Relationships between Starch and Physically Effective and Undegraded Fiber in Lactating Dairy Cows K. M. Smith1, R. J. Grant1, and A. Obata2 1William H. Miner Agricultural Research Institute, Chazy, NY 2Zennoh National Federation of Agricultural Cooperative Associations, Tokyo, Japan Introduction For several years we have been working at the Institute on how to integrate measures of fiber (un)degradability and particle size in an effort to better predict dry matter intake (DMI) and energy-corrected milk (ECM) production (Grant et al., 2018). To-date, we have focused mainly on physically effective neutral detergent fiber (peNDF) and undegradable NDF at 240 hours of in vitro fermentation (uNDF240). The resulting value – termed physically effective uNDF240 (peuNDF240) - can be calculated simply as the physical effectiveness factor (pef) multiplied by uNDF240, or perhaps more accurately over a wide range of diets, as a direct in vitro measure of uNDF240 on the pef fraction of particles (more about this topic later). The pef is measured by sieving the total mixed ration (TMR) sample: either using a 1.18-mm sieve when dry, vertical sieving (Mertens, 1997) or using a 4.0-mm sieve when horizontally sieving as-fed samples on the farm. At least for corn silage and haycrop silage-based TMR, using the Penn State Particle Separator with a 4.0-mm sieve yields similar pef values as the standard dry sieving method with a 1.18-mm sieve (Schuling et al., 2015). The objectives of this paper are to briefly review the progress to-date on integrating pef and uNDF240 to better predict DMI and ECM, and to present the lactation results from a recently completed study that investigated the interaction between dietary peuNDF240 and rumen fermentable starch (RFS). Physically Effective Undegradable NDF Miller et al. (2020) assembled a 5-study database from experiments using high- producing Holstein dairy cows at Miner Institute conducted between 2014 and 2019 to assess the relationship between uNDF240 and peuNDF240 with DMI and ECM. Details are provided in the abstract and the accompanying presentation from the 2020 American Dairy Science Association (ADSA) virtual annual conference (https://virtual2020.adsa.org/). Within this database, the range in dietary uNDF240 was 5.5 to 11.5% of ration dry matter (DM) and the range in peuNDF240 was 4.0 to 7.3 % of DM. This range in NDF undegradability spans what is commonly fed in the US with values of 10.0 to 11.5% more likely to limit DMI and values closer to 5 to 6% increasing the risk for subacute ruminal acidosis. The relationship between uNDF240 and DMI (lb/d) was moderate (y = -0.84x + 68.18, R2 = 0.32), but the relationship between peuNDF240 and DMI was stronger (y = - 2.16x + 72.42, R2 = 0.60). In particular, combining pef and uNDF240 allowed a better Tpher eredlatiicontsiohipn b eotwf eDenM uNID wF2h40e annd h DiMgI h(lbe/dr) wuaNs mDoFde2ra4te0 ( y d= ineetgsat ivwe e0.r8e4x m+ 6o8.r1e8, (fRin toe thlye 2c phowoepr)p =e 0.d32. ),O buut trh er reelsateioansrhciph b etween peuNDF240 and DMI was stronger (y = negative 2.16x + 72.42, (R to the 2 power) = 0.60). In particular, combining pef and uNDF240 allowed at boe-ttdera ptreed icstiuong ogf DeMsIt wsh etnh haigth ewr uhNeDnF2 4fo0 driaetgs ewe rfeib meorre dfinieglye cshotipbpeildi.t yO uirs re sloeawrche tro -dthatae snu gdgeessts itrheatd w,h ean ffoirnageer f ibfeor rdaiggesetib ility isp loawretri cthlaen desiriezde, a fiwneri lflo raegen pharaticnlec seiz e wDill MenhIa ncaen DdM I anEd CECMM propdruoctdionu. cThtieo imnp.r ovTedh laec tatiiomnapl preorfovremdan cel apcpetartsi onal top bee rafsosorcmiataedn wciteh leasps epaetinag rtsim et oan bd ae m aorses doesciriaabltee rudm wen iftehrm leenstastio ne aantdi nfibger ttuimrnoeve ra fonr dco aws mfedo hrigeh edr ueNsDiFr2a4b0 ldeie tr wuitmh en finer chop length. fermentation and fiber turnover for cows fed higher uNDF240 diet with finer chop length. The relatTiohnesh ripe lbaettiwoneesnh iupN bDeFt2w4e0 eann du ENCDMF (2lb4/0d) awnads sEtrConMg (y(l b=/ dne) gwataivse 2s.t2ro6nx g+ 1(y2 6=.3 8-,2 (.R2 6tox th+e 2 p1o2w6e.r3) 8=, 0R.528 )=, b0u.t5 s8im),i lbaru to s DimMiIl,a trh eto r eDlaMtioIn, sthiep bretlwateioen spheiupN bDeFt2w4e0e ann dp EeCuNMD (lFb/2d4) 0w aasn edv eEnC sMtro nger th(labn/ dth) awt aosb seevrveend s fotrro unNgDeFr 2th4a0 n(y t h=a nte ogbastieverv 4e.d92 fxo r+ u 1N33D.1F42, 4(R0 (toy t=h e-4 2. 9p2oxw e+r )1 =3 30.1784),. RA2 f i=el d0 .78). sAtu dfyie rledp osrttuedy b yr eGpeoisrteer da nbdy G Goesiseer (r2 a01n9d) uGsoinegs 5e5r c(o2m01m9e)r cuiasl idnagi ry5 5fa rcmosm wmherrec icaolr nd asiilrayg efa crommsp rised 3w6.h8e ±r e7 .c9o%r no fs tihlaeg reat icoonm DpMr isfoeudn d3 t6h.a8t ±a o7n.9e%-un oitf i nthcere arastei oinn uDNMDF f2o4u0n dof tthhae tc ao ronn seila-ugne itw iansc rease aisns oucNiaDteFd2 w4i0th oaf 0t.h5e9 lcbo/dr nd escirleaagsee wina DsM aI sasnodc aia 1t.e3d0 lwb/idth r ead u0c.t5io9n libn /EdC dMe.c Irne tahsee I nisnt itDutMe Id aatna d a b1a.s3e0, wlbe/ dob rseedrvuecdt ioa nre idnu cEtiConM o.f I0n. 8t4h elb /Idn soft iDtuMteI adnadt a2 .3b albs/ed ,o wf EeC oMb wseitrhv eeadc ha orneed-uuncitti oinnc roefa s0e. 84 inlb r/adti oonf DuNMDIF a2n4d0 2w.i3th lhbi/gdh -o pf rEoCduMci nwgi tcho wesa c(Mh iollenre e-tu anl.i,t 2in0c2r0e)a. Sseo, itnh eraret iiosn g eunNeDraFl 2ag4r0e ewmitehn ht igh- bpertwodeeunc ionugr cInoswtitsu t(eM dialletarb eats ea la.,n 2d 0th2i0s )f.ie Sldo s, ttuhdeyr we hisic hg egniveersa ul sa cgorenfeidmenecnet tbheattw theeesne oreular tIionnsstihtuiptse adrea tcaobnasissete nat nadn dt hcaisn bfiee luds esftuul diny thweh fiieclhd .gives us confidence that these relationships are consistent and can be useful in the field. We need to note that the diets in this database were primarily based on corn silage and haycrop silage with some chopped hay and straw. Importantly, there were no pure alfalfa diets, diets with larger amounts of non-forage fiber sources, or pasture. In the future, we intend to define the relationships between uNDF240, peuNDF240, and DMI and ECM for a wider range of diets and management scenarios. Nonetheless, there appears to be value in integrating two measures of fiber - uNDF240 and pef – when formulating rations. Interactions between Physically Effective uNDF240 and Rumen Fermentable Starch Our most recent work has evaluated the relationship between dietary peuNDF240 and RFS (Smith et al., 2020). Initial studies were