Recently Determined Requirement Recommendations

Based on the aforementioned database of treatment mean observations, metabolizable protein (MP) requirements of goats were estimated. To do so, the sum of ruminal outflow of feed and microbial protein digested in the small intestine (MPI) was predicted with an approach similar to that of AFRC[2] and NRC (Ref. 6, Level 1). However, because ruminal digesta passage rates in goats, relative to other ruminant species, have not been well characterized, and also passage rates vary with level of feed intake, extents of ruminal digestion of soluble and insoluble true protein potentially digested in the rumen were based on ruminal digesta passage rates (e.g., retention time) and rates of protein degradation. Fluid passage rate was determined as a function of particulate passage rate from an equation derived for cattle,[9] and particulate passage rate was estimated from an ARC[10] equation with the independent variable of level of feed intake relative to MEm.

The MP for maintenance (MPm) of non-Angora goats was calculated as the sum of metabolic fecal (2.67% of DM intake for diets not containing appreciable browse),[11] endogenous urinary (1.031 g/kg BW075)[12] and scurf crude protein (CP) losses (0.2 g/kg BW0 6, based on beef cattle data)[13] assuming an efficiency of MP use for maintenance protein of 1.0. To determine MP available for use in lactation (MP[), MPI was adjusted for BW change assuming a tissue protein concentration of 14.3% and efficiency of MP use for gain of 0.59.[2] A no-intercept regression of MPl against milk protein yield resulted in an MP requirement of 1.45 g/g milk protein, equivalent to a milk protein efficiency of 0.69.[14]

Accuracy of MP requirement expressions may be enhanced by research with goats concerning most important factors influencing MPI, notably feedstuff CP degradation properties, fluid and particulate passage rates, and ruminally fermentable energy (RFE) concentrations in feedstuffs. Another consideration in need of attention is consumption of plants with antiquality factors, such as condensed tannins, that may impact behavior of feedstuff constituents in the digestive tract as well as maintenance protein and energy losses.

Goats appear capable of recycling relatively high amounts of nitrogen;[2] therefore, research should be conducted to determine dietary CP levels and the ratios of CP:RFE below which ruminal availability of nitrogenous compounds, rather than RFE, limit microbial growth. Finally, accuracy of MEg and MPg determined by regressions against ADG is influenced by tissue concentrations of energy and protein, respectively, indicating the desirability of study of the composition of tissue accreted by goats as well as mobilized.

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