In experiment 1, 24 multiparous Holsteins and 24 multiparous Jerseys were assigned at random to one of four diets containing either 1.25, 1.38, 1.50, or 1.63 Mcal NEL/kg DM to determine the effects of prepartum energy density on productivity. Cows were assigned to diets 28 d before expected calving and received a similar diet 0 to 60 d postpartum. Prepartum DMI increased linearly as dietary energy density increased. Non-esterified fatty acids did not differ for energy density and were greater for Holsteins. Milk and 4% FCM increased linearly as dietary energy density increased, although postpartum DMI was quadratic for energy density. Fat yield increased linearly with energy density and protein yield did not differ. Increasing prepartum diet energy density increased yield of milk and 4% FCM. In experiment 2, 26 multiparous and 18 primiparous Jerseys were assigned to one of two CP and rumen undegradable protein (RUP) concentrations 30 d before expected calving to determine the effects of CP and RUP on productivity. Cows received a similar diet 0 to 60 d postpartum. Prepartum CP concentrations were 12 and 15% and prepartum RUP (% of CP) concentrations were 30 and 45%. Dry matter intake was not affected by CP or RUP. Milk yield and 4% FCM yield did not differ for CP or RUP. In addition, milk protein concentration and yield were similar for CP and RUP. Positive calculated metabolizable protein balance for 12% CP and 30% RUP concentrations may have precluded responses to increasing prepartum CP and RUP. In experiment 3, 12 Holstein and 12 Jersey multiparous cows were assigned at random to diets containing 0 or 500 mg FeCO3/kg DM to investigate the effects of dietary Fe on milk production and Cu status. Hepatic Cu did not differ for breeds or Fe supplementation. For Jerseys, hepatic Fe was greater for cows receiving supplemental Fe. Hepatic Zn was similar for breed and Fe supplementation. Plasma Cu was greater for Jerseys compared to Holsteins and did not differ for Fe supplementation. Plasma Fe and Zn did not differ for breeds or Fe supplementation. Iron supplementation did not affect yield of milk or 4% FCM. Dietary Fe did not affect Cu status of Holstein and Jersey cows in this experiment. / Ph. D.

The biological process of estrus in cattle is known to initiate behavioral responses as a result of fluctuations in hormones, which may alter time budgeted for feeding. The current study aimed to quantify these variations that may exist in feed intake and behaviors in the days pre and post estrus. For this study, 517 estrous cycles in crossbred beef heifers of divergent sizes were used to analyze the impact of estrus behavior on feeding behavior with or without the presence of a bull. There were few differences in estrus behavior among heifers of divergent sizes, however the presence of a bull influenced all estrus activity parameters. Our findings indicate that feed intake and behaviors were sharply decreased on the day of estrus, but returned to baseline levels the following day. Thus, decreases in feed intake and behavior may serve as an additional tool indicating the onset of estrus.

The current National Academies of Sciences, Engineering and Medicine (NASEM) dry matter intake (DMI) prediction models are inadequate for DMI prediction of beef cattle in the Northern Great Plains. Four studies were conducted to account for additional variation in DMI and average daily gain (ADG) caused by weather variables. Experiment 1 and 2 had 13,895 steer-weeks observations, experiment 3 had 13,739 steer-weeks observations, and experiment 4 had 2,161 cow-weeks observations, respectively. Experiment 1 examined the influence of ambient temperature and solar radiation on DMI of beef steers. In experiment 2, 3, and 4, we examined the influence of ambient temperature, range of temperature, dew point, solar radiation, wind speed and their lags (two-week lag and monthly lag) on DMI of beef steers, ADG of beef steers, and DMI of beef cows, respectively. After adjusting for week of the year, linear and quadratic relationships of predictor variables on response variables were evaluated. In experiment 1 and 2, body weight (BW) had both linear and quadratic relationship with DMI of steers. In experiment 3 and 4, BW had a linear relationship with ADG of steers and DMI of cows, respectively. Week of the year, BW, and dietary energy density (NEm) were accounted for in the base model in experiment 1, 2 and 4 while in experiment 3, DMI was also accounted for. For the models, stepwise regression procedure was utilized. In experiment 1, ambient temperature and solar radiation interacted (P = 0.0001) and accounted for additional variation in DMI of beef steers. In experiment 2, weather variables and their interactions (P = 0.0001) accounted for additional variation in DMI of beef steers. In experiment 3, weather variables (P = 0.0001) accounted for additional variation in ADG of beef steers. In experiment 4, wind speed interacted (P <0.001) with ambient temperature and range of temperature which all accounted for additional variation in DMI of beef cows. These studies show that weather variables interact and cause variation in DMI and ADG in beef cattle. This has helped in better understanding the relationship between weather variables with DMI and ADG. This will improve the accuracy of DMI and ADG prediction equations and help beef cattle producers in managing their feed resources more efficiently.