focused mainly on the middle to upper range of dietary uNDF240 concentrations to determine at what point DMI was constrained and how manipulating particle size affected DMI at a given uNDF240 content (Grant et al., 2018). In contrast, the study by Smith et al. (2020) was designed to determine the interaction between dietary starch (specifically RFS) and uNDF240 for diets that were on the lower end of the uNDF240 range commonly observed in the field. Consequently, the research focus shifted from gut fill and DMI constraints to maintenance of adequate dietary fiber and minimizing the risk of subacute rumen acidosis. The negative associative effect of starch on rumen fiber degradation and peNDF requirements is well known. Mertens and Loften (1980) were the first to observe that too much starch resulted in lengthened lag times prior to NDF degradation in vitro. Subsequent work showed that, as rumen starch fermentability increased, the negative effect on the lag and fractional rate of NDF degradation increased and lower rumen pH amplified this negative effect of starch (Grant and Mertens, 1992; Grant, 1994). However, we still need to understand how dietary starch content and RFS influence rumen NDF turnover in diets that differ in their fiber characteristics such as uNDF240, peuNDF240, and fast- and slow-degrading NDF (measured using 30-, 120-, and 240-h in vitro fermentations). Details oDf tehtea siltsu doyf bthye S mstiuthd eyt bayl. (S2m02i0th) aerte aalv. a(i2la0b2le0 )in a thre abvsatirlacbt laen idn atth teh ea AbDstSraAc at nannudal acto nthfeer ence wAeDb SsiAte . aBnrineuflya,l 1c6o lancftearteinngc Heo lwsteibn csoitwes. (8B ruiemfliyn,a ll1y6 fi sltaulcattaetdin) gth aHt woelsrtee ainp pcr oxwimsa t(e8ly ruminally 8f5is ptululas toerd m) itnhuast 1w5e draey as pinp mroilxki mwearte leyn 8ro5ll e±d ,1 b5l odcakeyds biny mpairlikty w, dearyes ei n rmoilllke, da,n bd lmocilk ed by parity, pdroadyusc itnio nm ailnkd, awnedre m usilekd p irno ad urecptiloicna taedn d4 wtimeeres ubys e4d L ainti na Sreqpualicrea dteedsi g4n x. T4h Lea sttiund Sy qhuada r2e8 -dde pseigrinod. s (T18h ed osf tauddayp tahtiaodn , 1208 d-d o f pceolrlieocdtiso n)(.1 A8 fadc toorifa l aadrraapngtaetmioenn,t o1f 0fo udr doieft s cwoallse ucstieodn t)o. eAva lfuaacteto rial thaerr eafnfegcet mofe dniet toarfy f opueur NdDieFts2 4w0a cso nutseentd, dtoie teavrya lRuFaSte c tohnete entf,f eacntd o thf ediire intateryra pcteiounN. TDaFb2le4 10 lcisotsn tent, thdeie ptarirmya RryF dSie tcaoryn itnegnret,d iaenndts tthhaeti rw ienrtee urasecdti oinn .th Te asbtuledy .1 D liifsfetsre tnhcee sp inri mdiaetrayr yd uieNtDarFy2 4in0g orre pdeieuNntDsF 240 ctohnatte nwt ewreer eu soebdta iinne tdh bey s utusidnyg. a D birffoewrne nmciedrsib i n(l odwieetra preyu uNNDDF2F4204 d0i eotrs )p veeursNuDs Fa2 c4o0nv ceonntiotennatl were coobrnta siinlaegde bhyyb urids i(nhgig ahe br rpoewuNnD mF2id4r0ib d i(elotsw).e Trh pe etwuoN dDieFt2a4ry0 R dFiSe tcso)n vceernstruasti oan sc wonervee onbtitoaninaeld c orn psriimlaagreily hbyyb vraidry in(hg itghhee cro npteenutN oDf fFin2e4ly0 g rdoiuentds )c. orTnh mee atwl too gedtiheetar rwyi thR thFeS s tcaorcnhc ien nthtrea ctioornns were obtained primarily by varying the