Relationship among thermal environment, stage of lactation, body characteristics, yield and milk constituents of dairy Gyr cows managed on pasture.

Our aims were to evaluate changes in body characteristics, milk yield and milk constituents as well as to determine the relationship between the thermal environment and production characteristics during the first lactation of dairy Gyr cows managed on pasture. Between 2013 and 2015, forty-five primiparous dairy Gyr cows were evaluated from prepartum to 10 months of lactation in Southeast of Brazil. Body weight, body condition score (BCS), subcutaneous fat thickness (SFT), milk yield (305 d), and milk constituents were collected monthly and progesterone was collected weekly. Additionally, we determined the temperature humidity index (THI) based on microclimate data. Overall, the cows lost body weight until six months of lactation and there was a progressive decrease in BCS, SFT, milk yield and milk lactose as the months in lactation progressed. In contrast, there was an increase in milk fat, milk protein and milk solids. The thermal environment did not pose a consistent heat challenge, nevertheless, we found a positive correlation between the average THI two days before milk collection with milk yield, fat and lactose contents, but in contrast a negative correlation was found with total solids and protein. In conclusion, the THI and months of lactation affected the yield and constituents of milk. However, more studies are necessary to understand the impacts of body characteristics and thermal environment on yield and milk constituents throughout the productive life of Gyr dairy cows.

Effects of Training for First Milking Involving Positive Tactile Stimulation on Post-Calving Maternal Behaviors in Primiparous Gyr Dairy Cows.

In dairy systems with Zebu breeds, calves are not immediately separated from their dams after calving; consequently, maternal care and protective behavior are important, influencing both productive performance and stockpeople's safety. Our objectives were to: (1) investigate the effects of a training protocol involving pre-calving positive stimulation, delivered prior to calving, on the maternal care of primiparous Gyr cows; and (2) evaluate the effects of this training protocol on maternal protective behavior towards handlers during the first calf handling. Primiparous dairy Gyr cows (n = 37) were allocated into two groups: training (n = 16) and control (n = 21). Animal behaviors were recorded in three periods: post-calving, first calf handling, and post-handling. Maternal protective behavior during calf handling was assessed from measures of aggressiveness, attention, displacement, and agitation. Calf latency to stand up (p < 0.01) and sex (p < 0.01) differed between the training and control groups. The training group had less touching (p = 0.03), more time not interacting with the calf (p = 0.03), tended to be less protective (p = 0.056), and moved less (p < 0.01) during the first handling of their calves. In conclusion, the primiparous dairy Gyr cows subjected to pre-calving training protocol displayed less maternal care and displacement during the first handling of their calves and tended to be less protective.

Is maternal defensiveness of Gyr cows (Bos taurus indicus) related to parity and cows' behaviors during the peripartum period?

The maternal care of cows can influence both the milk production and the performance of their calves, making this a topic of important relevance for the production industry that uses zebu cattle. The aims of this study were to 1) investigate the effects of parity on the behaviors of Gyr cows during the peripartum period; 2) characterize the maternal defensiveness of primiparous and multiparous cows towards handlers during the first handling of their calves; and 3) evaluate the relationships between cows' behaviors at the peripartum period and maternal defensiveness. Thirty-one Gyr cows (primiparous and multiparous), from Empresa de Pesquisa Agropecuária de Minas Gerais (Brazil), were used. The animals were placed in a maternity paddock monitored by video cameras. The behaviors of the animals were collected in four periods: Pre-calving, Post-calving, First handling of calf and Post-handling. Primiparous cows presented more pain signs, reflected in arched spine (P = 0.05), and tended to move more (P = 0.07) than the multiparous in the Pre-calving period. Trends were observed for both Maternal Composite Score (P = 0.06) and Maternal Protective Behavior score (P = 0.06), indicating that both primiparous and multiparous were protective, but only multiparous cows were aggressive toward the caretakers on the first handling of their calves. The most protective cows spent more time eating during the prepartum period (P = 0.03), while the least attentive cows spent more time lying down (P = 0.02) in the prepartum period. The cows who nursed and stimulated their calves more were also calmer (P = 0.02) and more attentive (P = 0.01). In conclusion, the peripartum behaviors of Gyr cows were related to maternal care and maternal defensiveness. Multiparous cows tended to be more aggressive than primiparous cows at the time of the first handling of their calves.

