José Fernando Piva Lobato

The objective of this work was to evaluate productive variables that can affect the reproductive performance for odds-ratio, in order to increase or reduce the pregnancy rate in beef cows at different ages. A logistic regression model was used to analyze the response variable “rate of pregnancy”. The productive variables were lactation period, milk production, interval between calving and end of reproductive season, weight, body score, and daily gain. The variables in the regression equation were tested for multicollinearity, and the parameters were evaluated by odds-ratio statistics. Adjusting the lactation period by seven days significantly influenced the pregnancy rate of three-year-old cows, since an extension of seven days increased the pregnancy rate by 27.3%, while a reduction of 7 days decreased it by 21.4%. During the reproductive period, changes in mean daily weight gain was the most consistent variable for pregnancy, with a significant effect on cows of three, four, and five years of age. Reproductive success is a result of different variables that depend on the age of the cow. For adult beef cattle, early calving and adequate weight gain are essential during the reproductive period. For younger categories, a short lactation period is essential.

The aim of this study was to evaluate the probability of pregnancy in heifers weaned at different ages and bred at 13 to 15 months old. A total of 121 Braford heifers were used, weaned as calves at 77 days (early) or 147 days (conventional) of age. To develop the statistical models of reproductive performance, factors related to the development of the heifers were analyzed. The analysis included a diagnosis of multicollinearity using the Pearson correlation matrix, adjusting the model by means of the Hosmer and Lemeshow test. The response variable, rate of pregnancy, was analysed using the LOGISTIC procedure. Beginning with a weight of 271 kg and an age of 402 days at the start of the breeding season, the pregnancy rates increased by 18.4% and 29.0%, respectively for every 15 kg increase in body weight and 10-day increase in age. However, a reduction of 15 kg in body weight and of 10 days in age reduced the pregnancy rates in the heifers by 15.5% and 22.5%. An increase or reduction of 0.100 kg in the average daily gain between early weaning and conventional weaning represented an increase of 44.6% and a reduction of 30.9% in the chances of pregnancy. Early-weaned heifers require correct nutritional management to allow satisfactory postweaning weight gains so as not to compromise their reproductive performance.

O objetivo do estudo foi avaliar o desempenho de novilhas de corte do desmame ao fim do período reprodutivo aos 13/15 meses de idade submetidas a três sistemas alimentares nos meses de inverno após o desmame: pastagem de aveia preta (Avena strigosa Schreb.) e azevém (Lolium multiflorum Lam.); pastagem de aveia preta e azevém mais suplementação com 1% do peso corporal com farelo de trigo (FT); e pastagem de aveia preta e azevém mais suplementação de 1% de peso corporal com farelo de trigo e 200 ml/cabeça/dia de glicerina bruta (FTG). As suplementações não afetaram a taxa de prenhez das novilhas púberes anteriormente ao período reprodutivo, mas determinaram mais novilhas púberes e maior concepção nos primeiros 21 dias do acasalamento. Novilhas prenhes, independentes dos tratamentos, possuem melhores características fenotípicas de conformação, precocidade, musculatura e tamanho já ao desmame, maior ganho diário médio e condição corporal ao início e fim dos sistemas alimentares e início da temporada reprodutiva, o que permite uma seleção prévia, a idades menores, das bezerras a serem postas em sistemas alimentares para prenhez aos 13/15 meses de idade. Rebanhos com prenhez estabilizada aos dois anos de idade necessitam de seleção para maior precocidade sexual e prenhez aos 13/15 meses de idade, podendo o peso ao desmame ser um bom preditor.