content of finely ground corn meal together with the silages. The corn meal contained 62% of DM ≤ 0.60 mm when dry sieved with a pef = 0.10. starch in the corn silages. The corn meal contained 62% of DM ≤ 0.60 mm when dry sieved with a pef = 0.10. Table 1. Ingredient composition of diets with varying concentrations of physically effective 240-h undegraded neutral detergent fiber (peuNDF240) and ruminal fermentable starch (RFS). Diets Low peuNDF240 High peuNDF240 Ingredients, % of DM Low RFS High RFS Low RFS High RFS Conventional corn silage - - 47.60 47.60 Brown midrib corn silage 47.60 47.60 - - Timothy hay, chopped 7.94 7.94 7.94 7.94 Wheat straw, chopped 1.59 1.59 1.59 1.59 Corn meal 2.78 7.94 3.57 8.73 Beet pulp pellets 7.14 5.16 6.35 4.37 Concentrate mix 32.95 29.77 32.95 29.77 Table 2 sTuambmlea ri2z e s uthme mchaermiziecasl ctohmep ocshiteiomn iocfa tlh ec foomurp troesaitmioenn t odfi ettsh. eU nfeoxupre cttreedalyt,m theen tw od iceotrsn. silage hUybnreidxsp deicdt neodtl yd,i fftehre a stw mou cho rans asnilaticgipea hteydb irnid thse dir iduN nDoFt 2d4i0ff ecor natesn tm ausc the ay sw earnet ifceidp aoutet d uirnin tgh eir thueN tDriaFl2: 84.06 %c onf DteMn tf oar sc otnhveeyn twionearel v efersdu so 6u.t7 %du orfi nDgM t hfoer thtreia bl:r o8w.n6 %mi dorifb DcoMrn fsoilra gceo (navltehnoutigohn al invietirasl usasm 6p.7le%s uosfe Dd Min froarti othne fo brmrouwlanti omni dhraidb icnodricna steilda g11e. 8(a%lt hanodu g5.h6 %in iotifa Dl sMa mforp cleosn vuesnetidon inal ration afnodrm bruolwatni omni dhriab,d r einspdeiccativteedly ).1 C1o.8n%se qaunedn tl5y,. 6th%e doief taDryM u NfoDrF c2o40n vceonnctieonntaral tiaonn da vberraogwend midrib, 6r.e8s5p%e ocft irvaetiloyn). D CMo fnors tehqeu loewnetlry ,u NthDeF 2d40ie dtaiertys aunNdD 7F.2204%0 o fc DonMc feonr ttrhaet ihoignh earv ueNraDgFe2d40 6d.i8et5s%; of inra ottiohenr DwoMrd fso, rt hthe eu NloDwFe2r4 0u NcoDnFte2n4t w0 adsi eqtusit ea nsidm 7ila.2r 0ac%ro sosf aDllM d iefotsr. tShiem ihlairglyh,e thr eu pNeDuNFD24F024 d0i ets; vianl uoetsh (epre fw tiomredss ,b yt hueN DuFN2D40F)2 w4e0r ec soinmtielanr ta nwda rsa nqgueidte f rosmim 3il.a8r8 atoc 4ro.1s6s% a ollf rdaiteiotns .D SMi.m ilarly, the Fpoer uaNll DdieFt2s,4 t0h ev aulNuDeFs2 (4p0e fa xn du tNheD pFe2u4N0D) Fw2e4r0e v sailmueilsa wr aerned o rna tnhgee lodw feror men 3d .o8f8 t htoe r4a.n1g6e% in o ofu rra 5ti-osntu dy DM. For all diets, the uNDF240 and the peuNDF240 values were on the lower end of the data base. range in our 5-study data base. BecauseB tehec acuoswes trhesep coondwesd rteos dpieotnadrye fdib etor cdhieartacryte rfisbteicrs c (hseaera Tcatbelreis t3ic asn (ds 4e)e, aTnadb ylets t h3e a mneda 4su),r ed uaNnDdF y2e4t0 t ahned m caelacusluarted pueNuNDDFF224400 a(pnedf ctimalecsu blayt eudN DpFe2u4N0D) vFa2lu4e0s (dpide fn xot udNiffDerF m2a4r0k)e dvlay,lu es did wneo td decififdeerd m toa drkireedctllyy, mweea dsuercei dtheed u tNoD dFir2e4c0t lcyo mnceeanstruartieo nth (eus uinNgD aFn 2in4 v0i tcroo fnecrmenetnrtaattiioonn) (iun sing an tihne vfritarcot iofenr mof eenatcaht idoient) tihna tt hwea sfr raectatiionne do of ne athceh ≥ d1i.e1t8 -tmhamt wsieavse r aentad intheed f roanct itohne t h≥a1t .p1a8s-smedm t hsrioeuvgeh tahnisd