Genetic parameters for body weight and milk production of dairy Gyr herds.

The aim of this study was to estimate genetic parameters for cow weight at calving (CW) and cumulative 305-day milk yield (MY305) in dairy Gyr cattle by two-trait analysis. The study used 1,847 CW records, in which 418 females presented more than one measure, and 4,048 MY305 records, wherein 1068 females provided repeated measures, from 2,339 females belonging to three herds, which calved between 1986 and 2019. Variance components were estimated by the restricted maximum likelihood method (REML) using a two-trait animal model. The model included direct additive genetic, permanent environmental and residual effects as random effects and the fixed effects of contemporary group, formed by animals that had calved on the same farm year and season, and age of cow at calving as covariates (fitted as linear and quadratic effect). The heritability estimates for CW and MY305 were 0.21 ± 0.06 and 0.29 ± 0.04, respectively, and repeatability estimates were 0.49 ± 0.03 and 0.49 ± 0.02. The genetic correlation between CW and MY305 was positive and of low magnitude (0.33 ± 0.18), indicating that selection for MY305 will cause little genetic change in the weights of dairy Gyr animals. The genetic trends of breeding values of analyzed traits showed marked genetic gains in MY305, with little changes in CW of dairy Gyr cows over the years in the herds studied, which is an important result considering the production systems adopted in the tropics.

Genetics of tolerance to heat stress in milk yield of dairy buffaloes assessed by a reaction norm model.

The objectives of this study were to assess the effects of heat stress on the milk yield and investigate the presence of genotype × environment interaction (G × E) in Brazilian Murrah buffaloes reared under tropical conditions. With this, 58,070 test-day (TD) records for milk yield from 3,459 first lactations of buffaloes collected between 1987 and 2018 were evaluated. A mixed model considering days in milk (DIM) and temperature-humidity index (THI) was applied to quantify milk yield losses due to heat stress. The most detrimental effect of THI on TD milk yield was observed in the mid-stages of lactation, after lactation peak, in DIM 105-154 and 155-204 days (-0.020 and -0.015 kg/day per THI, respectively). The least-squares means of TD milk yield were used to identify a heat stress threshold using a piecewise linear regression model. A substantial reduction in TD milk yield due to heat stress was observed for THI values above 77.8 (-0.251 kg/day per increase of 1 THI unit). An analysis using a single-trait random regression animal model was carried out to estimate variance components and genetic parameters for TD milk yield over THI and DIM values. Increased additive genetic variance and heritability estimates were observed for extreme THI values (THI = 60 and 80) combined with mid-lactation stages. The lowest genetic correlation (0.50) was observed between TD records at opposite extremes of the THI scale (THI = 60 vs. THI = 80). The genetic trends observed for the regression coefficients related to the general level of production (0.02) and specific ability to respond to heat stress (-0.002) indicated that selection to increase milk yield did not affect the specific ability to respond to heat stress until the present moment. These trends reflect the low genetic correlation between these components (0.05 ± 0.14). In this sense, monitoring trends of genetic components related to response to heat stress is recommended.

Estrus Prediction Models for Dairy Gyr Heifers.

Technological devices are increasingly present in livestock activities, such as identifying the reproductive status of cows. For this, predictive models must be accurate and usable in the productive context. The aims of this study were to evaluate estrus-associated changes in reticulo-rumen temperature (RRT) and activity (ACT) in Dairy Gyr heifers provided by reticulo-rumen boluses and to test the ability of different models for estrus prediction. The RRT and ACT of 45 heifers submitted to estrus synchronization were recorded using reticulo-rumen boluses. The means of RRT and ACT at different time intervals were compared between the day before and the day of estrus manifestation. An analysis of variance of RRT and ACT was performed using mixed models. A second approach employed logistic regression, random forest, and linear discriminant analysis models using RRT, ACT, time of day, and the temperature-humidity index (THI) as predictors. There was an increase in RRT and ACT at estrus (p < 0.05) compared to the same period on the day before and on the day after estrus. The random forest model provided the best performance values with a sensitivity of 51.69% and specificity of 93.1%. The present results suggest that RRT and ACT contribute to the identification of estrus in Dairy Gyr heifers.

Genetic principal components for reproductive and productive traits in Holstein cows reared under tropical conditions.

