Genetic parameters and genetic trends of conformation and management traits in Dairy Gir cattle / [Parâmetros genéticos e tendências genéticas de características de conformação e manejo de bovinos Gir Leiteiro]

The objective of this study was to estimate genetic parameters and genetic trends of different conformation and management traits regularly measured within the context of the National Dairy Gir Breeding Program (PNMGL). The estimation of genetic and residual variances for each trait was performed using average information restricted maximum likelihood (AI-REML) procedure in AIREMLF90 program software. The population was divided into three subpopulations constituted by measured females (with phenotype records), all females, and males. Linear regressions were applied for each trait, considering two periods of birth (1st period: 1938-1996; 2nd period: 1997-2012). The estimated heritability of conformation and management traits varied from 0.01 to 0.53, denoting a perspective of genetic improvement through selection and corrective matings for purebred Dairy Gir populations. The average genetic changes in conformation and management traits were, in general, variable and inexpressive, showing that the selection of Dairy Gir may have had been directed essentially to increase milk yield. The analysis of the two periods of birth indicated that some linear traits present progress (although inexpressive) in the 2nd period (more recent period).(AU)

O objetivo deste estudo foi estimar os parâmetros genéticos e as tendências genéticas para diferentes características de conformação e manejo de animais puros da raça Gir Leiteiro, pertencentes ao Programa Nacional de Melhoramento do Gir Leiteiro (PNMGL). A estimativa das variâncias genéticas e residuais para cada característica foi realizada usando-se o procedimento de máxima verossimilhança restrita (AI-REML), por meio do programa AIREMLF90. A população foi dividida em três subpopulações, constituídas por fêmeas mensuradas (com registros de fenótipo), todas fêmeas e machos. As regressões lineares para cada característica foram ainda divididas em dois períodos de anos de nascimento (1º período: 1938 a 1996; 2º período: 1997 a 2012). As herdabilidades estimadas variaram de 0,01 a 0,53 para as características de conformação e manejo, possibilitando a perspectiva de melhoramento mediante seleção e acasalamentos corretivos na população pura da raça Gir Leiteiro. As mudanças genéticas nas características conformação e manejo foram, em geral, variáveis e inexpressivas, sugerindo que a seleção no Gir Leiteiro possa ter sido direcionada essencialmente para maior produção de leite. Ao serem observados os dois períodos distintos de anos de nascimento, infere-se que algumas características lineares apresentaram progresso (embora inexpressivo) no 2º período analisado.(AU)

Estimation of genetic parameters for test-day milk yield in Girolando cows using a random regression model / [Estimação de parâmetros genéticos para produção de leite no dia do teste em vacas da raça Girolando utilizando modelo de regressão aleatória]

The objective of this study was to estimate the components of variance and genetic parameters of test-day milk yield in first lactation Girolando cows, using a random regression model. A total of 126,892 test-day milk yield (TDMY) records of 15,351 first-parity Holstein, Gyr, and Girolando breed cows were used, obtained from the Associação Brasileira dos Criadores de Girolando. To estimate the components of (co) variance, the additive genetic functions and permanent environmental covariance were estimated by random regression in three functions: Wilmink, Legendre Polynomials (third order) and Linear spline Polynomials (three knots). The Legendre polynomial function showed better fit quality. The genetic and permanent environment variances for TDMY ranged from 2.67 to 5.14 and from 9.31 to 12.04, respectively. Heritability estimates gradually increased from the beginning (0.13) to mid-lactation (0.19). The genetic correlations between the days of the control ranged from 0.37 to 1.00. The correlations of permanent environment followed the same trend as genetic correlations. The use of Legendre polynomials via random regression model can be considered as a good tool for estimating genetic parameters for test-day milk yield records.(AU)