Previous research on grazing behavior has shown that ruminants will select a mixed diet. The use of adjacent monocultures is an essential tool for determining dietary preference of forages. Much of the work to date has been conducted with white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.). Partial preference for white clover over ryegrass has been reported consistently and partial preference for legumes is thought to occur regardless of the legume and grass species being evaluated. Two forage species, tall fescue (Festuca arundinacea Schreb. or Lolium arundinaceum (Schreb.) S.J. Darbyshire) and alfalfa (Medicago sativa subsp. sativa L.), which had not been evaluated together previously as adjacent monocultures were grazed by beef steers in the present set of experiments. Steers exhibited a partial preference for alfalfa of 61 to 65% when given a choice of grazing alfalfa or tall fescue as adjacent monocultures, regardless of the ground area proportion of the two forages offered. Steers grazing tall fescue monocultures spent more time ruminating (P = 0.02) and tended to graze less time (P = 0.06) than steers in adjacent monoculture treatments. Time spent idling, number of prehensions and mastications, and bite rate were similar (P > 0.05) among treatments. Steers grazing tall fescue monocultures spent less time standing, more time lying, were less active and took fewer steps (P ≤ 0.05) than steers in adjacent monoculture treatments. Grazing behavior was examined when alfalfa had not been in the previous diet of the steers. Cattle without previous experience grazing alfalfa spent 78% of the time grazing alfalfa, whereas after having experience grazing it they spent a lower (P = 0.04) proportion of their time grazing alfalfa (72%). Overall proportion of the day spent grazing both forages was lower (P = 0.0001) when alfalfa was novel (40%), compared to when steers were experienced grazing both forages (46%). Proportion of the day spent idling was greater (P < 0.0001) when alfalfa was novel (35%), compared to when both forages were familiar to the steers (26%). Previous research has reported that ruminants exhibit a diurnal pattern of preference by decreasing the proportion of white clover consumed from morning to late afternoon while increasing the proportion of perennial ryegrass in the diet. This is thought to be a strategy to increase fiber intake before nightfall or as a response to higher carbohydrate levels in grass in the afternoon. In the present study, proportion of grazing time in alfalfa was higher (P = 0.02) in the afternoon (76.8 %) than in the morning (72.1 %). While fiber concentration was higher in the tall fescue, carbohydrate concentrations were similar. Steers were not attempting to increase fiber intake in the afternoon in the present study. Dry matter intake of steers grazing adjacent monocultures of alfalfa and tall fescue was estimated with n-alkanes. Diet composition was estimated using n-alkanes and long chain alcohols (LCOH) in several different combinations. The use of LCOH added additional characterization of the forages, but diet composition estimates were not different (P ≥ 0.22) than when estimated using four different n-alkanes. Laboratory analysis costs may be reduced if n-alkanes alone can adequately characterize the forages being consumed, depending on the forage species in question. Meteorological conditions impacted DMI with intake being less in hotter conditions. Steers had similar partial preferences for alfalfa over tall fescue (P = 0.13, 79% and 70% alfalfa in yr 1 and 2, respectively) even though total DMI differed between years (P = 0.002, 9.4 kg d-1 and 4.5 kg d-1 in yr 1 and 2, respectively). Lower DMI in yr 2 was attributed to hotter air temperatures. When animals are consuming two different forages as adjacent monocultures such as in the current experiments, it is important to determine the proportion of each forage in the diet before calculating DMI using odd chain n-alkanes of the forage along with a dose even chained n-alkane. Dry matter intake can be overestimated if the proportion of the forages consumed is not estimated and accounted for in the equation. This would apply to other studies utilizing mixed swards or any diet containing multiple components that differ in concentration of the n-alkane being used for DMI estimation. Analysis of n-alkane concentration should be performed on each item in the diet and the proportion of each item in the diet estimated so that the right value can be used in the calculation. Differences in marker concentrations between years also indicate the importance of analyzing those concentrations in the feed or forage at the time of fecal collection and not using values reported from previous research. / Ph. D.