The objective of this study was to compare the standard multi-trait model and five reduced-rank models fitted to the first principal components and genetic parameter estimates in order to determine the most appropriate method to model the covariance structure of reproductive and productive traits in Brazilian Holstein cows. Individual records of the following traits from 5217 cows were analyzed: 305-day milk yield (MY305), peak yield, milk yield per day of calving interval, days from calving to first estrus, days from calving to last service (CLS), calving interval (CI), and gestation length. Schwarz's Bayesian information criterion was used to compare the different models. The results indicated that four principal components were necessary to model the genetic (co)variance structure, reducing the number of parameters to be estimated. Analysis of genetic and phenotypic correlations showed that milk production-related traits were strongly correlated with each other (ranging from 0.74 to 0.99), while the correlation of these traits with the reproductive traits was weak (ranging from - 0.14 to 0.27). Heritability estimates for the traits ranged from 0.03 to 0.18. The reproductive traits CLS and CI and the production trait MY305 should be included as selection criteria in dairy cattle breeding programs because they are correlated with the first two principal components, retaining 91% of the genetic variability of the data.

Predictive potential of activity and reticulo-rumen temperature variation for calving in Gyr heifers (Bos taurus indicus).

Factors related to the thermal physiology and activity of Zebu animals close to calving are still unknown. The aims of this study were 1) to describe the pattern of reticulo-rumen temperature and activity variation in nulliparous Gyr heifers close to calving, and 2) to evaluate the predictive potential of these traits for calving in Gyr heifers. Forty pregnant Gyr heifers that had calved between August and December 2017 at the Getúlio Vargas Experimental Station, Empresa de Pesquisa Agropecuária de Minas Gerais (Epamig), Brazil, were used. The animals received a rumen bolus to monitor reticulo-rumen temperature (Trr) and activity (Act) at intervals of 10 min. Mixed linear models were used. A decrease in Trr and an increase in Act were observed on the days preceding calving. Differences in Trr and Act were more evident during the final 21 and 11 h previous to calving compared to 14 days before calving, measured at the same time of day. There was a decrease of about 0.20 °C in Trr at the time of calving when compared to baseline (14 days before calving measured at the same time of day). Environmental variables, such as temperature and air humidity, as well as biological factors such as circadian rhythm, may influence the interpretation of the data. In conclusion, variations exist in the Trr and Act of Gyr heifers in the hours before calving, which is preceded by a decrease in Trr and an increase in Act. Particularities in the thermal physiology of Zebu cattle must be considered when prediction devices previously validated only for European breeds are used.

Infrared thermography reveals surface body temperature changes during proestrus and estrus reproductive phases in Gyr heifers (Bos taurus indicus).

Our aim was to evaluate the application of infrared thermography (IRT) to detect body surface temperature variation of body regions during the proestrus and estrus phases of the reproductive cycle of Gyr heifers and investigate environmental factors that could affect these measurements. Fifty-seven heifers were submitted to an ovulation synchronization protocol. This was followed by monitoring the heifers every 12 h over 60 h. Heifers were monitored for rectal and vaginal temperature using a digital thermometer. The surface temperature of the eye, vulva, and muzzle regions were monitored by IRT. Meteorological data was recorded for temperature and humidity. Observation of sexual behavior was performed to monitor estrus onset. Transrectal ultrasonography was used to identify the dominant follicle and confirm ovulation of all heifers. We observed a decrease in temperature of the rectum and vagina, as well as in the eye and vulva regions within the first 12 h after the completion of the synchronization. This period coincides with the expected proestrus phase of the estrous cycle. A progressive increase in all temperatures was noticed in the following 36 h, which coincides with the estrus phase of the reproductive cycle. The regions evaluated around the vulva and eye exhibited the highest temperature and experienced less environmental distortion than the muzzle area thermographs. Environmental factors, such as rainfall and temperature-humidity index, influenced the IRT readings altering the radiation patterns detected. In conclusion, IRT is an effective method to detect temperature variation during the proestrus and estrus phases in Gyr heifers. Furthermore, biological and environmental effects should be considered when collecting and interpreting IRT data in livestock.

Effect of thermal stress on basic seminal characteristics of Gyr bulls.