O objetivo do experimento foi analisar o desempenho reprodutivo de 119 vacas Hereford primíparas aos 36 meses submetidas a diferentes níveis nutricionais durante a recria e pré-acasalamento, que foram estabelecidas a partir de diferentes cargas animais (CA) associadas ou não ao uso de pastagens melhoradas, e no pós-parto com a utilização ou não de pastagens melhoradas. Durante a recria as novilhas foram submetidas a duas CA (0,8 e 0,6 unidades animais [UA] por ha) e, previamente ao primeiro acasalamento, os grupos foram divididos e alocados (0,7 UA ha -1 ) em pastagem natural ou em pastagem natural melhorada. O melhor desempenho das diferentes CA e da pastagem melhoradas pré-acasalamento quando novilhas não se manteve durante a gestação, chegando as vacas ao parto com pesos e escores de condição corporal (ECC) semelhantes. Após o parto, as primíparas foram distribuídas, conforme os tratamentos anteriores, em dois lotes mantidos sob pastoreio contínuo em pastagem natural ou em pastagem natural melhorada. Com exceção do momento do parto, o peso e o ECC das vacas foram superiores para animais mantidos em pastagem natural melhorada, que também apresentaram maiores variações de peso até o final do período reprodutivo. O melhor nível nutricional ofertado para vacas mantidas em pastagem natural melhorada determinou maiores percentuais de prenhez comparadas com as vacas mantidas em pastagem natural, com valores de 44,3 e 13,8%, respectivamente. Quando os lotes foram estratificados em faixas de pesos em vacas muito leves, leves, médias e pesadas, as taxas de prenhez foram crescentes acompanhando o aumento de peso com valores de 0,0, 18,8, 21,2 e 88,0%, respectivamente. Vacas com ECC inferior a 2,6 pontos não emprenharam. Já vacas com ECC entre 2,6 e 3,4 pontos apresentaram taxa de prenhez de 48,1% e vacas com ECC igual ou superior a 3,5 pontos apresentaram 100% de prenhez. Pastagens naturais melhoradas no pós-parto determinam maior peso e melhor escore de condição corporal em primíparas, sendo os mesmos determinantes de maior taxa de prenhez.

Potential variables for the reproductive success of beef cows were evaluated. Included in the model were the age of the cow at calving; the interval between the Julian calving date and the end of the breeding season; the body weight and conditions at calving, at 75 days post-partum and at the end of the breeding season; and the mean daily variation in weight between these dates. Logistic regression was used in the analysis, with the parameters evaluated using the odds ratio statistic, estimating the chance of pregnancy. The mean rate of pregnancy was 84% and 55% for early and late-weaned cows, respectively. For early weaned cows, the regression variables were the Julian calving date, age of the cow, weight gain from calving to 75 days post-partum, and from 75 days post-partum to the end of the reproductive period. For late-weaned cows, there were only two regression variables, weight at calving and weight gain from calving to the end of the reproductive period. For every year above the average age of the herd, early weaned cows have an 80.9% greater chance of pregnancy, while a reduction of one year reduces the chance of pregnancy by 44.7%. In early weaned cows, every seven days after the mean Julian calving date reduces the chances of pregnancy by 22.6%, whereas every seven days before the mean calving date increases pregnancy by 29.2%. Greater gains in cow body weight, from calving to the end of the reproductive period, determine a greater probability of pregnancy.

The aim of this study was to evaluate the effects of conception weight and gestational weight gain on performance and productive efficiency in beef cows and their calves. Eighty-eight primiparous, three-year old Braford females were used, divided according to weight at conception and gestational performance (High and Low): cows with a low conception weight and low gestational weight gain (LL), cows with low conception weight and high gestational weight gain (LH), cows with high conception weight and low gestational weight gain (HL), and cows with high conception weight and high gestational weight gain (HH). At calving, HH cows were heavier than LH and HL and these were heavier than LL cows. Male calves born to HH cows were superior in body weight to those born to LL cows at 150, 210 and 365 days. Female calves born to HH, HL or LH cows were superior to those born to LL cows in pregnancy at 14 months of age. The LL and HL cows were more efficient in calf production. Actual fertility was influenced by the nutritional level of the herd, where HH cows were superior than LL cows. Better cow herd nutrition increases the development and performance of the progeny.

