Requirements for MP for
maintenance and growth of growing (≤ 1.5 years of age)
meat (50% Boer), dairy (selected for milk production, such
as Alpine and Saanen), and indigenous or local goats were
determined by multiple regression analyses (Luo et al., 2004).
The maintenance requirement via this method (3.07 g MP/kg
body weight0.75) is similar to that determined by the factorial
approach for mature goats. There was a difference among biotypes
in the MP requirement for gain or growth (0.409 g MP/g average daily gain for meat goats and 0.290 g MP/g average daily gain for dairy and indigenous goats). Thus, inputs necessary
to estimate MP needs for maintenance and growth of growing
goats are biotype, body weight (BW), and average daily gain
Little research has been conducted regarding how
body weight loss affects MP requirements. Nonetheless, with
assumptions of AFRC (1998) regarding the protein concentration
in live weight gain (Protein % = 15.722 - (0.069 x body weight
in kg)) and an efficiency of use of mobilized tissue protein
for maintenance of 1.0, estimates for BW loss can
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 examples above, the MP requirement for the Spanish wether, assuming DM intake of 3.25% of BW, equates to CP requirements of 83.5, 79.7, and 76.2 g, and 10.3, 9.8, and 9.4% of DM intake for diets with 20, 40, and 60% UIP, respectively. The MP requirement for the Boer wether, assuming DM intake of 3.5% of BW, equates to CP requirements of 133.8, 127.7, and 122.1 g, and 12.7, 12.2, and 11.6% of DM intake for diets with 20, 40, and 60% UIP, respectively.