The objective of this study was to evaluate the influence of thermal stress indices and minimum temperature on the semen quality of dairy Gyr bulls and to determine whether the minimum night temperature is able to balance the thermal stress that occurred during the day. A total of 922 semen samples were collected from 391 bulls aged between 15 and 49 months, which participated in progeny pretest trials, from December 2011 to April 2017. Semen characteristics (ejaculate volume, gross motility, sperm motility, sperm vigor, sperm concentration, and percentage of total defects) and scrotal circumference were analyzed. Meteorological data were recorded daily by the National Institute of Meteorology. The analyses were carried out using mixed models. The following fixed effects were included in the model: test, month of sample collection, scrotal circumference, the age of the bull at sampling as covariate (linear regression), and the climatic variables (equivalent temperature index [ETI], temperature and humidity index [THI], or minimum temperature). The effect of the bull nested within the test was included in the model as a random effect. Two periods were considered to evaluate the effect of the climatic variables on sperm quality: 7 days before collection (period 1) and the day of collection (period 2). There was an effect of ETI on sperm vigor and concentration in both periods and on volume and percentage of total defects only in the second period. The THI exerted a significant effect on volume in both periods and on sperm motility and concentration only in the second period. There was an effect of the minimum temperature on ejaculate volume, sperm motility, sperm concentration, and percentage of total defects only in the second period, which corresponds to the day of semen collection. Taken together, the results show that there was sufficient thermal stress to negatively affect semen quality. However, the minimum temperature during the night was a significant factor that balanced these negative effects of THI and ETI on semen traits of Gyr bulls.

Genetic parameters for somatic cell count (SCC) and milk production traits of Guzerá cows using data normalized by different procedures.

This study aimed to estimate the genetic parameters for somatic cell count (SCC) and the genetic association between SCC and milk production traits using two different methods of SCC normalization. The dataset contained information on 8870 lactation records of 6172 Guzerá dairy cows selected for dual-purpose from 95 herds. The lactation means of SCC were normalized in two ways: (a) SCC1 = log10 (SCC) and (b) SCC2 = log2 (SCC/100) + 3. Multivariate analyses were performed considering milk production traits over the course of 305 days of lactation. Estimates of the variance components and genetic parameters were carried out by the Bayesian inference method, applying Gibbs sampling. Single chains of 2,000,000 iterations were used, with sampling discards of the first 5000 chains and a sampling period of every 50 iterations. The deviation of information criteria (DIC) was used to evaluate the best transformation for standardization of the SCC data, comparing analysis 1 (milk production traits over 305 days and SCC1) with analysis 2 (milk production traits over 305 days and SCC2). According to the data structure of this study, SCC1 normalization was the most efficient method and produced better estimates than normalization by the SCC2 method. The heritability estimates for SCC were low regardless of the transformation method used, indicating a small possibility of expressive genetic gains from the direct selection of these traits. However, the repeatability indicated the potential for increasing heritability estimates if the effects of the permanent environment were reduced. The genetic correlations between the milk yield and SCC traits do not indicate the possibility of a correlated genetic gain from the direct selection of one trait. However, concomitant selection for milk production traits and SCC will likely not affect the individual response either.

Relationship between reproductive and productive traits in Holstein cattle using multivariate analysis.

Multivariate procedures are used for the extraction of variables from the correlation matrix of phenotypes in order to identify those traits that explain the largest proportion of phenotypic variation and to evaluate the relationship structure between these traits. The reproductive traits (days from calving to first estrus [CFE], days from calving to last service [CLS], calving interval [CI] and gestation length [GL]) and milk production traits (milk yield at 305 days of lactation [MY305], peak yield [PY] and milk yield per day of calving interval [MYCI]) of 5,217 Holstein females (primiparous and multiparous) were measured. Principal component analysis (PCA) and factor analysis of the correlation matrix were used to estimate the correlation between traits. Analysis grouped the seven traits into three principal components and four latent factors that retained approximately 81.5% and 88.9% of the total variation of the data, respectively. The production variables exhibited positive phenotypic correlation coefficients of high magnitude (>.67). The phenotypic correlation estimates between the productive and reproductive traits were low, ranging from .13 to .22. A strong association (.99) was observed between CLS and CI. Our results indicate that multivariate analysis was effective in generating correlations between the traits studied, grouping the seven traits in a smaller number of variables that retained approximately 81% of the total variation of the data.

Dual-purpose Guzerá cattle exhibit high dairy performance under heat stress.