s iethvee. Ifnratecrteiostnin gthlya, tth ep ausNsDeFd2 4th0 rwoausg nho tt huinsi fosrimevlye d. isItnritbeurteesdt iancgrolys,s tthhee twuoN sDizFe2 f4ra0c tiwonass not ausn hifaodr mbelye nd itshter icbaustee din ascormoes sp rtehveio tuwso r essizeaer cfrha (cGtiroannst eats a hl.,a 2d0 1b8e)e. nT hthe ed icreacstley ians ssaoymede pperueNvDioFu2s4 0 arveesreaagercdh 6 (.2G arandn t8 e.3t% a lo.,f r2a0ti1on8 )D. MT hfoer dthiree lcotwlye ra psesuaNyDeFd2 p4e0 uaNndD hFig2h4e0r paevueNraDgFe2d4 06 d.2ie tasn, d 8.3% roefs praectitoivne lyD. MTh fiso rr atnhgee lionw deirre cptleyu mNeDasFu2r4ed0 paenudN DhFig2h4e0r h peelpusN toD eFx2p4la0in dthieet sa,n irmeaslp reecsptiovneslye.s This range in directly measured peuNDF240 helps to explain the animal responses in Table 3 in Table 3 and 4. However, it does call into question the validity of simply calculating peuNDF240 and 4. However, it does call into question the validity of simply calculating peuNDF240 as apse pf exf utimNeDsF b2y4 u0N iDnF a2l4l 0d iient aalrl yd isectaerny asrcieonsa. rIinos m. Ian nmya innys itnasntacnecse, st,h tihsi ss simimppllee aapppprrooaacchh appears atopp weaorrsk t ow weollr,k b wuet llw, beu tn weee dn eteod btoe baew aawraer et hthaat,t, iiff tthhee uuNNDDF2F4204 i0s niso tn uonti fournmifloy rdmisltyri bduistetdri buted aaccrorosss sth teh epa prtaicrlteic sleiz es ifzraec tfiroancst,i othnesn, tthhee cna ltchuela cteadlc nuulmatbeedr mnuamy nboetr b me aapyp nroopt rbiaete a. Ipnp ardodpirtiioante, . In waed dnieteiodn t,o w bee snpeeecdifi ct oa bboeu ts hpoewc itfhice paebuoNuDt Fh2o4w0 itsh em epaesuuNreDd.F I2n 4th0i si sa rmticelea, swuer ewdil.l uIns et hthise article, twerem ws cilal lucsuela ttehde o tre armsssa yceadl cpueluaNteDdF 2o4r0 a. ssayed peuNDF240. The dietary starch content averaged 20.7 and 24.7% of DM for the high and low RFS diets, respectively. Starch degradability did not differ much across diets, but the RFS content averaged 16.8 and 19.1% of ration DM for the lower and higher RFS diets, respectively. It is important to put these starch measures into context. Although the diets differed by 4 units in starch percentage, the starch and RFS contents were moderate to low compared with many commonly fed diets in much of the US. The fact that the higher RFS diets were only moderately high is important to consider when interpreting the animal responses where negative effects on milk fat percentage were observed with relatively low RFS concentrations (see Table 4). Assessment of the interaction between RFS and fiber may be especially important with lower fiber diets with increased risk of subacute rumen acidosis (pH < 5.8; Stone, 2004). Finally, a post-hoc analysis of the intake of dietary carbohydrate fractions was performed using Cornell Net Carbohydrate Protein System (CNCPS) biology (NDS Professional, CNCPS biology v. 6.5, Reggio Emilia, IT) with Kurt Cotanch (Barn Swallow Consulting, LLC, Underhill, VT). This analysis used the ingredient compositional measures and animal measures from the study. Intake of uNDF240 was 2.2, 2.2, 2.5, and 2.4 kg/d for the lower peuNDF240/lower RFS, lower peuNDF240/higher RFS, higher peuNDF240/lower RFS, and higher peuNDF240/higher RFS