Determining intake of feeds in horses is an important factor in incorporating supplements in their diets. Fecal recoveries (R), fecal output (FO), dry matter digestibility (DMD) and dry matter intake (DMI) were estimated using alkanes as markers in 8 thoroughbred geldings. The experiment compared two diets in a 2 X 2 latin square experiment. The diets were mixed grass hay only (H) and the same hay plus a cereal by-product (H + CBP). The cereal by-product (CBP) was the high fat component added to feeds at Virginia Tech's Middleburg Agricultural Research and Extension Center. The apparent digestibility of ether extract (EE) and other nutrients in the H and H + CBP, as well as the partial digestibility of CBP were also determined. The periods were 21 d each with a dietary accommodation period followed by eight days of dosing the even chain alkanes dotriacontaine (C32) and hexatriacontane (C36) as external markers. Total collection (TC) was performed the last 4 d of dosing. The results show that mean recoveries of alkanes were close to 100%, but the range for individual alkanes was wide, and the pattern of recoveries for alkanes of different chain length was inconsistent from feed to feed. The results also indicate that mean estimates of the DMI, DMD and FO of a feed, such as H or H + CBP, are determined with reasonable accuracy by means of alkane markers. In contrast, alkane estimates of DMI and DMD in an individual horse fail to predict corresponding TC estimates. The alkane estimate of FO in an individual horse predicts a TC value with error of 16.4%. The CBP was found to be an excellent source of EE, CP and fiber but a poor source of Ca. / Master of Science

Submitted by Marcos Anselmo (marcos.anselmo@unipampa.edu.br) on 2017-06-12T13:44:14Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) OTÁVIO BACKES MEOTTI.pdf: 806223 bytes, checksum: d35606330f21e0cb45f98cc7474c455a (MD5) / Approved for entry into archive by Marcos Anselmo (marcos.anselmo@unipampa.edu.br) on 2017-06-12T13:44:34Z (GMT) No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) OTÁVIO BACKES MEOTTI.pdf: 806223 bytes, checksum: d35606330f21e0cb45f98cc7474c455a (MD5) / Made available in DSpace on 2017-06-12T13:44:34Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) OTÁVIO BACKES MEOTTI.pdf: 806223 bytes, checksum: d35606330f21e0cb45f98cc7474c455a (MD5) Previous issue date: 2017-03 / O período de transição é uma fase crítica na vida produtiva das vacas leiteiras, devido ao aumento do seu requerimento nutricional, concomitante a uma redução fisiológica na ingestão de matéria seca, e comprometimento imune. Devido a estes fatores, este período se associa a um estado catabólico, com alta incidência de enfermidades metabólico-nutricionais e infecciosas, que levam a perdas econômicas e deterioro do bem-estar animal. Como muitas dessas enfermidades são antecedidas por uma diminuição na ingestão de matéria seca (IMS), a avaliação de indicadores relacionados ao consumo poderiam servir para predizer o risco de doenças, durante o período de transição. Como o escore de enchimento ruminal (EER) apresenta uma boa correlação com IMS a presente dissertação teve dois objetivos: 1- correlacionar e comparar os escores de enchimento ruminal (EER, 1 a 5) com indicadores sanguíneos do balanço energético e mineral. 2- validar o escore de enchimento ruminal como ferramenta de monitoramento de saúde e taxa de retorno a ciclicidade em vacas leiteiras no período pré-parto. Para responder aos objetivos realizou-se um estudo de coorte com 113 vacas multíparas Holstein, e os resultados de cada objetivo são apresentados em 2 artigos separados. Para responder ao objetivo 1 foram coletados semanalmente nas semanas -2,-1, parto, +1,+2,+3, uma amostra de sangue para analisar as concentrações de βOH-butirato (BHBA), ácidos graxos não esterificados (NEFA), colesterol e cálcio, e aferido simultaneamente o EER e escore de condição corporal (ECC) das vacas. Nesse estudo comprovou-se que o EER correlacionou-se principalmente com o ECC. Também, se observou que o EER