ACCUMULATION OF DIGESTIBLE MATERIALS AND DECREASE IN DRY-MATTER DIGESTIBILITY IN THE GROWTH OF 2 TROPICAL GRASSES

This study was conducted to analyze the accumulation of digestible mat erials from digestible materials present in forage and new photosyntha tes and to analyze the decrease in dry matter digestibility with growt h of two tropical grasses, using the following two equations. 1/W . dD /dt = (1/D+dW/dt . dD/dt) . D+dW/dt/W, (A) D-1/W-1 - D-2/W-2 = (1 - W- 1/W-2) . (D-1/W1 - (ARD) over bar/RGR(w), (B) where W = dry weight of forage (W-1 not equal W-2), D = dry weight of digestible materials (D- 1 not equal D-2), dW/dt = new photosynthates, (D+dW/dt) = amount of so urce materials [S] for D accumulation, (1/W).(dD/dt) = accumulation ra te 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], RGR (w) = mean relative growth rate of forage, (1 - W-1/W-2) = forage grow th index [FG index], (D-1/W-1 - (ARD) over bar/(RGR) over bar(w) = ind ex 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 digestib ility [DDMD]. Two equations were applied to dallis grass (Dg) and berm uda grass (Bg) cut at 20, 59 and 80 days of regrowth. Each attribute i n 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 <(ARDS)over bar> in the 1st interval compared to the 2nd interval was due to higher <(A RDS)over bar> 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) over bar in the 1st interval than in the 2nd interval was due to lower <(ARDS)over bar> 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) over bar in D g than in Bg was mainly due to higher <(ARDS)over bar> in Dg. Almost t he same DDMD between Dg and Bg was due to the offset effect between FG index and DDMPD index. 4. 2nd interval: Lower (ARD) over bar in Dg th an in Bg was due to lower <(ARDS)over bar> in Dg. The larger DDMD in D g compared to Bg was due to higher DDMPD index in Dg. It was suggested that the proposed method gave macro analyses of the accumulation of d igestible materials and the decrease in dry matter digestibility with growth of tropical grasses.