O objetivo deste estudo foi estimar os componentes de variância e os parâmetros genéticos da produção de leite no dia do teste (TDMY) em vacas Girolando de primeira lactação, usando modelo de regressão aleatória. Foram utilizados 126.892 registros de produção de leite no dia controle de 15.351 vacas primíparas das raças Holandesa, Gir e Girolando, obtidas na Associação Brasileira dos Criadores de Girolando. Para estimar os componentes de (co) variância, as funções genéticas aditivas e de covariância ambiental permanente foram estimadas por regressão aleatória em três funções: Wilmink, polinômios de Legendre (terceira ordem) e polinômios splines lineares (três nós). A função polinomial de Legendre apresentou melhor qualidade de ajuste. As variâncias genéticas e de ambiente permanente para produção de leite no dia do controle variaram de 2,67 a 5,14 e de 9,31 a 12,04, respectivamente. As estimativas de herdabilidade aumentaram gradativamente do início (0,13) para o meio da lactação (0,19). As correlações genéticas entre os dias do controle variaram de 0,37 a 1,00. As correlações de ambiente permanente seguiram a mesma tendência das correlações genéticas. A utilização dos polinômios de Legendre via modelos de regressão aleatória pode ser considerada como uma boa ferramenta para estimação de parâmetros genéticos da produção de leite no dia do teste.(AU)

Effect of inclusion or non-inclusion of short lactations and cow and/or dam genetic group on genetic evaluation of Girolando dairy cattle.

The objective of this study was to evaluate the effects of inclusion or non-inclusion of short lactations and cow (CGG) and/or dam (DGG) genetic group on the genetic evaluation of 305-day milk yield (MY305), age at first calving (AFC), and first calving interval (FCI) of Girolando cows. Covariance components were estimated by the restricted maximum likelihood method in an animal model of single trait analyses. The heritability estimates for MY305, AFC, and FCI ranged from 0.23 to 0.29, 0.40 to 0.44, and 0.13 to 0.14, respectively, when short lactations were not included, and from 0.23 to 0.28, 0.39 to 0.43, and 0.13 to 0.14, respectively, when short lactations were included. The inclusion of short lactations caused little variation in the variance components and heritability estimates of traits, but their non-inclusion resulted in the re-ranking of animals. Models with CGG or DGG fixed effects had higher heritability estimates for all traits compared with models that consider these two effects simultaneously. We recommend using the model with fixed effects of CGG and inclusion of short lactations for the genetic evaluation of Girolando cattle.

Correlações genéticas entre algumas características de tipo e intervalo de partos em vacas da raça Holandesa / Genetic correlations for some type traits and calving interval in Holstein cows

Estimaram-se correlações genéticas entre algumas características de tipo e intervalo de partos (IP) em vacas da raça Holandesa no Estado do Paraná. Foram analisados registros de conformação de 23.014 vacas, classificadas no período de 2000 a 2010, oriundas de 248 rebanhos, filhas de 797 touros. Os componentes de variância necessários para obtenção das estimativas das herdabilidades e das correlações genéticas foram obtidos pelo método da Máxima Verossimilhança Restrita (REML), utilizando-se o procedimento MIXED do pacote estatístico SAS. As estimativas de herdabilidade para as características de tipo variaram de 0,09 a 0,24 e para IP foi 0,05. As estimativas de correlação genética entre as características de tipo foram, em geral, de pequena magnitude, sendo o maior valor observado entre largura de peito e profundidade corporal (0,52). As estimativas de correlação genética entre as características de tipo e IP foram, em sua maioria, de pequena magnitude. A baixa correlação genética entre ângulo de garupa e IP (0,02) sugere que a seleção para ângulo de garupa pode não ser efetiva em determinar ganho genético para fertilidade.