associou-se com indicadores sanguíneos estudados tanto no período pré como pós-parto, sendo que vacas com maiores EER (EER3) apresentaram maiores concentrações de cálcio, BHBA e colesterol no pré-parto. Já, no pós-parto vacas com EER1 tiveram valores maiores de NEFA e menores de colesterol que vacas com EER3. Para responder ao objetivo 2 mensurou-se o EER nas semanas -2, -1 e parto, diagnosticou-se a ocorrência de doenças clínicas e avaliou-se o regresso a ciclicidade entre a 4º e 8º semana pós-parto. Determinou-se o EER como preditor das doenças mais prevalentes (metrite, mastite, retenção placentária, deslocamento de abomaso e afecções podais). Observou-se que o EER≤2 duas semanas antes do parto foi um bom preditor de todas as doenças acima descritas, com sensibilidade variando de 53% a 75% e especificidade de 80%. Por outro lado, vacas com EER1 durante o pré-parto e parto voltaram mais tardiamente a ciclicidade e em menor porcentagem quando comparadas com vaca com EER3. Portanto o escore de enchimento ruminal correlacionou-se com os indicadores sanguíneos durante período de transição, bem como serviu como preditor para as doenças do período de transição e regresso a ciclicidade, podendo ser utilizado como ferramenta de monitoramento da saúde de vacas leiterias multíparas. / The transition period is a critical phase in the productive lives of dairy cows, due to the increase in their nutritional requirement, concomitant with a physiological reduction in dry matter intake, and immune compromise. Due to these factors, this period is associated with a catabolic state, with a high incidence of metabolic-nutritional and infectious diseases, leading to economic losses and deterioration of animal welfare. Since many of these diseases are preceded by a decrease in dry matter intake (DMI), the evaluation of consumption indicators could serve to predict disease risk during the transitional period. As the rumen fill score (RFS) presents a good correlation with DMI, the present dissertation had two objectives: 1 – To correlate and compare rumen fill (RFS, 1 to 5) with blood indicators of negative energy balance. 2 – To validate the rumen fill score as a health monitoring tool and rate of return to cyclicality when it comes to dairy cows’ pre-calving. In order to answer the objectives, a cohort study was carried out with 113 Holstein multiparous cows, and the results of each objective are presented in 2 separate articles. To respond to objective 1 blood samples were collected to analyze the concentrations of βOH-butyrate (BHBA), non-esterified fatty acids (NEFA), cholesterol and calcium at week -2, -1, calving, + 1, + 2, +3, and simultaneously, RFS and cow’s body condition score (BCS) were measured. It was verified in this study that the RFS is correlated mainly with the BCS. Also, it was observed that the RFS was associated with blood indicators studied in pre and post partum periods, and cows with higher RFS (RFS 3) had higher concentrations of calcium, BHBA and cholesterol in prepartum. In the postpartum period, cows with RFS 1 had higher NEFA values and lower cholesterol than cows with RFS 3. To respond to objective 2, the RFS was measured at week -2, -1 and calving, clinical diseases were diagnosed and the return to cyclicity between the 4th and 8th week postpartum was evaluated. RFS was determined as a predictor of the most prevalent diseases (metritis, mastitis, placental retention, displaced abomasum and lameness). It was observed that EER≤2 two weeks before calving was a good predictor of all diseases described above, with sensitivity varying from 53% to 75% and specificity of 80%. On the other hand, cows with EER1 during prepartum and calving returned to cyclicity later and in a lower percentage when compared to cows with EER3. Therefore, the ruminal filling score correlated with the blood indicators during the transitional period, and served as a predictor for the diseases of the transition period and return to cyclicity. It can be used as a monitoring tool for the health of multiparous dairy cows.

