Age at first calving (AFC) is a key parameter in dairy farming referring to the time elapsed from birth to the day on which the heifer gives birth to its first offspring. AFC is an indicator that allows farmers to quantify the costs of milk production, with effects on economic performance (Cielava et al., Reference Cielava, Jonkus and Paura2017). Controlling AFC is an important management factor in achieving a lower risk of dystocia, higher lactation performance, lower somatic cell score and shorter length of the calving interval (Atashi et al., Reference Atashi, Asaadi and Hostens2021). Costs associated with the time required for heifers to become productive direct breeding programs toward development of more precocious animals, as costs of raising replacement animals represent the second largest source of expenses in a dairy farm, accounting for 15–20% of total activity costs (Heinrichs et al., Reference Heinrichs, Tozer, Gabler and Schriefer2022). Heinrichs et al. (Reference Heinrichs, Coleen, Gray, Heinrichs, Cornelisse and Goodling2013) studied the replacement costs of dairy heifers and observed that as AFC decreases, the period that the animal remains on the property also decreases, reducing rearing costs.
Many breeders in Brazil still search for optimal AFC. Some deliberately delay the mating age of their heifers, resulting in AFC at 30 months, while others advance the mating of heifers. However, doubt regarding the best AFC for Holstein cows persists. This study aimed to analyze the relationship between AFC, productive characteristics and milk composition in Holstein dairy cattle. The hypothesis is that younger animals have different production parameters than animals that start their productive lives at a later age.
Materials and methods
The experimental protocol was approved by the Animal Ethics Committee at the State University of Maringá (protocol No 7388211020). First lactation data from 737 purebred Holstein heifers, calving between the years 2019 and 2021, were used. Farms were in Carambeí, PR, Brazil. Immediately after birth, calves were identified and received colostrum. During the first few days after birth, calves were transferred to another farm also located in Carambeí. For 30 d during the weaning period (days 1–70 after birth), calves were housed in cages and fed 6 l of milk a day, divided into two feedings (morning and afternoon). From 30 to 70 d of age, the amount of milk was gradually reduced until calves were fully weaned. Oat or ryegrass hay was available to all calves from 30 d of age onwards. After weaning, the calves were transferred to collective pens, in batches according to age. In these lots, the animals had access to concentrated feed, mineral salt, hay and pre-dried alfalfa or oats. The heifers were fed according to body development. Diets were based on protein ingredients, mineral salt and corn or oat silage. All animals when there body score was between 2.8 and 2.9. Two months before the expected date of calving, heifers were transferred back to where they were born, divided into pre-calving batches and housed in maternity stalls. Anionic diets were fed to all animals 30 d before expected calving.
After birth, cows were allocated to a free-stall confinement production system, where they were kept stable throughout their productive life. Animals were milked twice a day and fed three times a day. A balanced diet consisted of roughage (corn silage, oat, or alfalfa and pre-dried oat or alfalfa) and concentrate (soybean meal and ground corn). Mineral salt and water were available ad libitum. Information on milk production and peak yield was obtained from a milking parlor system equipped with Delpro® software (DeLaval®). Data were collected by individual computerized meters during milking. The number of days in lactation was calculated from the time interval between parturition and drying. Peak production was defined as the highest daily production value in kilograms of milk recorded during lactation. Somatic cell count (SCC) analyses were performed by flow cytometry. Protein and fat contents were determined by Fourier-transform infrared spectroscopy. Milk samples were analyzed at the milk analysis laboratory (PARLEITE) of the PR Association of Holstein Cattle Breeders (APCBRH) as part of quality control.
Production characteristics of the herd were assessed and analyzed using descriptive statistics (minimum, maximum, and standard deviation, data shown in online Supplementary Table S1). This step was necessary to characterize the herd as a whole. During the second stage, animals were divided into groups according to AFC. The subdivision was performed using hierarchical cluster analysis. Cluster analysis is a multivariate statistical technique that results in the formation/classification of groups of individuals or cases. Within each group, cases are similar to each other with regard to the variables chosen for classification, and between groups, cases differ according to the classifying variables (Hair et al., Reference Hair, Black, Babin and Anderson2009), in this case, AFC.
