Effect of Stage of Maturity on the Chemical Composition and In Vitro Digestibility of Sorghum Grain

Transcription

1 Effect f Stage f Maturity n the Chemical Cmpsitin and In Vitr Digestibility f Srghum Grain C.A. Hibberd, D.G. Wagner and R.L. Hintz Stry in Brief Dwarf Redlan (waxy), Redlan (nrmal) and Darset (bird-resistant) varieties f grain srghum were cilected at weekly intervals starting 35 days preharvest. The early maturing Darset (BR =Bird-resistant) variety cntained 78.6 percent dry matter n day 35 while the Dwarf Redlan (waxy) was much less mature (58.6 percent dry matter) at the same pint, indicating black layer frmatin and physilgical maturity had nt yet ccurred fr Dwarf Redlan. Mrever, dry seed weight did nt plateau until day 28 preharvest fr the Dwarf Redlan althugh the earlier maturing Darset (BR) and Redlan (nrmal) varieties had reached final dry matter depsitin bv day 35 preharvest. Dry matter depsitin, as measured by dry seed weight, was essentiaily cmplete when the dry matter cntent f the grain reached 70 percent. Althugh the cncentratin f starch, ash and crude prtein was very similar frm day 35 thrugh harvest fr all three varieties, changes in sluble prtein (Landry-Mureaux Fractin I) and tannin cntent cntinued thrughut maturity, suggesting that physilgical changes in the kernel may cntinue even after dry matter depsitin is cmplete. In vitr dry matter disappearance (IVDMD) f the Dwarf Redlan (waxy) remained unaffected thrugh maturity. Relative digestibility (IVDMD) f the Darset (BR) variety, hwever, cntinued t increase as maturity apprached and as tannin levels decreased. These studies suggest that the success f early-harvesting srghum grain may be partiaily dependent n the particular variety that is utilized. Intrductin Undergrund water depletin in many areas, saring csts f irrigatin water and recent drught cnditins in the Suthern Great Plains have increased the appeal f srghum grain as a prductin alternative. Mre livestck prducers in Oklahma wiil be faced with the pprtunitv r necessity f using srghum grain in their feeding prgrams. In rder t insure adequate utilizatin f srghum grain, sme frm f prcessing is required. Cnfrtunately, srghum grain is a highly variable prduct due t numerus factrs such as variety, surce, endsperm type and envirnmental cnditins during grwth. This variatin may influence the success f srghum grain prcessing effrts. One energy etlicient grain prcessing alternative f current interest is t harvest grain befre maturity (abut 30 percent misture) and stre the material in a sil. Little is knwn, hwever, f the physilgical changes that ccur as the kernel matures, especiaily with BR srghums. Cnsequently, the bjective f this study was t mnitr changes in chemical cmpsitin and in vitr digestibility f several varieties f srghum grain as they prgess thrugh maturity. 178 Oklahma Agricultural Experiment Statin