Genetic correlations between type traits and calving interval (CI) in Holstein cows in Parana State - Brazil were estimated. The data comprised linear classification of 23,014 cows, from 248 herds, classified from 2000 to 2010, daughters of 797 sires. Variance components for heritability and genetic correlation were estimated with Restricted Maximum Likelihood method (REML), using the MIXED procedure of statistics package SAS. Heritability estimates for type traits ranged from 0.09 to 0.24 and for CI it was 0.05. Genetic correlation estimations among type traits were, in general, of low magnitude, with the higher value between chest width and body depth (0.52). Estimates of genetic correlations between type traits and CI were mostly small. The low genetic correlation between rump angle and CI (0.02) suggests that the selection for rump angle may not be effective in determining genetic gain for fertility.

Genetic parameters for test day somatic cell score in Brazilian Holstein cattle.

Selection for lower somatic cell count has been included in the breeding objectives of several countries in order to increase resistance to mastitis. Genetic parameters of somatic cell scores (SCS) were estimated from the first lactation test day records of Brazilian Holstein cows using random-regression models with Legendre polynomials (LP) of the order 3-5. Data consisted of 87,711 TD produced by 10,084 cows, sired by 619 bulls calved from 1993 to 2007. Heritability estimates varied from 0.06 to 0.14 and decreased from the beginning of the lactation up to 60 days in milk (DIM) and increased thereafter to the end of lactation. Genetic correlations between adjacent DIM were very high (>0.83) but decreased to negative values, obtained with LP of order four, between DIM in the extremes of lactation. Despite the favorable trend, genetic changes in SCS were not significant and did not differ among LP. There was little benefit of fitting an LP of an order >3 to model animal genetic and permanent environment effects for SCS. Estimates of variance components found in this study may be used for breeding value estimation for SCS and selection for mastitis resistance in Holstein cattle in Brazil.

Georeferenced evaluation of genetic breeding value patterns in Brazilian Holstein cattle.

The aim of this study was to analyze the relationship between environmental and genetic values for milk production and type traits in Holstein cattle in Brazil. The genetic value of 65,383 animals for milk production and 53,626 for type classification were available. Socioeconomic and environmental data were obtained from the Brazilian Institute of Geography and Statistics, the Food and Agriculture Organization of the United Nations, the National Aeronautics and Space Administration, and the National Institute of Meteorology. Five to six clusters were generated for each of the groups of type traits and production levels. The relationships between these traits were assessed using the STEPDISC, DISCRIM and CANDISC procedures in SAS(®). Traits within the clusters behaved differently, but, in general, animals with lower genetic values were found in environments that were more stressful for animal production. These differences were mainly associated with temperature, humidity, precipitation and the Normalized Difference Vegetative Index. Genetic values for milk production showed best discrimination between different environments, while type traits showed poor discrimination, possibly because farmers mainly select for milk production. Environmental variations for genetic values in dairy cattle in Brazil should be further examined.

Origins and genetic diversity of British cattle breeds in Brazil assessed by pedigree analyses.

Pedigree information available for Angus (ANG), Devon (DEV), Hereford (HER), and Shorthorn (SHO) cattle in Brazil was analyzed to appraise the genetic diversity and population structure of these breeds. Pedigree records collected from the beginning of the 20th century until 2010 were used in the analyses. Over time, the number of herdbook registrations declined in HER after a peak in the 1970s, remained low in DEV and SHO, and increased steadily in ANG since the 1990s, such that it the latter is now the leading British cattle breed in Brazil. The average number of offspring registered per sire ranged from about 12 (SHO) to 20 (DEV) and the mean generation interval ranged from about 6.0 (HER and SHO) to 6.4 (ANG) years. In the reference population (calves born in 2009 and 2010, plus those born in 2008 for SHO) the mean equivalent number of generations known ranged from about 7 (SHO) to 9 (HER). In the 4 breeds studied, nearly all animals born over the last few years are inbred, even though the mean level of inbreeding in the reference population is below 4% in all breeds. The rate of inbreeding per generation, computed from the individual increase in inbreeding, ranged from about 0.2 (ANG) to 0.5% (DEV), with a corresponding effective population size of 245 and 92, respectively, which is above the recommended minimum critical threshold. The number of founders/ancestors contributing with 50% of the reference population gene pool was 211/26 for ANG, 41/14 for DEV, 164/25 for HER, and 79/10 for SHO, with effective number of founders/ancestors/founder genomes of 470/68/36, 89/33/16, 289/59/30, and 200/28/18 for ANG, DEV, HER, and SHO, respectively. The genetic contribution of different countries to the gene pool of each breed indicated that, throughout the period studied, DEV genes originated predominantly from the United Kingdom, while for the other breeds there was a changing pattern over time. Until the 1970s Argentina was the major supplier of ANG, while HER and SHO genes were mostly from Uruguay, but since then the United States took the leading role as supplier of ANG, HER, and SHO genes to Brazil. Our results reveal a mild increase in inbreeding in all breeds studied, with effective population size estimates indicating that reasonable levels of genetic diversity have been maintained in all 4 breeds. Continuous monitoring of inbreeding trends and of parameters derived from probability of gene origin should be ensured to warrant the long-term maintenance of genetic diversity.