Orientador: Danilo Domingues Millen / Resumo: O objetivo deste estudo foi avaliar a classificação das baias com base no consumo logo nas semanas iniciais do confinamento de machos castrados e novilhas, sobre o desempenho, e desenvolver modelos de predição da ingestão de matéria seca (IMS), procurando melhorar sua acurácia por meio da inclusão da fase de confinamento em que os animais se encontram. Isso possibilitaria a identificação de “baias problemas”, ou de desempenho superior, facilitando a tomada de decisão na gestão. Para isso foram utilizados dados de 3.650 baias de confinamentos comerciais dos USA (2.256 de machos castrados e 1.394 de novilhas), os quais foram confinados entre os anos de 2009 a 2014. Os dados de IMS e de energia líquida de ganho (ELg) médios do período de terminação foram ajustados de acordo com o peso vivo inicial e dias em alimentação (DEA) pelo PROC NLIN do SAS (2009) separadamente para machos castrados e novilhas. Tanto a IMS como a ingestão de ELg médias ajustadas do período de terminação foram correlacionados com a ingestão das semanas de 5 a 12 pelo PROC CORR, e de acordo com a semana de maior correlação as baias foram classificadas em quartis: Superior ¼, Alta ¼, Média ¼ e Baixa ¼ do maior para o menor consumo e avaliado o desempenho pelo PROC MIXED. Para o desenvolvimento dos modelos foi utilizado o PROC CORR para identificar a correlação de cada variável (PVI, IMS de 8 a 28 dias, sexo, IMS da semana anterior) com a IMS e ordenar a inclusão delas no modelo, e por meio do PROC MIXED foi t... (Resumo completo, clicar acesso eletrônico abaixo) / Mestre

Pasture-based dairies have become more prevalent in recent years due to a higher proportion of organic milk demand and production. Organic certification requires that animals must graze at least 120 days in each growing season. However, dry matter intake is often limited when dairy animals receive most of their herbage from pasture, resulting in lower animal performance and milk production. The purpose of this study was to analyze the complimentary effect of high energy grasses with birdsfoot trefoil (BFT) tannins to improve intake of dairy heifers. Jersey heifers were rotationally grazed for 105 days in 2017 and 2018 on eight different pasture treatments, which included monocultures of perennial ryegrass (PR), orchardgrass (OG), meadow bromegrass (MB), and tall fescue (TF), with each respective grass also planted in mixture with BFT. Intake was measured by sampling herbage before and after each seven-day grazing period and was from greatest to least as follows: MB+BFT, OG+BFT, OG, MB, PR+BFT, TF+BFT, PR, TF. Physical characteristics such as pasture bulk density, herbage height, herbage allowance, leaf pubescence, leaf softness, and birdsfoot trefoil content as well as nutritional properties such as fat, non-fibrous carbohydrates, fiber, and energy were all associated with intake. Crude protein and ash were also associated with intake. While PR+BFT did not have the greatest overall intake, it was the only treatment that consistently had greater intake than its respective grass monoculture (PR). Since it had more energy and tannins than all other grasses, a complimentary effect between energy and tannins to increase intake was likely. The fact that both physical and chemical herbage characteristics were associated with intake shows the importance of planting the right species in pasture as well as making proper management decisions to maximize nutritive value and herbage intake.