diets, respectively. In the same dietary order, the intake of RFS was 5.0, 5.6, 5.0, and 5.5 kg/d. The ratio of dietary RFS:uNDF240 was 2.42, 2.82, 2.32, and 2.68 which may potentially have usefulness as a benchmark for milk fat depression (see discussion for Table 4). Table 2. Composition of diets with varying concentrations of physically effective undegraded neutral detergent fiber after 240-h fermentation (peuNDF240) and rumen fermentable starch (RFS). Diets Low peuNDF240 High peuNDF240 Item Low RFS High RFS Low RFS High RFS Dry matter (DM), % 55.3 55.3 54.4 54.2 Crude protein (CP), % of DM 16.1 15.3 16.0 15.2 Soluble protein, % of CP 40.6 39.8 43.4 42.5 aNDFom1, % of DM 33.1 32.4 33.3 32.6 Lignin, % of DM 3.21 3.1 3.5 3.42 Starch, % of DM 20.7 24.6 20.8 24.7 Starch degradability2, % of 80.5 78.1 81.4 77.0 starch Rumen fermentable starch, 16.7 19.2 16.9 19.0 % of DM3 Sugar, % of DM 3.9 4.5 4.7 4.5 Ether extract, % of DM 3.83 3.76 3.81 3.75 uNDF30om, % of DM 13.5 15.2 15.1 15.5 uNDF120om, % of DM 7.5 7.6 8.5 8.5 uNDF240om, % of DM 6.9 6.8 7.3 7.1 pef4 0.60 0.57 0.57 0.57 Calculated peuNDF240 (pef 4.14 3.88 4.16 4.05 x uNDF240), % of DM Assayed peuNDF240om, % 6.35 6.07 8.60 8.00 of DM5 1Amylase- and sodium sulfite-treated neutral detergent fiber, ash corrected. 2The 7-h starch degradability value was measured on the entire total mixed ration. 3Rumen fermentable starch: starch content multiplied by starch degradability. 4Physical effectiveness factor: measured by dry sieving with the 1.18-mm sieve (Mertens, 1997). 5Physically effective undegraded neutral detergent fiber after 240 h of in vitro fermentation, ash corrected. The uNDF240om from composited diet that was retained on ≥1.18-mm sieve. This value is sensitive to differences in uNDF240om distribution across dietary particle size fractions. Table 3 summarizes the intake responses to the diets. There were no interactions between dietary peuNDF240 and RFS on DMI or intake of starch and uNDF240.There was no effect of either peuNDF240 or RFS on DMI in kg/d, but the higher peuNDF diets did slightly reduce DMI as a percentage of BW similarly for both RFS concentrations. The higher RFS diets reduced the intake of aNDFom which reflected the small differences between the diets in aNDFom content (Table 2). As expected, the higher RFS diets increased starch intake by approximately 18 to 20%. Likewise, the higher peuNDF240 diets increased uNDF240 intake by 9 to 14%; the content of dietary RFS also affected uNDF240 intake although the effect was very small. Table 4 summarizes the milk and milk component responses to the diets. The higher peuNDF240 diets reduced milk yield by approximately 1.2 kg/d compared with the lower peuNDF240 diets. The daily yield of 3.5% fat-corrected milk (FCM) and ECM were both reduced by greater RFS content. Although there was no significant interaction between dietary peuNDF240 and RFS, the higher RFS reduced 3.5% FCM by 2.3 kg/d for the lower peuNDF diets versus only 0.7 kg/d for the higher peuNDF diets. It appears that the negative associative effect of RFS on FCM yield was more pronounced with the lower peuNDF240 diet. Again, it is important to remember that the uNDF240 and peuNDF240 (pef x uNDF240) values