Accumulation of Digestible Materials and Decrease in Dry Matter Digestibility in the Growth of Two Tropical Grasses

This study was conducted to analyze the accumulation of digestible materials from digestible materials present in forage and new photosynthates and to analyze the decrease in dry matter digestibility with growth of two tropical grasses, using the following two equations. 1/W * dD/dt = {(1D + dW/dt)*(dD/dt)}*(D + dW/dt)/W, (A) D_1/W_1 - D_2/W_2 = (1- W_1/W_2)*(D_1/W_1 - ARD/RGBw), (B) where W = dry weight of forage (W_1 ≠ W_2), D = dry weight of digestible materials (D_1 ≠ D_2), dW/dt = new photosynthates, (D+dW/dt) = amount of source materials [S] for D accumulation, (1/W). (dD/dt) = accumulation rate of D per unit W [ARD], (1/(D+dW/dt))· (dD/dt) = accumulation rate of D per unit S [ARDS], (D+dW/dt)/W = the ratio of S to W [S ratio], RGRw = mean relative growth rate of forage, (1-W_1/W_2) = forage growth index [FG index], (D_1/W_1 - ARD/RGRw) = index for the decrease in dry matter partition into digestible materials [DDMPD index], D_1/W_1 - D_2/W_2 = the decrease in dry matter digestibility [DDMD]. Two equations were applied to dallis grass (Dg) and bermuda grass (Bg) cut at 20, 59 and 80 days of regrowth. Each attribute in two equations was given a bar on it to show mean value over the 1st interval of 39 days and over the 2nd interval of 21 days, respectively. The following results were obtained: 1. Dg: Higher ARD in the 1st interval compared to the 2nd interval was due to higher ARDS in the 1st interval. Similar DDMD between two intervals was due to the offset effect between FG index and DDMPD index. 2. Bg: Lower ARD in the 1st interval than in the 2nd interval was due to lower ARDS in the 1st interval. The larger DDMD in the 1st interval compared to the 2nd interval was due to higher DDMPD index in the 1st interval. 3. 1st interval: Higher ARD in Dg than in Bg was mainly due to higher ARDS in Dg. Almost the same DDMD between Dg and Bg was due to the offset effect between FG index and DDMPD index. 4. 2nd interval: Lower ARD in Dg than in Bg was due to lower ARDS in Dg. The larger DDMD in Dg compared to Bg was due to higher DDMP index in Dg. It was suggested that the proposed method gave macro analyses of the accumulation of digestible materials and the decrease in dry matter digestibility with growth of tropical grasses