Made available in DSpace on 2014-06-11T19:28:24Z (GMT). No. of bitstreams: 0 Previous issue date: 2010-08-10Bitstream added on 2014-06-13T19:36:55Z : No. of bitstreams: 1 natel_as_me_botfmvz.pdf: 491232 bytes, checksum: aceb2af278452335d6fe98f4fd9978be (MD5) / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Universidade Estadual Paulista (UNESP) / Avaliou-se o efeito da inclusão de fibra em detergente neutro (FDN) sobre o consumo de matéria seca (CMS), eficiência alimentar, produção, composição e o perfil de ácidos graxos no leite de ovelhas Bergamácia. Dezesseis ovelhas em lactação foram distribuídas em quadrados latinos, quádruplo, 4x4. As rações experimentais eram compostas por concentrado protéico e feno de alfafa. As proporções concentrado:volumoso variavam conforme o tratamento: (T1) 23%, (T2) 32%; (T3) 41% e (T4) 50% de FDN na ração total. As ovelhas foram ordenhadas duas vezes ao dia. Amostras de leite foram colhidas por dois dias para a determinação da composição centesimal, por infravermelho, e do perfil de ácidos graxos, por cromatografia gasosa. Houve efeito linear negativo (P<0,01) na produção de leite, na eficiência de produção (P<0,02) e nos teores de ácidos graxos de cadeia curta, com a inclusão de fibra na ração. Efeito quadrático foi observado para o CMS (1,69; 1,71; 1,58 e 1,32 kg). Os ácidos graxos desejáveis (poliinsaturados + esteáricos) constituíram 35,5% do perfil de ácidos graxos. O aumento da fibra na ração reduziu a produção de leite, afetou a composição dos ácidos graxos, porém, não alterou a composição centesimal dos constituintes deste. O máximo CMS foi atingido ao nível de 37,8% de FDN na ração / This study evaluated the effect of the inclusion of neutral detergent fiber (NDF) on the dry matter intake, weight gain, feed efficiency, production, composition and fatty acid profile of Bergamacia sheep milk. Sixteen lactating ewes were divided into four 4x4 Latin squares. The experimental rations were composed of protein concentrate and alfalfa hay. The proportions of concentrate:forage varied according to treatment to achieve the required level of NDF: (T1) 23%, (T2) 32%, (T3) 41% and (T4) 50% NDF in the total ration. The ewes were milked in milking machine twice a day. Milk was sampled for two days in each subperiod for the determination of the composition and the of fatty acids by infrared by gas chromatography, respectively. There was a linear effect (P<0.001) in milk production, average daily production efficiency (P<0.02) and levels of short chain fatty acid with the inclusion of fiber in the ration. A quadratic effect was observed for DMI (1.69, 1.71, 1.58 and 1.32 kilograms). The desirable fatty acid (poliunsaturated + stearic) constituted 35.5% of the fatty acid profile. Increased fiber in the ration reduced milk production, affect the fatty acid profile, without alter its composition. The maximum intake was achieved at 37,8% NDF in the ration

Objetivou-se avaliar o efeito da digestibilidade da fibra do milho sobre o consumo, ganho de peso e composição da carcaça de novilhas Nelore em crescimento, de modo a validar este índice como indicador da qualidade nutricional de híbridos de milho em programas de melhoramento genético e de seleção de híbridos para silagem. Foram plantados quatro híbridos de milho com diferente digestibilidade da fibra, IAC 8390, 30F90Y, 30S40Y e DKB 390Y, sendo a digestibilidade da FDN da dieta determinada após incubação de 30 horas. Os milhos foram colhidos aos 62, 82 e 104 dias crescimento, para determinar principalmente a digestibilidade da folha e do colmo dos híbridos, para determinar quais híbridos fariam parte da dieta experimental. Foram utilizadas quarenta e oito novilhas da raça Nelore com aproximadamente 250 kg, confinadas por 63 dias, em delineamento de bloco casualizados, em grupo de 3 novilhas por baia, de acordo com o peso vivo inicial. A cada 21 dias, os animais eram pesados, sendo a avaliação da carcaça feita no início do período A dieta experimental foi constituída de quatro rações, com relação volumoso/concentrado de 20:80 e 40:60, com volumoso de duas silagens de milho, 30F90Y, de alta digestibilidade da FDN e, 30S40Y, de baixa digestibilidade da FDN. Foi analisado o tamanho de partícula de amostras