The following model was used for cluster analysis. It calculates the shortest distance between elements i and j using the dij matrix (Hair et al., Reference Hair, Black, Babin and Anderson2009).
$$d[ {k, \;( {ij} ) } ] = {\rm max}[ {d( {k, \;i} ) , \;\;d( {k, \;j} ) } ] $$Animal clusters were analyzed using analysis of variance (ANOVA) and subjected to means tests using Tukey's test (P < 0.05). This procedure allowed classifying groups according to precocity at first calving.
In the next step, animal groups with different precocities were assessed with regard to milk production and quality variables. Statistical analyses were performed using IBM SPSS Statistics version 9.0. This software was chosen because of the ease with which it generates statistical calculation results and data crossings.
Results and discussion
The results of cluster analysis are shown in Table 1. Three groups of animals were formed according to AFC. The first group consisted of younger animals with high precocity, the second of older animals with medium precocity and the third group of oldest animals with low precocity. The AFC mean in this study was 740.16 ± 70.28 d, or 24.3 ± 2.3 months, with significant differences between groups. High-precocity heifers (n = 473) had a mean AFC of 697.52 d (approximately 22.9 months), which was significantly lower than that of medium-precocity heifers (n = 234: 800.85 d, approximately 26.3 months). Medium-precocity group, in turn, had a significantly lower AFC than low-precocity heifers (n = 30, 939.10 d, approximately 30.8 months). Sawa et al. (Reference Sawa, Siatka and Krezel-czopek2019) in Poland, found that the ideal AFC of Holstein heifers is 22–26 months of age for enhanced production and productive longevity, so our groups have experimental validity for examining effects of AFC, including loss of productivity at extended AFC. Cielava et al. (Reference Cielava, Jonkus and Paura2017), in a study conducted in Latvia, found that the AFC of heifers reared in a loose housing system was 834 ± 11.3 d and that of heifers reared in a tie-stall housing system was 821 ± 21.4 d. Mohd Nor et al. (Reference Mohd Nor, Steeneveld, Van Weven, Mourits and Hogeveen2013) categorized Holstein heifers into three different groups on the basis of AFC (in months), namely: ≤24 months, >24 months and <27 months. The AFC of Holstein heifers may vary according to the population and location of the herd.
Table 1. Results of cluster analysis of Holstein cows based on age at first calving (AFC)
1 Tukey's test.
Table 2 shows mean values of peak milk production, daily production, days in lactation, SCC, 305-day fat content, 305-day protein content, 305-day fat yield, 305-day milk yield and 305-day protein yield of each group. Peak milk production differed significantly between groups with values being lowest in the high-precocity group (P < 0.05). Mean daily milk production of medium- and low-precocity groups did not differ (P > 0.05) although low-precocity heifers were numerically the highest, being 0.95 and 2.05% higher than that of medium- and high-precocity groups, respectively (Table 2). Studies carried out in the United States demonstrated that each month added to the breeding period of Holstein heifers can represent an additional cost of US$50–US$75 in rearing (Hutchison et al., Reference Hutchison, Vanraden, Cole and Bickhartt2017). However, as observed in the current study, animals in the high-precocity group (AFC approximately 23 months) were less productive. Milk yield and composition were improved in the medium-precocity group (26.2 months). Mohd Nor et al. (Reference Mohd Nor, Steeneveld, Van Weven, Mourits and Hogeveen2013) demonstrated that decreasing the AFC of Holstein heifers to less than 23 months of age may lead to a 310 kg decrease in milk production during first lactation, as observed here. Several authors described that milk production is maximized when AFC occurs between 24 and 26 months (Wathes et al., Reference Wathes, Pollott, Johnson, Richardson and Cooke2014; Atashi et al., Reference Atashi, Asaadi and Hostens2021), and our results corroborate this. Heifers that calve at a young age (<22.9 months) have low production and profitability in the first lactation, possibly due to incomplete mammary gland development (Eastham et al., Reference Eastham, Coates, Cripps, Richardson, Smith and Oikonomou2018). Other factors might also be in play: Atashi et al. (Reference Atashi, Asaadi and Hostens2021) compared heifers calving for the first time between 23 and 26 months of age and showed that both increased and decreased AFC were associated with increased risk of dystocia.
