Feeding Recommendations for Dietary Carbohydrates and Protein

            The preceding discussion has focused on understanding the rumen system and presented concepts on how to better feed the microbial populations to improve milk production efficiency.  To move into this new era of dairy cattle feeding, more comprehensive methods of feed composition analysis are needed.  With a better understanding of feed composition, we can apply concepts of rates of degradation and passage to a dynamic rumen system to achieve improved feeding practices that will not only improve milk production efficiency, but decrease wasted dietary nitrogen from entering the environment.  The following tables provide some practical feeding guidelines in applying these concepts to ration formulation.

NDF and Dry Matter Intake

            As previously described, NDF has been shown to influence dry matter intake in ruminant animals (17).  Mertens had shown optimum intake of NDF as a percent of body weight (%BW) was  1.2 " 0.1 %BW.  Neutral detergent fiber intake capacity is influenced by physiologic state and age (Table 7). Cows can consume more or less dietary NDF, however, there will be negative consequences on production or performance as a result of reduced intake.

Table 7.          Suggested intake levels for NDF on a percent of body weight basis for differing production groups by lactation number.

		Lactation Number
		1	2	3+
Dry Cow
	Far Off	0.8 - 0.9	0.9 - 1.0	1.0 - 1.1
	Close-up	0.6 - 0.7	0.65 - 0.75	0.8 - 0.9
Lactating Cow		All values " 0.1
	0-30 DIM	0.85	0.95	1.05
	30-60 DIM	0.9	1.0	1.1
	>60 DIM	1.05	1.1	1.2

            In taking these recommendations for NDF intake into consideration, dietary concentration of NDF should range from 25 to 35% in lactating cows and 30 to 45% in dry cows.  This would represent total dietary NDF, of which 75 to 85% should be from forages.  How does one decide on just how much NDF to incorporate into a diet?  To determine dietary NDF within this range one needs to consider a number of factors that mitigate the effectiveness of dietary NDF on rumen conditions.  It would be reasonable to formulate diets with lower NDF content when they contain long forage particles; buffers, additional fat; ingredients with moderate starch digestibility and are fed as total mixed rations.  On the other hand, formulated for higher dietary NDF content would be reasonable if no long forage particles are in the diet; high fiber byproducts are used; grain is fed  in meals; ingredients have high starch fermentability; high rate of fiber degradation and passage is present; or forage dry matter and quality is variable.

Nonfiber Carbohydrates

            The NFC portion of the diet is primarily responsible for rapid ruminal fermentation in support of milk production.  Milk production will be severely compromised in the face of low dietary NFC content.  Conversely, excess dietary NFC content may result in ruminal dysfunction, namely subclinical or clinical acidosis.  The balancing act with NFC is to provide sufficient amounts to support maximal microbial growth, yet not providing excess amounts that will reduce ruminal pH and compromise fermentation activity.  Table 8 presents suggested dietary concentrations for total NFC and NSC (sugar and starch) components over differing physiologic states.

Table 8.          Suggested dietary concentrations for nonfiber carbohydrates (NFC) and sugars and starches in dairy cattle diets.

		Total NFC Content		Sugar and Starch Content
		Mean	Range	Mean	Range
Dry Cow		% of dietary Dry Matter
	Far Off	22	18 - 25	18	16 - 20
	Close-up	35	32 - 38	28	26 - 30
Lactating Cow
	Early	38	36 - 42	30	28 - 32

Read Also

Mid

36

32 - 38

29

27 - 31

Late

34

28 - 36

28

26 - 30

            For lactating cows, dietary NFC content can range from 33 to 45 % of dry matter.  Similar to NDF recommendations, the nutritionist must consider a number of factors to decide what will potentially be the optimum level of NFC in the diet.  If the NFC are primarily coming from cereal grains (corn, barley, wheat) and are processed (steam-flaked, finely ground, ensiled), then one should formulate to the lower range of total NFC.  The reason is to account for the greater degradability of these starch sources and the potentially negative impact on rumen pH.  If one is using fiber byproducts and starch sources are low to moderate degradability, then one can formulate to higher levels of NFC in the diet.  Nutritionists should also consider amount of physically effective NDF in the diet and feeding program (TMR vs. Meal feeding) to fine tune dietary NFC content.

Protein Fractions

            From a dietary protein formulation perspective, nitrogen needs of the rumen should be met first, then metabolizable protein requirements for the cow should be addressed.  Dietary crude protein is a poor method by which to formulate diets in this manner.  However, one needs some sophisticated dynamic models to adequately predict dietary metabolizable protein delivery.  If one formulates diets to the specific protein fractions shown in Table 9, the amount of metabolizable protein should be reasonably adequate.  Again, one can only truly predict the degradability and undegradablility of protein sources with a dynamic modeling system, which accounts for dry matter intake and ingredient interactions on availability and contribution to either microbial protein yield or digestible undegraded protein.

Table 9.          Suggested dietary concentrations for various protein fractions in diets for dairy cattle.

		Crude Protein	Soluble CP	RDP1	RUP1
Dry Cows		% DM	% of CP	% of CP	% of CP
	Far Off	12 - 13	35	70 - 72	28 - 30
	Close-up	14 - 15	30	63 - 65	35 - 37
Lactating Cows
	Fresh	19	30	60 - 62	38 - 40
	22-80 DIM	18	31	64	36
	81-200 DIM	16	32	66	34
	>200 DIM	14	34	68	32

¹Abbreviations: RDP = rumen degradable protein; RUP = rumen undegradable protein.

            In formulating for dietary protein, rumen nitrogen needs must be considered first.  Ruminal degrable protein should be between 10.5 and 11% of dry matter to support microbial nitrogen needs.  One can never separate rumen nitrogen from CHO needs.  To address this interaction between dietary CHO and protein in the rumen, one should formulate for a NFC:RDP ratio between 3.0 - 3.5 to 1.  This suggests as NFC content goes up, then more degradable protein can be assimilated.  Rumen protein needs should be met with forage and plant source proteins.  If additional bypass protein is necessary, then heat-treated plant proteins or animal proteins (if allowable) can be used to meet the cow’s metabolizable protein needs.  Use of rumen protected amino acids should be left to only fine tune a diet if necessary, given their cost.