Bioeconomic model and selection indices in Aberdeen Angus cattle.

A bioeconomic model was developed to calculate economic values for biological traits in full-cycle production systems and propose selection indices based on selection criteria used in the Brazilian Aberdeen Angus genetic breeding programme (PROMEBO). To assess the impact of changes in the performance of the traits on the profit of the production system, the initial values ​​of the traits were increased by 1%. The economic values for number of calves weaned (NCW) and slaughter weight (SW) were, respectively, R$ 6.65 and R$ 1.43/cow/year. The selection index at weaning showed a 44.77% emphasis on body weight, 14.24% for conformation, 30.36% for early maturing and 10.63% for muscle development. The eighteen-month index showed emphasis of 77.61% for body weight, 4.99% for conformation, 11.09% for early maturing, 6.10% for muscle development and 0.22% for scrotal circumference. NCW showed highest economic impact, and SW had important positive effect on the economics of the production system. The selection index proposed can be used by breeders and should contribute to greater profitability.

In-depth pedigree analysis in a large Brazilian Nellore herd.

A large herd of Nellore cattle was evaluated using in-depth pedigree analyses. Taking into account the incomplete pedigree due to the use of multiple young sires for mating, the average inbreeding coefficient was calculated as 1.73% for the last generation, which was higher than the regular inbreeding coefficient (0.25%). The effective population size was estimated to be 114, 245, and 101 for the time periods 1995-1999, 1999-2003, and 2003-2007, respectively. Parameters based on the probability of gene origin were used to describe the genetic diversity over time in the herd. The effective number of founders, ancestors, and founder genomes decreased over time, showing an overall loss of genetic diversity. In the last five-year period (2003-2007), based on available pedigree information, one prominent ancestor contributed 10.6% to the gene pool of the herd, and 30% of this pool was contributed by 31 ancestors. The analysis of inbreeding under random mating indicated that the mating strategies used in the herd are slowing down inbreeding rates. However, it is advisable to continue monitoring the inbreeding rates and genetic diversity in this herd in the future.

Efeito da suplementação com diferentes níveis de selênio orgânico e inorgânico na produção e na composição do leite e no sangue de vacas em lactação / Effects of different sources and levels of selenium supplementation on milk yield and composition and selenium blood concentration in lactating cows