Table 2. Performance parameters of groups of primiparous Holstein cows (n = 737) according to age at first calving
1 Tukey's test
Values are mean ± standard deviation
a,bMeans followed by different letters indicate significant between differences between groups within a parameter.
Lactation duration did not differ between our groups (P > 0.05: Table 2). The highest numerical mean was observed among low-precocity heifers, being 4.96% higher than the high-precocity group, representing a 17-day greater lactation period. SCC, 305-day fat content and 305-day protein content did not differ between groups (P > 0.05). However, 305-day fat and protein yields were highest in medium- and low-precocity groups, not differing between them (P > 0.05). High-precocity group had the lowest value, differing significantly from other groups (P < 0.05).
Animals in the medium-precocity group had a 305-day milk yield of 10 827.56 kg, not differing (P > 0.05) from animals in the low-precocity group (10 655.07 kg). High-precocity group had the lowest mean milk production. The difference between medium- and high-precocity groups amounted to 492.26 kg of milk per animal. In the current study, 305-day milk yield, fat yield, and protein yield were higher in medium- and low-precocity groups than in the high-precocity group (P < 0.05). This is presumably due to the fact that older animals require less energy for growth and can thus direct more energy toward milk production. About 96% of heifers started their productive life at 24 months (range, 22–26 months), in agreement with Wathes et al. (Reference Wathes, Pollott, Johnson, Richardson and Cooke2014). Medium-precocity animals had the highest mean milk production during the peak period (50.37 ± 8.33 kg), showing that high- or low-precocity animals do not always have the best performance. Furthermore, the medium-precocity group had the highest 305-day milk yield (10 827.56 ± 2103.00 kg), with a daily mean of 35.5 kg, and the highest 305-day fat yield (402.90 ± 81.95 kg). Medium-precocity animals are more desirable, particularly for farmers that are paid per liter of milk and receive a bonus for milk solids content (Eastham et al., Reference Eastham, Coates, Cripps, Richardson, Smith and Oikonomou2018). In dairy farming, milk production during the first lactation is an important economic indicator, serving as a predictor of productive performance and longevity throughout the animal's life (Atashi et al., Reference Atashi, Asaadi and Hostens2021). AFC is also strongly associated with milk production during the first lactation.
In conclusion, age at first calving must be taken into account during decision-making, as it affects milk production during the first lactation. To maximize the production of Holstein cows, it is suggested that heifers have an AFC of 24 to 26 months, starting their reproductive life between 14 and 16 months of age. Results indicated that the medium-precocity group has promising economic characteristics for dairy production systems.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0022029923000729
Acknowledgements
The authors would like to thank the Department of Animal Science and the Post-Graduate Program in Animal Science of the State University of Maringá for their technical support. This study was financed in part by the Brazilian Federal Agency for Support and Evaluation of Graduate Education (CAPES, Finance Code 001) through a scholarship awarded to the first author, the Brazilian National Council for Scientific and Technological Development (CNPq), the National Institute of Science and Technology of Dairy Production Chain (INCT-LEITE/UEL-PR, Londrina, Brazil) (grant No. 465725/2014-7), and the Araucaria Foundation (Curitiba, Brazil). The authors also thank the Melkstad Farm, particularly Márcio Hamm, for having welcomed and received us so well, with great joy, patience, and expert advice throughout our stay.
