Our example would require a total of 44.9 g MP for maintenance (26.7 g for metabolic fecal CP + 16.4 g for endogenous urinary CP + 1.8 g for scurf CP), which equates to a dietary MP concentration of 4.5% of DM.

The maintenance MP requirement for mature (≥ 1.5 years of age) meat (≥50% Boer), dairy (selected for milk production, such as Alpine and Saanen), and indigenous or local goats, non-lactating and lactating, is based on estimates of metabolic fecal (Moore et al., 2004; 0.0267 g/g DM intake) and endogenous urinary crude protein (Luo et al., 2004; 1.031 g/kg body weight^0.75), as well as the scurf protein loss of beef cattle (NRC, 1984; 0.2 g/kg body weight^0.6). In addition, it was assumed that the efficiency of MP use for these maintenance proteins is 100% (AFRC, 1998).

Body weight (BW) is necessary to determine MP required for endogenous urinary and scurf proteins. Conversely, metabolic fecal CP is predicted from DM intake. The simplest way to address DM intake is expression as a % of BW, which can be entered directly or derived from a feed intake calculator. However, feed intake prediction equations were derived with goats not in the last 56 days of gestation. Therefore, use of this calculator for goats in late gestation should be with caution.

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 know, 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, the requirement for 44.9 g of MP equates to CP requirements of 66.8, 63.8, and 61.0 g, and 6.7, 6.4, and 6.1% of dietary DM, for 20, 40, and 60% UIP diets, respectively. These values reflect that, as long as ruminally available CP is adequate for normal microbial protein growth and digestion, CP requirements decrease as the dietary UIP concentration increases.

The calculations above were for no change in the body protein content other than possibly for gestation. However, often mature goats decrease in BW, such as with low feed availability during dry seasons. Similarly, after a period of appreciable BW loss, including proteineous tissue (lean) in addition to adipose (fat), with a higher nutritional plane BW and protein stores can be replenished. Therefore, this calculator also is fitted to estimate reduced maintenance MP needs during periods of BW loss as well as MP requirements for BW gain by goats, which does not pertain to pregnancy tissues. This is achieved by assuming 14.3% protein in tissue loss or gain, an efficiency of mobilized tissue protein for maintenance of 1.0 (AFRC, 1998). The requirement of MP for BW gain was determined by assuming 14.3% protein in tissue gained and an efficiency of MP use for tissue protein accretion of 0.59% (AFRC, 1998).