The MP requirement for lactation is based on estimates of, and assumptions used by, Nsahlai et al. (2004). The MP requirement was derived via a factorial approach in which MP required for lactation was partitioned from that required for maintenance and tissue growth and mobilized tissue protein used for milk protein synthesis. The maintenance MP requirement is based on estimates of metabolic fecal (Moore et al., 2003) and endogenous urinary CP (Luo et al., 2004), as well as the scurf protein loss of beef cattle (NRC, 1984). In addition, it was assumed that the efficiency of MP use for these maintenance proteins is 100% (AFRC, 1998).

BW is necessary to determine MP required for endogenous urinary and scurf CP. Conversely, metabolic fecal CP is predicted from DM intake. The simplest way to address DM intake is expression as a % BW.

For example milk concentrations noted by Nsahlai et al. (2004) for different goat breeds and stages of lactation, click on this button:

Our example would require 74.9 g MP for maintenance (53.4 g for metabolic fecal CP + 19.4 g for endogenous urinary CP + 2.1 g for scurf CP), which equates to a dietary MP concentration of 3.7% of DM consumed. To estimate the MP requirement for milk production, required inputs are kg of milk and milk protein concentration. As an example, if a doe produces 3 kg of milk with 3% protein, the MP requirement for lactation is 130.5 g (3 kg × 3% milk protein × 1.45 g MP/g milk protein). If there is no change in BW, the total MP requirement is 205.4 g (74.9 g for maintenance + 130.5 g for lactation), or 10.3% of DM intake.

In this example, it was assumed that BW change or average daily gain (ADG) was 0 g/day. In other words, neither use of dietary MP for tissue accretion nor use of mobilized tissue protein for milk production have been considered yet. A doe gaining weight will require more MP than based simply on milk production and protein concentration. Likewise, a doe losing BW will require less dietary MP for a given quantity and composition of milk than predicted above.

For an increase in BW, positive ADG (g) is multipled by 14.3% (AFRC, 1998) and then divided by an efficiency of 0.59 (AFRC, 1993) to estimate dietary MP used for tissue accretion (e.g., 20 g ADG × 0.143 / 0.59 = 4.8 g/day of dietary MP required for this 20 g ADG). This added to the total requirement for 0 ADG yields a total MP requirement of 210.2 g for +20 g ADG.

For a decrease in BW, negative ADG (g) is multiplied by 14.3% protein in tissue to estimate mobilized tissue protein (2.9 g). With tissue mobilization to support lactation, even though the dietary MP supply was adequate for the maintenance MP need, MP was partitioned to the mammary gland for lactation. Therefore, mobilized tissue protein is subtracted from the maintenance MP requirement to estimate true dietary MP used for maintenance (i.e., 74.9 – 2.9 = 72.0 g). The next step is to estimate milk protein arising from dietary MP and mobilized tissue protein. Total milk protein is calculated by multiplying milk yield by milk protein concentration (3 kg × 3% protein = 90 g). Milk protein originating from mobilized tissue protein is estimated assuming an efficiency of MP use for lactation (0.69 or 1.45 g MP required per 1 g of milk protein; for both dietary MP and mobilized tissue protein) (2.9 g × 0.69 = 2.0 g), which is subtracted from total milk protein to derive milk protein from dietary MP (90.0 – 2.0 = 88 g). The requirement of dietary MP is then determined as 88 g / 1.45 g MP/g milk protein = 127.6 g. The total requirement is estimated by summing requirements for maintenance and lactation (72.0 + 127.6 = 199.6 g).

MP requirements are preferable compared with CP because they consider how feedstuffs vary in the extent to which proteins are degraded in the rumen (or the extent of passage to the small intestine of intact feed protein) and dietary and animal characteristics that impact the quantity of microbial protein that is synthesized in the rumen and flows to the small intestine.

However, in many instances there may not be adequate knowledge about these factors to directly predict the amount of MP resulting from a given level of consumption of a particular diet. If such information is known, then the Calculator entitled "Metabolizable Protein (MP) Intake Based on Estimates of Ruminally Undegraded Protein (UIP) and Microbial Protein Synthesis" can be employed. If not, then a simple means of determining how requirements for MP relate to those for CP can be used. In this regard, NRC (2000) suggested that MP requirements can be reasonably well translated or projected to CP needs for most practical purposes with some assumptions regarding the extent of ruminal degradation of dietary CP. It was suggested that CP requirements can be determined through dividing MP needs by values from 0.64 to 0.80, which apply to diets with 0 and 100% rumen undegraded protein (UIP), respectively. Typically, diets with 0 or 100% of CP degraded (or undegraded) in the rumen are not fed. Thus, CP requirements have been calculated from MP for diets containing CP that is digested in the rumen (DIP; degraded intake protein) with extents of 80, 60, and 40%, which
equate to concentrations of UIP of 20, 40, and 60%, respectively. A diet with 20% UIP would probably be one of fresh forage that typically has CP extensively degraded in the rumen. A diet with 40% UIP might be one with a mixture of concentrate (e.g., high level of corn) and forage. A diet with 60% UIP would have a moderate to high level of concentrate, and perhaps
would contain some feedstuffs such as blood, feather, fish, or corn gluten meals that have considerable protein
passing from the rumen intact. Likewise, pelletizing usually increases the dietary UIP concentration.

Based on the example above with 20 ADG, the MP requirement assuming DM intake of 2.0 kg equates to CP requirements of 312.8, 298.6, and 285.6 g, and 15.6, 14.9, and 14.3% of DM intake for diets with 20, 40, and 60% UIP, respectively.