*Lawrence R. Jones is with American Farm Products Inc., and Joanne Siciliano-Jones is with FARME Institute Inc.

WELL-designed dairy rations contain forage to maximize rumen function without severely compromising intake. In a recent article, we presented "NDFu30" as a predictor of gut fill (Feedstuffs, July 21), relating the undigested neutral detergent fiber (NDF) pool size to dry matter (DM) intake.

As early as 1966, Conrad (1966) theorized that dry matter digestibility regulated DM intake. In a 2010 paper presented at the Cornell Nutrition Conference, Mertens (2010) discussed "Combining the Mechanisms of Intake Regulation Using a Common Feed Characteristic," summarizing earlier work relating dry matter digestibility (DMD) to the following linear equation: DMD = 87.1 - (0.98 - NDF digestibility) * NDF.

We have further reduced this equation to demonstrate that DM disappearance is a linear function of undigested NDF quantity: DMD = 87.1 - NDFu.

Combining the notion that DM intake is related to DMD with Mertens' derivation that DMD is a linear function of the undigested NDF pool leads to the logical conclusion that the undigested NDF pool (i.e., NDFu30) is related to DM intake.

We have chosen a 30-hour in vitro incubation to represent gut fill (Siciliano-Jones, unpublished data). This assumption is based on our own experience, the commercial availability of NDFd30iv estimates and the assertion by some that the static in vitro NDFu30iv mimics the in situ NDFu24 given typical passage rates.

The purpose of this article is to extend this concept of gut fill with the assumption that NDFu30iv is one of the most constraining factors in designing a forage base. It is important to reiterate that NDFu30iv is a constraining factor and not a limiting factor. Consequently, we would argue that forages that provide higher value per unit of NDFu30iv are the most desirable. Therefore, an index that expresses value per unit of NDFu30iv is presented.

It is important to temper value with acquisition price. Otherwise, rankings of high-value forages with high costs are deceptively inflated. Generically, the index has the following form: (value - acquisition price)/gut fill.

Determining forage value should be based on the forage functionality. Forages are principally included in a dairy ration to promote a stable ruminal environment. Additionally, forages provide nutrients for rumen bacteria.

In a 1988 research review, Nocek and Russell (1988) presented the relationship between rumen available carbohydrate and rumen available protein. Rumen available carbohydrate can be estimated from the combination of non-forage carbohydrates (NFC) and digestible NDF (NDFdiv). For forages that are not heat damaged, crude protein (CP) serves as a practical estimate for rumen available protein. Our approach is to calculate the forage commodity value using these three indices: CP, NDFd30ivand NFC.

Forage value is essentially the commodity values in terms of CP, NDFd30iv and NFC that are being replaced in the diet. Expressing this value on a unit of NDFu30iv provides the basis for our index. To demonstrate the practically of this index, two forages from a large, high-producing herd will be examined (Table 1). The corn silage is a textbook 38% NDF/35% starch corn silage. The alfalfa appears to be harvested at early bloom with an analysis of 18% CP and 47.8% NDF. The situation on the farm is that inventory is not an issue, and it is currently feeding 7.5 lb. of corn silage (DM) and 15 lb. of alfalfa haylage (DM).

To assess these forages, we will use current commodity prices expressed as component prices. For example, if soybean meal is $450 per ton, 88% DM and 48% CP, then the component price for CP is 53 cents/lb. Of course, this assumes that soybean meal is providing only CP. The point here is not to define how to calculate commodity values but to argue that they are needed to standardize different feedstuffs.

Table 2 contains the current component prices used in this example. Another important point about standardizing feeds on a commodity component value is that as the commodity markets change, forage values change.

Given commodity component prices, it becomes a matter of straightforward mathematics to calculate the value of a feed. To coincide with a producer's perspective, value is given on a 35% DM basis. These values are shown in Table 3. In this example, the corn silage is 7.2% CP, which translates to 50.4 lb. of CP per ton. Given the value of 53 cents/lb. for CP, this component has a value of $25.20 per ton of 35% DM corn silage. After accounting for the quantity and value of all three components, the corn silage value is estimated at $106.90 per ton, and the alfalfa value is $116.57 per ton.

On a commodity component basis, the alfalfa is worth 10% more than the corn silage. However, underlying this calculation is the assumption that dairy cows can consume the same amount of each of these forages.

Table 3 also contains the NDFu30iv content. A ton of the example corn silage provides 120.4 lb. of NDF that is undigested after 30 hours of incubation, while the alfalfa provides a much higher level of 221.2 lb. The gut fill paradigm suggests that the higher NDFu30iv level for the alfalfa will limit intake more than the corn silage.

To account for this difference, the commodity component value is divided by the NDFu30iv content. For each pound of NDFu30, the corn silage provides 89 cents of commodity components, while the alfalfa provides only 53 cents. In this analysis, the corn silage is substantially better (68%) than the alfalfa. The underlying assumption of this calculation is that NDFu30iv intake is an important dietary constraint.

