food" has become one of the main societal criticisms of livestock farming. This concept draws an unfavorable material balance between feed given to animals and those produced by them for human consumption.

From the protein point of view, it is important to consider this observation holistically, taking into account the share of proteins consumed by animals that are not valorized by humans: this is the case for example of grasslands, or processing by-products.

The net protein conversion efficiency is an indicator (developed by LAISSE et al. in 2018) that allows assessing this balance.

Formulas used for calculating net protein conversion efficiency (A) and net quantity of proteins provided by human food (B). G. DURAND, BSA, 2020.

It is thus considered that, depending on ruminant farming systems, net protein efficiency is often close to or even less than 1. This means that about 1 kg of plant proteins consumable by humans is needed to produce 1 kg of animal proteins, and that under certain conditions animals are net producers of proteins consumable by humans.

Our series of guides on protein self-sufficiency

• Autonomie en protéines
• Appréhender les besoins protéiques des ruminants
• Valoriser les prairies pour accroitre l'autonomie protéique
• Produire des cultures fourragères riches en protéines
• Produire et consommer localement des concentrés protéiques
• Transformer et stocker les ressources alimentaires protéiques

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