Avaliaram-se o efeito da suplementação de selênio, na dieta ofertada aos animais, sobre a concentração do mineral no sangue e no leite e as alterações nas características físico-químicas, contagem de células somáticas (CCS) e produção de leite. O experimento durou 63 dias, dos quais os primeiros 21 foram pré-experimental. Foram utilizadas 32 vacas em lactação da raça Jersey, as quais apresentavam, ao início, peso corporal de 402,5+58,4kg, escore de condição corporal de 3,19+0,31, produção de leite de 10,4+2,1kg e número de dias em lactação de 141,4+69,3. Os tratamentos foram: sem suplementação (grupo-controle); com suplementação de selênio inorgânico 0,3 (dieta-padrão + 0,3mg selenito de sódio/kg de concentrado - SI0,3); com suplementação com selênio orgânico 0,3 (dieta-padrão + 0,3mg seleniometionina/kg de concentrado - SO0,3) e com suplementação de selênio orgânico 0,6 (dieta-padrão + 0,6mg seleniometionina/kg de concentrado - SO0,6). As quantidades totais de selênio das dietas foram, respectivamente, 2,38; 4,18; 4,18 e 5,98mg/dia para os tratamentos controle, SI0,3, SO0,3 e SO0,6. O delineamento experimental foi o completamente ao acaso. O número de dias em lactação e os valores obtidos no início do experimento foram usados como covariáveis. Foram realizadas avaliações da produção de leite, do peso, da condição corporal, da composição do leite e do sangue nos dias 0, 14, 28 e 42 do período experimental. Entre os tratamentos, não foram detectadas alterações quanto à produção de leite, peso, condição corporal, características físico-químicas e microbiológicas do leite, e perfil bioquímico do sangue, exceto em relação à concentração de selênio no sangue entre o tratamento-controle e os tratamentos suplementados. Não houve diferenças quanto aos teores de selênio no sangue entre as fontes de selênio e as doses. Os teores de selênio no sangue evoluíram distintamente durante o experimento conforme a dose e a fonte. A suplementação com selênio ...

The effects of the dietary supplementation with selenium were evaluated on the concentration of the mineral in blood and milk, as well as changes in milk yield, physical and chemical characteristics, and somatic cells count (SCC). The trial lasted 63 days, the first 21 were designed to adaptation of animals to experimental conditions and standard diet. Thirty-two lactating Jersey cows were used and, at the beginning of the trial, they presented body weight of 402.5+58.4kg, body condition score of 3.19+0.31, milk yield of 10.4+2.1kg/day, and 141.4+69.3 days in milking. Treatments were: control (standard diet without added selenium), inorganic selenium (standard diet + 0.3mg sodium selenite/kg concentrate - SI0.3), organic selenium 0.3 (standard diet + 0.3mg selenomethionine/kg concentrate - SO0.3), and organic selenium 0.6 (standard diet + 0.6mg selenomethionine/kg concentrate - SO0.6). Total daily amounts of selenium were 2.38, 4.18, 4.18, and 5.98mg/cow, respectively, for control, SI0.3, SO0.3, and SO0.6 treatments. The trial was conducted as a completely randomized design. The number of days in milking and the values for all attributes measured at the end of the adaptation period were used as covariates. Measurements of body weight and condition score, milk yield and composition, and blood composition were performed on days 0, 14, 28, and 42 of the experimental period. No differences were detected among treatments for milk yield and composition, body weight and condition score, physical-chemical characteristics of milk, somatic cells count, and biochemical profile of the blood, except for Se contents of blood of control compared to supplemented. There were no differences caused by selenium sources or levels. Selenium supplementation did not alter neither milk nor blood components.

Estimação de parâmetros genéticos para produção de leite de vacas da raça Holandesa via regressão aleatória / Estimation of genetic parameters for Holstein cows milk production by random regression

