Feed tags for supplements typically list the CP concentration. Likewise, the CP concentration in basal forage can be determined by a commercial laboratory, or perhaps this can be estimated based on past levels in similar forage. For simplicity, the CP concentration in forage along with an assumed factor for converting CP to metabolizable protein (MP; MP concentrate (%) = 64.0 + (0.16 × rumen undegraded protein concentration, % of total CP) will be used.
A number of measurements are necessary to most accurately determine the rumen undegraded protein concentration (UIP) in forages and supplemental concentrates. However, as noted for calculation of MP requirements, knowledge of the general type of feedstuff can allow an adequate estimate of the UIP concentration for most practical purposes. For example, many fresh forage diets would probably have a UIP concentration of 20% (on a total CP basis), which means that 80% of the CP is degraded in the rumen. Dried forages would have a slightly higher UIP level (e.g., 30% for grasses). In general, as the dietary level of concentrate increases, the UIP level increases. Since not a large number of feedstuff UIP concentrations have been determined with goats, below is a table with UIP levels for cattle (Preston, 2000) that can be used until more values determined with goats become available. Typically, feed tags list the major ingredients, which then can be used along with the table below, to derive a reasonable estimate of the UIP concentration. However, default UIP values of 20% for the basal forage and 40% for the supplemental concentrate have been used in the input box below.
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To estimate the need for supplemental concentrate, an estimate of forage intake is needed. The feed intake calculators can be used for this, unless there is already knowledge of how much of a particular type of forage is consumed by class of goat of interest. The next factor to be considered is potential associative effects. An associative effect can be defined as a response, such as in total feed intake, digestibility, performance, etc., when mixtures of feedstuffs are consumed that are not as would be expected based on consumption of the feedstuffs alone. Associative effects can be positive or negative. That is, in some instances feed intake, for example, might be greater or less than expected. An example of a positive associative effect is when a very low protein forage (e.g., 4 or 5% CP in weathered and(or) mature prairie hay) is fed and a high-protein supplemental concentrate (e.g., soybean meal) is given, resulting in an increase in forage intake. An example of a negative associative effect is when a moderate to low digestibility forage (e.g., TDN concentration of 40-50% is supplemented with a moderate to high level (e.g., 0.75-2.0% BW) of concentrate high in cereal grains like corn. In this case, the supplemental concentrate elicits a decrease in forage intake, or there is ‘substitution' of concentrate for forage.
There has not been a great deal of research on associative effects with goats compared with cattle or sheep. Thus, it is not possible to use a well accepted method of addressing the issue. Nonetheless, because associative effects are known to occur in goats, a simple system being used in another project of the Institute will be employed until further information is available.
When the supplemental concentrate level is at least 0.5% BW (DM basis), the following formula is suggested to address negative effects on forage intake of supplementation with high energy feedstuffs.
SUBN = -1 * ((-0.0007 - (0.2491 × DMIBWS) + (0.7641 × DMIBWS2) - (0.2006 × DMIBWS3)) - ((55 - DIGF) × 0.04))
SUBN = change (negative or a decrease) in forage intake, expressed as a fraction of supplement intake (for example, if supplement is given at 1% BW (DM basis), and SUB is -0.6, then hay intake is decreased by 0.6% BW.
DMIBWS = supplement intake as % BW (DM basis)
DMIBWS2 = DMISBW squared
DMIBWS3 = DMISBW cubed
DIGF = digestibility of forage
DIGF can be calculated from ME concentration as: DIGF = (ME concentration, MJ/kg + 1.8749) / 0.1867.
For forages < 6% CP (DM basis) and supplements 15% or greater in CP concentration (DM basis), and a supplemental concentrate less than 0.5% BW (DM basis), the following formula is suggested to address the positive associative effect of high-CP supplements on intake of low-protein forages.
SUBP = (1.0398 - (0.2323 × DMIBWS) - (0.7047 × DMIBWS2) + (0.3757 × DMIBWS3)) - ((6 - PTCPF) × 0.05)
SUBP = change (positive or an increase) in forage DM intake expressed as a fraction of supplement intake (for example, if supplement is given at 0.25% BW (DM basis), and SUB is 0.6, then hay intake is increased by 0.15% BW.
DMIBWS = supplement intake as % BW (DM basis)
DMIBWS2 = DMIBWS squared
DMIBWS3 = DMIBWS cubed
PTCPF = % CP in forage or basal diet
In most practical supplementation settings, associative effects will be minor and not worth considering. However, they will be considered here when sgnificant, perhaps resulting in change in projected forage intake and need for supplemental concentrate.
In some settings MP or CP will be relatively more limiting than energy. However, goats appear relatively more efficient in recycling nitrogen than cattle or sheep, which suggests that the likelihood of MP being more limiting than ME is at least slightly less for goats. Also, it is important to note that with low-quality forage, both ME and ME may be limiting, and frequently supplemental concentrate given to correct a ME deficiency will be more than adequate to also correct for a MP shortfall. Nonetheless, this should be tested, with use of whichever estimate of supplemental concentrate used (i.e., based on ME vs MP) that is greater. The corresponding estimate of forage intake is employed as well.
If the combination of the particular animal forage, and supplemental concentrate conditions results in an unrealistically high proportion of concentrate in the diet, then a change in one of these three factors is warranted. For example, perhaps the forage is simply too low in quality whatever the supplemental concentrate for relatively high animal requirements for ME and(or) MP. Or, a supplemental concentrate with a much higher level of ME and(or) MP could be warranted.
In this regard, this calculator also displays the concentrations of MP and CP in the supplement necessary to exactly meet the need when the amount of supplement is being based on ME. Likewise, the optimal ME concentration in the supplement is listed when the supplement amount was determined by the MP requirement. If no value is listed in a box then supplemental concentration was based on this factor (ME or MP) or, forage alone without supplemental concentrate satisfied the requirement.
In addition to calculation of MP intake, the requirement for and intake of rumen degraded intake protein (DIP) are estimated. In most instances, however, because MP intake is determined as the sum of microbial protein and feed protein passing from the rumen intact, when MP intake is adequate so too will intake of DIP be.
To estimate the dietary ME concentration of the basal forage as well as supplemental concentrate, often feed tags list the Total Digestible Nutrient (TDN) concentration. Likewise, most commercial feed laboratories estimate the TDN concentration based on analyses, such as for crude protein (CP) and various fiber fractions.
As an example of this calculator, the following conditions can be used.
|Forage MEC (MJ/kg)
|MP: Forage CP (%)
|Concentrate TDN (%)
|Concentrate CP (%)
|Forage UIP (% CP)
|Concentrate UIP (% CP)
The feed intake calculator with optional adjustments should be used. With 5.5% CP in the basal forage, the amount of supplement is based on MP rather than ME. Total ME intake is 8.03 MJ, which is slightly greater than the requirement of 7.95 MJ. Hence, the optional ME concentration in the supplement is 11.00 MJ, slightly less than the assumed concentration of 12.08 MJ/kg. Very importantly, since the basal forage had a CP concentration of 5.5% (less than 6%) and the supplement CP level was above 15% (i.e., 20%), the supplement had a positive associative effect on basal forage intake (initial projected intake of 1.62% BW without supplementation compared with 1.79% BW when the supplement was given).
Conversely, with a basal forage CP concentration of 7%, the amount of supplement is based on ME rather than MP. MP intake now is slightly greater than the requirement (i.e., 55.24 vs 43.11 g). This results in optimal MP and CP concentrations in the supplement markedly less than assumed. This example brings out the importance of knowing forage composition in order to design appropriate supplementation strategies.