The present study evaluated the heat stress response pattern of dual-purpose Guzerá cattle for test-day (TD) milk yield records of first lactation and estimated genetic parameters and trends related to heat stress. A total of 31,435 TD records from 4,486 first lactations of Guzerá cows, collected between 1986 and 2012, were analysed. Two random regression models considered days in milk (DIM) and/or temperature × humidity-dependent (THI) covariate. Impacts of -0.037, -0.019 and -0.006 kg/day/THI for initial and intermediate stages of lactation were observed when considering the mean maximum daily temperature and humidity to calculate THI. Heritability estimates ranged from 0.16 to 0.35 throughout lactation and THI values, suggesting the possibility to expect gains from selection for such trait. The variable trajectory of breeding values for dual-purpose Guzerá sires in response to changes in THI values confirms that the genotype × environment interaction due to heat stress can have some effect on TD milk yield. Despite the high dairy performance of Guzerá cattle under heat stress, estimated genetic trends showed a progressive reduction in heat tolerance. Therefore, new strategies should be adopted to prevent negative impacts of heat stress over milk production in Guzerá animals in future.

Mapping the body surface temperature of cattle by infrared thermography.

Infrared thermography (IRT) is an alternative non-invasive method that has been studied as a tool for identifying many physiological and pathological processes related to changes in body temperature. The objective of the present study was to evaluate the body surface temperature of Jersey dairy cattle in a thermoneutral environment in order to contribute to the determination of a body surface temperature pattern for animals of this breed in a situation of thermal comfort. Twenty-four Jersey heifers were used over a period of 35 days at APTA Brazil. Measurements were performed on all animals, starting with the physiological parameters. Body surface temperature was measured by IRT collecting images in different body regions: left and right eye area, right and left eye, caudal left foreleg, cranial left foreleg, right and left flank, and forehead. High correlations were observed between temperature and humidity index (THI) and right flank, left flank and forehead temperatures (0.85, 0.81, and 0.81, respectively). The IRT variables that exhibited the five highest correlation coefficients in principal component 1 were, in decreasing order: forehead (0.90), right flank (0.87), left flank (0.84), marker 1 caudal left foreleg (0.83), marker 2 caudal left foreleg (0.74). The THI showed a high correlation coefficient (0.88) and moderate to low correlations were observed for the physiological variables rectal temperature (0.43), and respiratory frequency (0.42). The thermal profile obtained indicates a surface temperature pattern for each region studied in a situation of thermal comfort and may contribute to studies investigating body surface temperature. Among the body regions studied, IRT forehead temperature showed the highest association with rectal temperature, and forehead and right and left flank temperatures are strongly associated with THI and may be adopted in future studies on thermoregulation and body heat production.

Random regression models to account for the effect of genotype by environment interaction due to heat stress on the milk yield of Holstein cows under tropical conditions.

The present study had the following objectives: to compare random regression models (RRM) considering the time-dependent (days in milk, DIM) and/or temperature × humidity-dependent (THI) covariate for genetic evaluation; to identify the effect of genotype by environment interaction (G×E) due to heat stress on milk yield; and to quantify the loss of milk yield due to heat stress across lactation of cows under tropical conditions. A total of 937,771 test-day records from 3603 first lactations of Brazilian Holstein cows obtained between 2007 and 2013 were analyzed. An important reduction in milk yield due to heat stress was observed for THI values above 66 (-0.23 kg/day/THI). Three phases of milk yield loss were identified during lactation, the most damaging one at the end of lactation (-0.27 kg/day/THI). Using the most complex RRM, the additive genetic variance could be altered simultaneously as a function of both DIM and THI values. This model could be recommended for the genetic evaluation taking into account the effect of G×E. The response to selection in the comfort zone (THI ≤ 66) is expected to be higher than that obtained in the heat stress zone (THI > 66) of the animals. The genetic correlations between milk yield in the comfort and heat stress zones were less than unity at opposite extremes of the environmental gradient. Thus, the best animals for milk yield in the comfort zone are not necessarily the best in the zone of heat stress and, therefore, G×E due to heat stress should not be neglected in the genetic evaluation.

A new way to measure milk yield persistency: a genetic point of view with application to Gyr (Bos indicus) cattle.