Foram utilizados 21.702 registros de produção de leite no dia do controle de 2.429 vacas primíparas da raça Holandesa, filhas de 233 touros, coletados em 33 rebanhos do Estado do Rio Grande do Sul, para estimar parâmetros genéticos para produção de leite no dia do controle. O modelo de regressão aleatória ajustado aos controles leiteiros entre o sexto e o 305º dia de lactação incluiu o efeito de rebanho-ano-mês do controle, idade da vaca no parto e os parâmetros do polinômio de Legendre de ordem quatro, para modelar a curva média da produção de leite da população e parâmetros do mesmo polinômio, para modelar os efeitos aleatórios genético-aditivo e de ambiente permanente. As variâncias genéticas e de ambiente permanente para produção de leite no dia do controle variaram, respectivamente, de 2,38 a 3,14 e de 7,55 a 10,35. As estimativas de herdabilidade aumentaram gradativamente do início (0,14) para o final do período de lactação (0,20), indicando ser uma característica de moderada herdabilidade. As correlações genéticas entre as produções de leite de diferentes estágios leiteiros variaram de 0,33 a 0,99 e foram maiores entre os controles adjacentes. As correlações de ambiente permanente seguiram a mesma tendência das correlações genéticas. O modelo de regressão aleatória com polinômio de Legendre de ordem quatro pode ser considerado como uma boa ferramenta para estimação de parâmetros genéticos para a produção de leite ao longo da lactação.

A total of 21,702 records of milk production from 2,429 first-lactation Holstein cows, sired by 233 bulls, collected in 33 herds in the State of Rio Grande do Sul from 1991 to 2003, were used to estimate genetic parameters for that characteristic. The random regression model adjusted to test day from the 6th and the 305th lactation day included the effect of herd-year-month of the test day, the age of the cow at parturition, and the order fourth Legendre polynomial parameters, in order to obtain the average curve for the milk production of the population and parameters from the same polynomial to estimate the additive genetic and permanent environmental random effects. The genetic and permanent environmental variances for test day milk yield ranged from 2.38 to 3.14 and from 7.55 to 10.35, respectively. The estimated heritabilities gradually increased from the beginning (0.14) to the end (0.20) of the lactation period, indicating that test day milk yield is a characteristic with moderate heritability. The genetic correlation between milk yield in different phases of lactation ranged from 0.33 to 0.99 and was higher between the adjacent test days. The permanent environmental correlations followed the same tendency of the genetic ones. The random regression animal model using Legendre polynomials of order four can be considered as a good tool to estimate genetic parameters for milk production throughout the lactation.

Utilização dos polinômios de Legendre e da função de Wilmink em avaliações genéticas para persistência na lactação de animais da raça Holandesa / Use of Legendre polynomials and Wilmink function in genetic evaluations for persistency of lactation in Holstein cows

Os registros de produção de leite de 11.023 primeiras lactações de vacas da raça Holandesa pertencentes a 251 rebanhos distribuídos no estado de Minas Gerais foram usados para comparar os polinômios de Legendre e a função Wilmink em modelos de regressão aleatória (MRA) quanto aos seus efeitos na estimação de parâmetros genéticos e predição de valores genéticos para nove medidas de persistência na lactação e produção de leite até 305 dias. Os modelos de regressão aleatória ajustados aos controles leiteiros entre o sexto e o 305(0) dia de lactação incluíram o efeito de rebanho-ano-mês do controle, os parâmetros dos polinômios de Legendre de ordens 3 a 5 ou da função de Wilmink, para modelar as curvas fixas da regressão dentro das subclasses de idade-estação de parto da vaca e os parâmetros dos polinômios de Legendre de ordens 3 a 5, para modelar os efeitos aleatórios genético-aditivo e permanente de ambiente. Os testes do critério de informação de Akaike e Bayesiano indicaram o modelo com maior número de parâmetros como o que melhor se ajustou aos dados de produção de leite. Observaram-se grandes variações nas estimativas de herdabilidade para a maioria das medidas de persistência na lactação, com uso dos modelos que envolveram ajustes dos polinômios de Legendre. As estimativas de herdabilidade variaram de 0,11 a 0,33 para produção de leite ao longo da lactação, de 0,33 a 0,36 para a produção de leite até 305 dias e de 0,00 a 0,32 para persistência na lactação. As correlações genéticas entre persistência e produção de leite até 305 dias diferiram com o modelo e a medida de persistência. A utilização da função de Wilmink, comparada aos polinômios de Legendre, proporcionou mudanças expressivas na ordem dos animais quando classificados para persistência na lactação.

