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Título

Effects of β-casein A1 and A2 milk and β-casein genotype on milk intake, growth, mean fecal score, and days with diarrhea in dairy calves

Titulo em português

Efeitos do leite β-caseína A1 e A2 e do genótipo β-caseína na ingestão de leite, crescimento, escore fecal médio e dias com diarreia em bezerros leiteiros

Introdução (obrigatório)

The growing consumer interest in A2 milk, driven by its purported significance in human health and commercial potential, has resulted in an increased selection of bulls and cows for homozygous β-CN A2 milk (Arens et al., 2023). However, before incorporating the β-CN A2 genotype as a criterion in bull selection, it is crucial to explore its effects on milk production and composition, reproductive performance, health, and survival. Scott et al. (2023) and Arens et al. (2023) evaluated these variables in cows. However, no studies have assessed the performance and health considering calves β-CN genotype.
Comparing the effects of feeding β-CN A1 and A2 milk, Hohmann et al. (2021) reported a higher milk intake and feed efficiency, a tendency for higher average daily gain (ADG), and lower fecal score and prevalence of diarrhea in calves fed β-CN A1 milk compared with calves fed A2 in the first 21 days of life. Kappes et al. (2024) did not observe any difference in the first two weeks of life. The aim of this study is to assess the performance, mean fecal score, and days with diarrhea in calves fed β-CN A1 and A2 milk carrying the β-CN genotype A1A1, A1A2, or A2A2

Material e métodos (obrigatório)

In total, 104 German Holstein (GH), German Simmental (GS), and crossbred GH x GS (CR) calves were evaluated. The calves were randomly assigned to one of two feeding groups according to their birth order. The feeding groups were: β-CN A1 milk (n = 52), or β-CN A2 milk (n = 52). After birth, the calves were weighed and received ad libitum colostrum (IgG > 50 mg/ml), not considering the β-CN type of the colostrum. On the second day, the calves were housed in pairs in double-igloo systems, according to their random birth order. They were offered 7.5 liters of either β-CN A1 or A2 milk divided into three times daily, measuring the leftovers. Fecal score was recorded daily from 1 to 4. Fecal scores 3 and 4 were considered diarrhea. On day 15th, the calves were weighed to determine the end body weight, ADG, total milk intake, total energy-corrected milk intake (TECMI), and feed conversion based on TECMI. The calves were genotyped for β-CN using a 50k bovine oligonucleotide (SNP) chip analysis, and additionally the inbreeding value was calculated based on the SNP data. A variance analysis was performed by using the MIXED procedure. The model included the fixed effects β-CN milk, β-CN genotype, sex, breed, and β-CN milk x β-CN genotype interaction. The significance level was set to an error probability of 5%.   

Resultados e discussão (obrigatório)

The results are presented in table 1. No interaction was observed between milk type and β-CN genotype. Table 1. Least squares means, standard errors (SE) for body weight, milk intake, ADG, feed conversion, mean fecal score, and days with diarrhea for β-CN milk type and β-CN genotype. Variables β-CN milk type P-value β-CN genotype P-value A1 SE A2 SE A1A1 SE A1A2 SE A2A2 SE Birth body weight, kg 41.9 0.8 41.1 0.8 0.52 41.7 1.3 41.1 0.8 41.6 0.8 0.90 End body weight, kg 52.1 0.7 51.0 0.8 0.31 52.2 1.2 51.6 0.8 50.9 0.8 0.62 ADG, g 704 29 678 32 0.54 710 49 723 30 639 31 0.10 Total milk intake, l 96.6 1.3 94.9 1.5 0.40 98.0 2.3 95.2 1.4 94.1 1.4 0.38 TECMI, l 105.0 1.8 104.7 2.0 0.93 107.6 3.1 103.1 1.9 103.9 2.0 0.50 Feed conversion, l/kg 10.5 0.3 10.7 0.3 0.73 10.5 0.6 10.3 0.3 10.9 0.3 0.40 Mean fecal score 1.4 0.05 1.5 0.05 0.20 1.4ab 0.08 1.3b 0.05 1.5a 0.05 0.02 Days with diarrhea 1.7 0.2 2.2 0.2 0.15 1.7ab 0.3 1.6b 0.2 2.5a 0.2 0.01   Differing from Hohmann et al. (2021), we did not observe differences for any trait evaluated when calves were fed β-CN A1 or A2 milk, maybe related to the shorter period of evaluation (14 days). Calves with the β-CN A1A2 genotype had almost one day less of diarrhea than A2A2 calves, with no difference from A1A1 calves. A similar pattern was observed for the mean fecal score, which tended to have a negative effect on ADG and was slightly better in A1A2 and A1A1 calves than in A2A2 calves (P=0.10). The increased selection towards the A2 allele may lead to an increase in inbreeding, negatively affecting the calf immunity and performance. However, we did not observe differences for the inbreeding coefficient (3.6%) among the three β-CN genotypes (P=0.25). Therefore, the fecal score and frequency of diarrhea were not affected by the inbreeding coefficient, suggesting the effect of other factors related to the genotype. More studies need to be performed to better understand the role of the β-CN genotype in calf diarrhea.

Conclusão (obrigatório)

The β-CN A1 and A2 milk did not affect total milk intake, ADG, mean fecal score, and days with diarrhea. Calves with the β-CN A1A2 genotype had a lower mean fecal score and fewer days with diarrhea than A2A2 calves in the first two weeks of life, not differing from A1A1 calves, which resulted in a slightly better ADG for A1A2 and A1A1 calves.

Referências bibliográficas (opcional)

Arens, S.C. et al. