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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior Vários desafios ocorrem no status fisiológico normal da vaca para suportar a formação de um novo indivíduo da espécie. Dados sobre exigências nutricionais e aspectos quantitativos do crescimento, desenvolvimento e digestão de vacas Bos indicus são limitados. Objetivando preencher pelo menos uma parte da lacuna existente, foi conduzido um experimento de abate comparativo com vacas gestantes e não gestantes. Os resultados são aqui apresentados em quatro capítulos. No primeiro capítulo, quarenta e nove vacas Nelore (32 gestantes e 17 não gestantes) com peso médio inicial de 451 ± 10 kg foram usadas num experimento para avaliar o crescimento e desenvolvimento de tecidos maternos e fetais durante a gestação em vacas zebuínas. As vacas foram divididas em dois grupos de acordo com o nível alimentar, que foi ALTO (ad libitum) ou BAIXO (alimentação restrita a 1,2 vezes a mantença, segundo o NRC). As 32 vacas gestantes foram separadas em 4 grupos com 8 vacas em cada e abatidas aos 136, 189, 239 e 269 dias de gestação para coletas de tecidos e análises. Vacas do alto nível alimentar tiveram maior ganho de peso (P0,10). Vacas do nível alimentar alto aumentaram o peso do úbere após 239 dias de gestação (P 0.10). HIGH-fed cows increased the weight of fresh udder after 239 d of gestation (P < 0.10) while LOW-fed cows did not present significant udder increase due to pregnancy (P > 0.10). LOW-fed cows had proportionally (g/kg of EBWnp) lower liver, visceral fat and mesentery (P < 0.01) and greater heart (P < 0.10) than HIGH-fed cows. The proportional weights of rumen- reticulum, omasum and total stomach were affected (P ≤ 0.05) by pregnancy and were lower at 269 d than in early gestation. HIGH-fed cows had greater fetuses at 269 d of gestation than LOW-fed cows. Fetuses from HIGH-fed cows had greater carcasses (P = 0.03) but did not differ from fetuses from LOW-fed cows in organs and viscera (P = 0.27). Placenta weight was greater in HIGH-fed cows (P < 0.01) and the relationship between placenta and fetus weight was different between feeding levels (P < 0.01). The weights of the gravid uterus and fetus, as well as their contents of energy and N, were lower in this study compared to previous studies with Bos taurus cattle. In the second chapter, data from the same 49 cows used in the first chapter were used to develop a set of equations and relations for BW adjustments in pregnant or not pregnant beef cows, improving the options and the reach of the research in the area. Cows were weighed every 28 d (0700 h, before feeding) to obtain the BW, and reweighed at the same time the following day after 16 h of fasting to obtain the shrunk body weight (SBW) values. Pregnant cows were separated into four groups of eight cows each and harvested at 136, 189, 239 and 269 d of pregnancy to obtain the empty body weight (EBW) and the weight of components related to pregnancy. To establish the relationships between the BW, SBW and EBW of pregnant and non-pregnant cows as a function of time of gestation, a set of general equations was tested, based on theoretical suppositions. The pregnant compound (PREG) was defined as the weight that is genuinely related to pregnancy, that includes the gravid uterus minus the non-pregnant uterus plus the accretion in udder related to pregnancy. The PREG was deducted from the SBW or EBW of a pregnant cow to estimate the non-pregnant SBW and EBW (SBWnp and EBWnp) and to calculate the body gain of only maternal tissues (rSBG and rEBG). The overall equation to predict SBW from BW was 0.8084 × BW 1.0303. The equation to predict gravid uterus weight in function of days of pregnancy (DOP) was 0.2106 × e((0.03119 0.00004117 × DOP) × DOP) . There was no accretion of weight in udder up to 238 d of pregnancy and the accretion after 238 d was exponentially correlated with DOP. We conclude that the weight related to the pregnancy can be estimated in a live cow allowing estimating the non-pregnant EBW and SBW of a pregnant cow. This allows estimate and compare the weight gain of tissues not related to the pregnancy and the comparison of weight gain of a pregnant and non-pregnant cow. The objective of the third chapter was to evaluate the effects of pregnancy and feeding level on intake, digestibility and efficiency of microbial N production in Nellore cows. Forty-four multiparous Nellore cows (32 pregnant and 12 non-pregnant) with average initial body weight of 451 ± 10 kg were fed either HIGH (ad libitum) or LOW (restricted feeding 1.2 times maintenance according to the NRC) feeding level. The diet consisted of corn silage (85%), ground corn, soybean meal, urea and mineral mixture. The intake was controlled daily and the DM intake (DMI) was evaluated weekly. In vivo apparent total digestibility was estimated using indigestible NDF as an internal marker and microbial N synthesis was estimated from the technique of the purine derivatives in urine. The voluntary feed intake reduced as the pregnancy advances in Nellore cows and can be calculated as DMI (kg/d) = (16 0.0093 × days in pregnancy) / 1000 × SBWp or DMI (kg/d) = (16.4 0.0093 × days in pregnancy) / 1000 × SBWnp. The average DMI of LOW-fed cows corresponded to 102, 98 and 67% of the amount of energy necessary to attend the maintenance and pregnancy energy requirements suggested by NRC for a cow at 0, 135 and 270 d of pregnancy, respectively. LOW-fed cows had 0.26 kg/d of average shrunk body gain indicating that the nutrient energy requirements of Zebu cows are likely lower than those suggested by NRC. The interaction between the feeding level and days of pregnancy was significant (P

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior This work was developed based on three experiments conducted aiming to evaluate animal performance and determine the nutritional requirements of growth in Nellore female cattle. The first two experiments aimed evaluate the dry matter intake (DMI) and nutrients, average daily gain (ADG), feed efficiency (FE), diet digestibility, microbial efficiency and cold carcass yield (CCY) and primal cuts (PC) of young Nellore heifers receiving two levels of concentrate. Nineteen heifers with initial average weight of 165.6±29.8 kg were used. Three animals were used in a trial to determine the digestibility and microbial efficiency of the two levels of concentrate diet (22.5 and 45%) and also including treatment with a restricted intake maintenance level. The remaining 16 heifers were confined for 168 days for evaluation of performance and intake, divided into two levels of concentrate, being 22.5 and 45% concentrate diet. Every 42 days, four animals, two from each level of concentrate, were slaughtered to determine carcass characteristics, making a factorial arrangement 2 × 4. The digestibility and microbial efficiency were not affected (P>0.05) by concentrate level and not by the level of intake, ad libitum or restricted. Heifers fed 45% concentrate diet had greater ADG (P0.05) between levels of concentrate. ADG (kg / day), DMI (% body weight/day) and EA were higher (P0,05) pelo nível de concentrado na dieta e nem pelo nível de consumo, ad libitum ou restrito. Novilhas recebendo 45% de concentrado na dieta apresentaram maior GMD (P0,05) entre os dois tratamentos. O GMD (kg/dia), CMS (% do peso corporal/dia) e EA foram maiores (P