2 Materials and Methds Three varieties f srghum grain crable I) were grwn and harvested under similar cnditins at the Suthwestern Livestck and Frage Research Statin, EI Ren, Oklahma. Grain samples were cllected at weekly intervals starting abut 5 weeks preharvest. Each sample was hand-threshed, cleaned and grund thrugh a 20-mesh screen in a labratry Wiley mill. Starch cntent was measured as a-linked glucse plymers using an enzymatic methd, and tannin cntent was determined by a mdified vanillin-hci assay. Crude prtein was determined by Kjeldahl analysis and sluble prtein by the first stage f the Landry-Mureaux Fractinatin Sequence D. Relative digestibility f each sample was estimated by an in vitr dry matter disappearance (IVDMD) technique. Grund grain samples (.4 g dry matter) were placed in 50-ml centrifuge tubes and inculated with buffered rumen fluid (15 ml McDugall's bufler: 15 ml strained rumen fluid) frm a cncentrate (80 percent crn) fed steer. After 18 hurs f incubatin at 39 C (102 F), the samples were centrifuged, decanted and dried. Percent IVDMD was calculated by difference. Table 1. Descriptive characteristics f srghum grain Endsperm Seedcat Testa" starch Variety clr layer clr type Dwarf Redlan red absent white waxyb Redlan red absent white nrmalc Darset brwn present white nrmal "Presence f testa layer indicative f high tannin le1vels. "Waxy starch cntains essentially 100% amylpectin. cnrmal starch cntains abut 75% amylpectin and 25% amylse. Classificatin waxy nrmal bird-resistant Results and Discussin The three varieties used in this study appeared t mature at different rates (Figure I). The Darset (BR) srghum matured the earliest (78.6 percent dry matter n day 35 preharvest) while the Dwarf Redlan (waxy) matured much mre slwly (58.6 percent dry matter n day 35). This pattern is als reflected in dry seed weight (Figure I). The dry weight f 100 kernels fr the Darset and Redlan varieties had reached harvest levels by day 35 preharvest, except fr a slight deviatin fr the Redlan n day 21 preharvest, suggesting that the depsitin f seed cmpnents (starch, prtein, etc.) was cmplete. The slwer maturing Dwarf Redlan, hwever, increased dry matter depsitin until day 28 preharvest. These bservatins reinfrce the thery that the depsitin f dry seed cmpnents is cmplete when the dry matter f the kernel reaches abut 70 percent. The cncentratin (percent f dry matter) f starch and ash in each f the three varieties was very similar frm day 35 preharvest t maturity (Figure 2). Crude prtein cntent (percent f dry matter) was als fairly cnstant during this perid (Figure 3). In cntrast, the cncentratin f Landry-Mureaux Fractin I prtein (highly sluble albumins and glbulins) appeared t decrease during maturatin fr the Darset and increase fr the Redlan (Figure 3). Althugh ttal prtein depsitin may be cmplete, changes in the physilgical characteristics f that 1981 Animal Science Research Reprt 179

3 3 2 -~- ~._.~ -=- - ::::, w l- I- < ~ > DAYS PREHARVEST Figure 1. Effect f stage f maturity (mean:!: S.E.) n dry matter cntent and dry seed weight (Dwarf Redlan -, Redlan-.-, Darset. -) prtein appear t cntinue. Tannin cntent f the Darset (BR) variety decreased between days 35 and 28 pre harvest (Figure 4). If tannin cntent des decrease with maturity, as this study suggests, perhaps bird-resistant (high tannin) srghums shuld nt be harvested t early. The digestibility (IVDMD) f the Darset (BR) variety appeared t increase as maturity prgressed (Figure 4). The decrease in tannin cntent nted earlier may be partially respnsible fr this effect. The lvdmd f the Dwarf Redlan (waxy) was very cnstant frm day 35 preharvest thrugh maturity as was the Redlan variety except fr an unexplainable depressin n day 20 pre harvest. Cnsequently, the digestibility f sme varieties (Darset) may be affected by physilgical changes in certain kernel cmpnents, i.e., tannin, while digestibility f ther srghum grain varieties remain unchan~ed. 180 Oklahma Agricultural Experiment Statin

4 72 70 '\ :I: 0 II: 66 c( l- I/) T_._ ~ _ : ::::,~.-X DAYS PREHARVEST Figure 2. Effect f stage f maturity (mean :t S.E.) n starch and ash cntent (Dwarf Redlan -, Redlan...,Darset...) 1981Animal Science Research Reprt 181

5 z W t- ~ w :) (,) 8 6L:E- 4L ~--~ ~- f /I +.-.~. " DA YS PREHARVEST Figure 3. Effect f stage f maturity (mean :t S.E.) n crude prtein and Landry-MureauxFractin I prtein (Dwarf Redlan-, Redlan ---, Darset - - -) 182 Oklahma Agricultural Experiment Statin

6 DAYS PREHARVEST Figure4. Effectf stage f maturity(mean :f:s.e.) n tannin cntent and IVDMD (Dwarf Redlan -, Redlan -'-,Darset ---) High-misture harvest f srghum grain is nrmally initiated when the grain reaches abut 30 percent misture. These studies indicate that this practice is sund, at least fr nrmal and waxy srghums. The tannin cntent f the Darset variety declined in early maturity, suggesting that early harvesting f birdresistant srghums may nt be feasible. Hwever, in previus studies f recnstitutin, fermentatin f srghum grain appeared t either inactivate r destry mst f the tannin in srghum. If ne can insure adequate fermentatin f early harvested bird-resistant srghum during ensiling, this cncern may be f little cnsequence Animal Science Research Reprt 183