da dieta e das sobras/baia pela peneira Penn State Forage Particle. Houve diferença na digestibilidade da FDN dos híbridos na fração colmo, entre os dias de coleta, sendo que a digestibilidade in vitro da matéria seca (DIVMS) reduziu após o período de florescimento dos híbridos e houve uma variação entre os híbridos, em cada coleta. A digestibilidade in vitro da fibra em detergente neutro (DIVFDN), também reduziu com a maturidade da planta, porém houve uma diferença entre os híbridos apenas na coleta aos 82 dias de crescimento. O híbrido 30F90Y foi superior aos demais ao analisar a DIVMS e DIVFDN. Houve diferença entre o tamanho de partículas da dieta oferecida e o tamanho de partícula que restou na sobras, sendo que houve efeito de seleção apenas para a dieta com 40% de concetrado. Houve interação milho*dieta tanto para consumo de matéria seca (CMS) quanto para consumo da fibra em detergente neutro (CFDN), sendo que o híbrido 30F90Y, de maior digestibilidade, teve consumo maior, inclusive na dieta com 20% de concentrado. Para desempenho, foi observado efeito da dieta no ganho médio diário (GMD), sendo maior o ganho de peso (GP) na dieta com a silagem do híbrido 30F90Y Já a conversão alimentar e a composição de carcaça não tiveram efeito de dieta, híbrido de milho ou interação dieta*milho. A utilização de híbridos com melhor digestibilidade da fibra, é uma indicador de híbr do destinado a nutrição animal, pois alterou o consumo, o ganho de peso, porém não alterou a conversão alimentar e composição de carcaça. / The objective of this study was to evaluate the effect of fiber digestibility on intake, weight gain and carcass of growing Nellore heifers, aiming to validate this rate as an indicator of nutritional quality on corn hybrids in programs of genetical improvement and selection of hybrids for silage. Four corn hybrids on different fiber digestibility were raised, IAC 8390, 30F90Y, 30S40Y and DKB 390Y, being the FDN digestibility determined on the diet after a 30-hour-incubation period. The harvest was at the 62, 82 and 104 growing days to mainly determine the leaf and stem digestibility in hybrids to identify which ones would be part of the experimental diet. Forty-eight Nellore heifers of approximately 250 kg BW were housed for 63 days, in randomized blocks, in groups of three heifers per pen, according to its initial body weight. Each 21 days the animals were weighed and the carcass evaluation was done in the beginning of the period. The experimental diet was composed of four rations at 20:80 and 40:60 of roughage/concentrate respectively, in which the corn silage was used the 30F90Y, the one with high digestibility in FDN, and the 30S40Y, of low digestibility in FDN. The particle size of samples collected from the diets and ortis of each pens were analyzed by the Penn State Forage Particle sieve. There was difference in the FDN digestibility for hybrids in the stem fraction among the collection days, being the dry matter digestibility in vitro (DIVMS) reduced after the hybrid blossom and there was a variancy among the hybrids, in each collection. The digestibility in vitro of fiber detergente neuter (DIVFDN), was also reduced according to tha plant maturity, however there was a difference among the hybrids only in the collection at the 82 day of growth. The hybrid 30F90Y was superior to the other when analyzed the DIVMS and DIVFDN. There was difference between the particle size on the diets and the particle size on the ortis, with a selection affect only for the diet at 40% concentrate. There was interaction on the corn*diet for the dry matter intake (CMS) and for the fiber detergent neuter intake (CFDN), beingt the hybrid 30F90Y of higher digestibility, intake, and also on the diet with 20% concentrate. For performance was observed the diet effect on the average in daily weight gain (GMD), in which the highest weight gain (GP) was obtained on the silage of the hybrid 30F90Y. The feed conversion and carcass had no diet effect, neither corn hybrid nor interaction diet*corn. The hybrids with higher fiber digestibility is an indicator for its usage in animal nutrition once they altered the intake and the weight gain, although it hasn\'t altered the feed conversion and the carcas.