for all diets were at the lower range (approximately 7 and 4% of ration DM, respectively). Table 3. Dry matter intake (DMI) and carbohydrate intake responses to experimental diets. Diets 1Low peuNDF240 High peuNDF240 P-value Low High Low High peuNDF Starch Variable RFS RFS RFS RFS DMI, kg/d 29.7 29.4 29.4 29.2 0.27 0.40 DMI, % of BW/d 4.31 4.28 4.24 4.20 0.04 0.41 aNDFom2 intake, kg/d 9.9 9.5 9.8 9.6 0.75 0.03 aNDFom intake, % of 1.44 1.39 1.42 1.37 0.37 0.03 BW/d Starch intake, kg/d 6.1 7.2 6.0 7.2 0.74 <0.0001 Starch intake, % of 0.88 1.06 0.87 1.04 0.35 <0.0001 BW/d uNDF240om intake, 2.25 2.16 2.45 2.40 <0.0001 0.008 kg/d uNDF240om intake, % 0.322 0.315 0.354 0.345 <0.0001 0.0078 of BW/d 1There was no significant (P > 0.10) interaction between peuNDF240 and rumen fermentable starch. 2Amylase- and sodium sulfite-treated neutral detergent fiber, ash corrected. Milk fat percentage was greater for the higher peuNDF240 than the lower peuNDF240 diets (Table 4). Similarly, milk fat percentage and daily output were depressed by the higher RFS versus the lower RFS diets. There was no significant interaction between peuNDF240 and RFS, although it is useful to note that numerically the highest milk fat percentage was for cows fed the higher peuNDF/lower RFS diet and the lowest milk fat percentage was with cows fed the lower peuNDF240/higher RFS diet. A negative associative effect existed between peuNDF240 and RFS that expressed itself in reduced milk fat. Overall, milk fat percentage was lower for all diets in this study compared with the typical milk fat percentage for the Institute dairy herd of approximately 4.0%. This general depression in milk fat likely reflected the lower uNDF240 and calculated peuNDF240 for all diets. Table 4. Milk and milk component responses to experimental diets. Diets Low peuNDF240 High peuNDF240 P-value1 Low High Low High peuNDF Starch Variable RFS RFS RFS RFS Milk, kg/d 53.1 52.0 51.2 51.5 0.01 0.35 3.5% FCM2, kg/d 53.8 51.5 52.9 52.2 0.85 0.01 ECM3, kg/d 53.4 51.5 52.5 51.9 0.56 0.02 Fat, % 3.59 3.48 3.74 3.60 0.05 0.06 Fat, kg/d 1.90 1.79 1.90 1.84 0.41 0.01 True protein, % 2.83 2.87 2.85 2.86 0.61 0.12 True protein, kg/d 1.50 1.48 1.45 1.47 0.02 0.94 Lactose (anhydrous), 4.57 4.57 4.59 4.61 0.04 0.58 % Lactose (anhydrous), 2.43 2.38 2.35 2.37 0.09 0.60 kg/d Urea nitrogen, mg/dL 12.0 10.1 12.4 10.5 0.08 <0.0001 De novo FA4, g/100 g 0.80 0.76 0.81 0.80 0.15 0.26 milk Mixed origin FA, 1.34 1.31 1.43 1.38 0.008 0.13 g/100 g milk Preformed FA, g/100 1.31 1.26 1.34 1.29 0.17 0.02 g milk De novo and mixed 2.14 2.07 2.24 2.18 0.03 0.17 origin FA, g/100 milk Unsaturation, double 0.288 0.294 0.281 0.280 0.005 0.43 bonds/FA 3.5% FCM/DMI, 1.81 1.75 1.81 1.79 0.41 0.06 kg/kg 1There was no significant (P > 0.10) interaction between peuNDF240 and rumen fermentable starch. 