Records of 11,023 first-parity Holstein cows belonging to 251 herds in the State of Minas Gerais were used to compare the Legendre polynomials and Wilmink function in random regression models (RRM) as for their effects in the estimate of genetic parameters and prediction of breeding values for nine types of persistency measurements and 305-day milk yield. The random regression test day models included the effect of herd-year-month test day, parameters of the function of Wilmink or 3th to 5th order Legendre polynomials to model fixed curves of the subclasses and 3th to 5th order Legendre polynomials to model genetic and permanent environmental effects. The AkaikeÆs Information Criterion (AIC) and Bayesian Information Criterion (BIC) indicated the model with larger number of parameters as the one that best fitted the data of milk yield. Using the Legrendre polynomial model large variation was observed in the estimates heritabilities for most of the persistency measures. The estimates herdabilities varied from 0.11 to 0.33 to milk yield throughout the lactation, from 0.33 to 0.36 for the 305-day milk yield and, from 0.00 to 0.32 for persistency. Genetic correlations between persistency and 305-day milk yield differed according to the model and persistency measure. Compared the Legendre polynomials to the Wilmink function provided expressive changes in rank of animals for persistency of lactation.

Controles alternados das ordenhas da manhã e da tarde para estimação da produção de leite na lactação / Morning and afternoon tests for estimating lactation milk yield

Determinaram-se procedimentos para estimar produção diária de leite usando controles da manhã (AM) ou da tarde (PM) e comparar sistemas de controle para previsão da produção de leite em 305 dias usando várias combinações de controles AM e PM. Utilizaram-se 20.328 produções do dia de controle de 1.516 lactações de vacas ordenhadas duas vezes ao dia, em 96 rebanhos supervisionados pela Associação dos Criadores de Gado Holandês de Minas Gerais e registradas no período de 2000 a 2003. Sete procedimentos foram usados para estimação da produção diária. Correlações entre produções observada e estimada, médias e desvios-padrão das diferenças entre elas e desvios-padrão das produções estimadas foram os critérios para comparação de procedimentos. As estimativas obtidas com a ordenha da manhã foram mais acuradas do que as obtidas com as da ordenha da tarde. O procedimento em que se incluiu uma regressão para cada classe de intervalo de ordenhas e uma regressão sobre dias em lactação foi o selecionado para estimação da produção diária. Quatro sistemas para previsão da produção de leite em 305 dias foram comparados. Os sistemas com alternância mensal dos controles AM-PM apresentaram melhores resultados do que aqueles em que se utilizaram somente o controle AM ou somente PM. As correlações entre produção estimada e observada foram 0,99 e 0,97, respectivamente, para os primeiros e os últimos sistemas. Alternância dos controles AM-PM, iniciando-se com o controle AM, foi o sistema mais acurado de estimação.

Procedures to estimate daily yield from either morning (AM) or evening (PM) test weighings and to compare different systems of testing for accuracy of predicting 305-day milk yield using various combinations of AM and PM production records were determined. Data of 20,328 test-day records of 1,516 cows milked twice-a-day, from 96 Holstein Dairy Cattle Association herds in Minas Gerais State and recorded from 2000 to 2003. Seven procedures were used to estimate daily yields. Comparisons were made on the basis of correlations between true and estimated daily yields, average difference between the estimated and actual values and also the variances of differences. Morning weighings predicted daily yield more accurately than evening weighings. The procedure which included a regression for each milking interval class and a regression on days in milk were selected for estimation of daily yields. Four systems for predicting 305-day milk yield were compared. Alternating AM-PM testing systems on monthly frequency showed better results than testing always in the AM or always in the PM. Correlations between observed and estimated 305-day lactation yields were 0.99 and 0.97, respectively, for the first and later systems. Alternating AM-PM testing starting with an AM test was the most accurate system of estimation.