There are both strategic and tactical views when selecting a forage base for a diet. The tactical view is to minimize added commodity costs. The strategic view is to ensure that the right forages in the proper amount are available for the diet. This view involves achieving the lowest total feed costs, not necessarily the lowest added commodity costs. For the tactical view, value per NDFu30iv will be a useful guide to achieve the lowest added commodity costs.

Adding the forage acquisition price as the third dimension will guide us to the lowest total ration costs. Specifically, considering the acquisition price will discount high-value forages that are expensive to produce. An interesting application of this index is comparing conventional corn silage to a brown mid-rib corn silage where there are differing NDF levels, NDF digestibility values, starch levels and production costs.

In Table 3, we have assigned an acquisition price of $40 per ton for the corn silage and $60 per ton for the alfalfa haylage. Our corn silage index was lowered to 56 cents per unit of NDFu30iv, and the alfalfa index was lowered to 26 cents per unit of NDFu30iv. Now, the corn silage is twice as valuable as the alfalfa given the gut fill index and the price of acquisition.

To make the use of these indices more intuitive, we have developed the concept of a Forage Box (copyrighted by FARME Institute in 2014) that needs to be filled without breaking. Consider the bottom of the box as the NDFu30iv level. Too much NDFu30iv, and the bottom will fall out. The box dimensions are the amounts of CP, NDFd30iv and NFC that are desired from the forage base to optimize rumen function. The added commodity costs are the components that are added to the forage to fill the box. The size of the Forage Box will depend on the nutritionist's feeding philosophy.

Getting back to the example farm, we have defined our Forage Box to be 6 lb. of NDFu30iv, 5 lb. of CP, 7 lb. of digestible NDF and 13 lb. of NFC. We are not advocating these specific numbers; the herd is currently eating 6 lb. of forage NDFu30iv, and the other amounts were dictated by the nutritionist from ration parameters.

The current forage diet is depicted in the first line of Table 4. With 7.5 lb. of corn silage and 15 lb. of alfalfa, the forage commodity component value is $3.64 per cow per day. The forage acquisition price is $1.71 per cow per day, which represents a 2:1 return on forage cost. To achieve the targeted 5 lb. of CP, 7 lb. of NDFd30iv and 13 lb. of NFC, $2.36 per cow per day worth of added commodities are needed. Consequently, the total cost to fill the Forage Box is $4.07 per cow per day. Again, the tactical view only generally considers the $2.36 added commodity cost, while the strategic view should consider the $4.07 total cost.

Compare this to the final line, where 25 lb. of corn silage and 5.47 lb. of alfalfa are fed. This provides a forage commodity component value of $4.73 per cow per day at an acquisition price of $1.90 per cow per day. This forage level provides a 2.5 times return on forage acquisition price.

The added commodity costs to fill the Forage Box are $1.27 per cow per day for a total cost of $3.17 per cow per day. By increasing the forage level for the base diet to the 25 lb. of corn silage and 5.47 lb. of alfalfa, the purchased feed cost declined by $1.09 per cow per day, and the total cost of the Forage Box decreased by 90 cents per cow per day.

By adjusting the forage base within the parameters specified by the Forage Box, we would not expect any negative effects on herd productivity.

Summary

Asserting that gut fill is a major constraint in designing forage-based rations and indexing forages based on their gut fill factor seems appropriate. For practical purposes, the amount of undigested fiber after 30 hours of in vitro incubation (NDFu30iv) is consistent with the current understanding of rumen physiology and passage rates to serve as a useful proxy for gut fill.

The economic value of a forage in a dairy ration is dependent upon its contribution to rumen function as well as offsetting commodity prices. The three major forage components for determining ruminal nutrient production are the amounts of digestible fiber (estimated by NDFd30iv), NFC and CP. Assigning commodity-based valuations to these components provides a commodity value for the forage.

The index presented in this article is expressed as the commodity value divided by the pounds of NDFu30. This index is useful for determining which forage quantities provide the highest commodity value without violating the gut fill constraint.

When selecting among different categories of forages, this index is biased toward the highest value without regard to cost. Including the acquisition price in the index by subtracting it from the commodity value provides a better index for farm planning. This is especially important when comparing crops that have significantly different costs associated with their production.

Finally, the concept of a "Forage Box" that needs to be filled is presented. The Forage Box is defined as the amount of CP, NFC and NDFd30iv while having an NDFu30iv constraint. The optimal levels of specific forages in combination with added commodities will either provide the lowest added commodity cost or the lowest total costs.

References

Conrad, H.R. 1966. Symposium of factors influencing the voluntary intake of herbage by ruminants. J. Anim. Sci. 25:227-235.

Mertens, D.R. 2010. NDF and DMI — Has anything changed? Proceedings of the Cornell Nutrition Conference.

Nocek, J.E., and J.B. Russell. 1988. Protein and energy as an integrated system. Relationship of ruminal protein and carbohydrate availability to microbial synthesis and milk production. J. Dairy Sci. 71:2070-2107.

Volume:86 Issue:53