2Fat-corrected milk. 3Energy-corrected milk. 4 Fatty acids. Although milk fat yield was unaffected by peuNDF240 content, the yield of true protein was reduced slightly with higher peuNDF240 (Table 4). Milk urea nitrogen content tended to be increased by higher peuNDF240 and RFS substantially reduced milk urea nitrogen at either concentration of peuNDF240. These responses reflect greater efficiency of nitrogen use for cows fed the lower peuNDF240 and particularly the positive effect of moderately greater RFS on rumen nitrogen efficiency. Mixed origin and mixed + de novo fatty acids were reduced by lower peuNDF240 diets versus higher peuNDF240. Likewise, the unsaturated fatty acids were increased for cows fed the low peuNDF240 diets. Numerically, cows fed the lower peuNDF240/higher RFS diet that produced milk with the lowest milk fat percentage also had the least mixed + de novo fatty acids and highest unsaturated milk fatty acids. Overall, these changes in milk fatty acid composition track with the changes in milk fat percentage and indicate the onset of trans fatty acid-induced milk fat depression (Barbano et al., 2018). As a bottom line measure of herd performance, efficiency of FCM production (3.5% FCM/DMI) was lower for cows fed the higher RFS diets and it was least numerically for cows fed the lower peuNDF240/higher RFS diet. As a final “food for thought”: in the post hoc analysis with CNCPS biology, it appeared that a RFS:uNDF240 ratio >2.8 might be a useful indicator for diets that have greater risk of milk fat depression. This idea requires further research to validate, but it seems to fit this data set. Take Home Messages As this research story unfolds, we plan to better define the interactions between RFS and fiber particle size and degradability to provide target values and benchmarks to use when formulating rations. To-date, take home messages of this research include: • There is value in integrating forage particle size and uNDF240, and useful relationships exist between uNDF240 and peuNDF240 with DMI and ECM for high producing dairy cows. • For corn silage-based diets, when uNDF240 exceeds 10 to 11% of ration DM, DMI may decrease; consider a finer chop length. • uNDF240 less than 7% of ration DM may increase the risk of subacute rumen acidosis; maintain peNDF at least 19 to 20% of ration DM. Don’t chop low uNDF240 forage too fine: cows still need effective NDF. • peuNDF240 (pef x uNDF240) is a work-in-progress, but a range of 4.5 to 6% of ration DM seems to be a target for high producing cows fed corn silage- based diets. • Associative effects among RFS, uNDF240, and peNDF are important. When peuNDF240 is approximately 4 to 6% of ration DM for corn silage-based diets (depending on how measured), and uNDF240 is <7.0% of ration DM, then negative effects of RFS on milk fat at only 19 to 20% of ration DM may occur. • If dietary uNDF240 is not uniformly distributed across particle sizes, then direct measurement of uNDF240 in pef particle fraction may be a better approach. It will be critical not to confuse the two methods for measuring peuNDF240. Stay tuned. References Barbano, D. M., H. Dann, C. Melilli, and R. Grant. 2018. Milk analysis for dairy herd management: Today and in the future. Pages 103-115 in Proc. Cornell Nutr. Conf. Feed Manufac. East Syracuse, NY. Cornell Univ., Ithaca, NY. Geiser, J., and J. Goeser. 2019. Midwestern US commercial dairy survey results: Corn silage kernel processing, rumen starch digestibility and fecal starch content. J. Dairy Sci. 102:(E Suppl.):371 (Abstr.). Grant, R. J. 1994. 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