This work was conducted to study the efficiency of using nitrogen on Alexander Grass (Urochloa plantaginea (Link.) Hitch) through information about the characteristics of the pasture and forage intake of beef heifers. The experiment was carried out at the Federal University of Santa Maria (Universidade Federal de Santa Maria, UFSM) in Santa Maria, Rio Grande do Sul, Brazil, from January to April of 2014. The treatments consisted of doses of zero, 150, or 300 kg/ ha nitrogen (N) in the form of urea. The study used a rotational pasture method and 16 Angus heifers with a mean initial age and body weight (BW) of 15 months and 276 ± 17.4 kg, respectively. Forage intake was evaluated by treating two picketed test heifers with chromium oxide (Cr2O3) for eleven days as an external indicator of fecal production. The experimental design was completely randomized, and the measurements were repeated over time with three treatments and two repetitions per area. Crude protein content was 3.4% higher under 300 kg/ha nitrogen fertilization compared to 150 kg/ ha (18.7%). Independent of the N application rate, the heifers ingested 2.2 ± 0.09 kg DM 100/ kg BW of forage. Nitrogen fertilization of Alexander grass modified the structure of the pasture, increasing the quality and total production of the forage. Heifers pastured on Alexander grass fertilized with 300 kg/ha nitrogen harvested forage with higher levels of crude protein. The structural change in the canopy let the heifers reduce their consumption of forage at the end of the Alexander grass life cycle.

O objetivo do experimento foi analisar o desempenho reprodutivo de 42 vacas mestiças Angus primíparas aos 24/25 meses que receberam ou não um suplemento a 0,8% do peso corporal. O suplemento, com 14% proteína bruta e 70% de nutrientes digestíveis totais, foi fornecido por 30 dias pré-acasalamento, no período de transição entre pastagem cultivada para natural, sendo 23 dias em pastagem de aveia (Avena strigosa Schreb) e azevém (Lolium multiflorum Lam.), e sete dias em pastagem natural. O método de pastoreio foi o contínuo com oferta de forragem média de 1450 e 2150 kg/ha de matéria seca para pastagem cultivada e natural, respectivamente. As primíparas foram distribuídas em um delineamento inteiramente casualizado nos grupos não suplementado (controle) e suplementado conforme peso, escore de condição corporal e data de parto. Foram avaliados o desenvolvimento corporal das vacas e seus bezerros entre o início da suplementação até o diagnóstico de gestação. A suplementação não influenciou o peso e a condição corporal final das vacas (P>0,05). No entanto, o ganho diário médio das suplementadas e de seus bezerros (0,465 e 0,829 kg, respectivamente) foi superior (P<0,05) ao das não suplementadas (0,227 e 0,644 kg, respectivamente), resultando em maior peso ao desmame aos cinco meses dos bezerros (130,9 vs. 117,3 kg, respectivamente). As vacas suplementadas Aos 42 dias do acasalamento tinham maior prenhez (80,9 vs 28,6%) e ao término dos 63 dias do período a prenhez foi 80,9% vs 57,1% das não suplementadas. Vacas primíparas aos 24/25 meses de idade e que concebem têm maior peso e condição corporal ao início do acasalamento.

The objective of this study was to evaluate, for two years, the performance of 124 female and 105 male Braford calves born during the first and second halves of the same calving season and the effects of birth period on the development of males until slaughter, as yearlings, and of females until calving, after having mated at 13-15 months of age. Early-born females were heavier than those born late at weaning (119.3 vs 109.9kg; P<0.05), at the start of the breeding season (275.0 vs 263.0kg; P<0.05), and at the end of the breeding season (300.0 vs 289.5kg; P0.05) in the development of males born early in relation to those born late, except for weaning weight, which was higher in the former. Steers born early were ready for slaughter at a younger age (459.6 vs 490.1 days; P<0.05), and both groups (early- and late-born) had a body condition classified as fat (4.21 points). In intensive production systems, both male and female calves perform better if they are born during the first half of the calving season.