The objective of this work was to estimate genetic parameters for a measure of persistency of milk yield and to evaluate its association with 305-d cumulative milk yield and lactation length. 12 346 records from 8202 dairy Gyr cows including lactations up to fifth calving were used. The measure of persistency was obtained from one of the parameters of a quadratic model that describes the cumulative yield across lactation as a function of days in milk. A three-trait multivariate analysis was done. Heritability and repeatability for persistency were 0.08 and 0.21, respectively. Deviance Information Criterion provided evidence that the additive genetic covariance between the measure of persistency studied and 305-d cumulative yield is zero. Genetic correlations between persistency and lactation length were 0.50 and 0.27 for first or all lactations, respectively. Milk yield persistency as measured in this study has low heritability. Selection for persistency can increase lactation length. The measure of milk yield persistency studied here is genetically independent of total milk yield and can be included in routine genetic evaluations of dairy cattle.

Reaction norm model to describe environmental sensitivity across first lactation in dairy cattle under tropical conditions.

Reaction norm models have been widely used to study genotype by environment interaction (G × E) in animal breeding. The objective of this study was to describe environmental sensitivity across first lactation in Brazilian Holstein cows using a reaction norm approach. A total of 50,168 individual monthly test day (TD) milk yields (10 test days) from 7476 complete first lactations of Holstein cattle were analyzed. The statistical models for all traits (10 TDs and for 305-day milk yield) included the fixed effects of contemporary group, age of cow (linear and quadratic effects), and days in milk (linear effect), except for 305-day milk yield. A hierarchical reaction norm model (HRNM) based on the unknown covariate was used. The present study showed the presence of G × E in milk yield across first lactation of Holstein cows. The variation in the heritability estimates implies differences in the response to selection depending on the environment where the animals of this population are evaluated. In the average environment, the heritabilities for all traits were rather similar, in range from 0.02 to 0.63. The scaling effect of G × E predominated throughout most of lactation. Particularly during the first 2 months of lactation, G × E caused reranking of breeding values. It is therefore important to include the environmental sensitivity of animals according to the phase of lactation in the genetic evaluations of Holstein cattle in tropical environments.

Genetic association between milk yield, stayability, and mastitis in Holstein cows under tropical conditions.

The objective of this study was to estimate genetic parameters for milk yield, stayability, and the occurrence of clinical mastitis in Holstein cows, as well as studying the genetic relationship between them, in order to provide subsidies for the genetic evaluation of these traits. Records from 5,090 Holstein cows with calving varying from 1991 to 2010, were used in the analysis. Two standard multivariate analyses were carried out, one containing the trait of accumulated 305-day milk yields in the first lactation (MY1), stayability (STAY) until the third lactation, and clinical mastitis (CM), as well as the other traits, considering accumulated 305-day milk yields (Y305), STAY, and CM, including the first three lactations as repeated measures for Y305 and CM. The covariance components were obtained by a Bayesian approach. The heritability estimates obtained by multivariate analysis with MY1 were 0.19, 0.28, and 0.13 for MY1, STAY, and CM, respectively, whereas using the multivariate analysis with the Y305, the estimates were 0.19, 0.31, and 0.14, respectively. The genetic correlations between MY1 and STAY, MY1 and CM, and STAY and CM, respectively, were 0.38, 0.12, and -0.49. The genetic correlations between Y305 and STAY, Y305 and CM, and STAY and CM, respectively, were 0.66, -0.25, and -0.52.

Genetic parameters for milk, fat and protein yields in Murrah buffaloes (Bubalus bubalis Artiodactyla, Bovidae)

The objective of the present study was to estimate genetic parameters for test-day milk, fat and protein yields and 305-day-yields in Murrah buffaloes. 4,757 complete lactations of Murrah buffaloes were analyzed. Co-variance components were estimated by the restricted maximum likelihood method. The models included additive direct genetic and permanent environmental effects as random effects, and the fixed effects of contemporary group, milking number and age of the cow at calving as linear and quadratic covariables. Contemporary groups were defined by herd-year-month of test for test-day yields and by herd-year-season of calving for 305-day yields. The heritability estimates obtained by two-trait analysis ranged from 0.15 to 0.24 for milk, 0.16 to 0.23 for protein and 0.13 to 0.22 for fat, yields. Genetic and phenotypic correlations were all positive. The observed population additive genetic variation indicated that selection might be an effective tool in changing population means in milk, fat and protein yields.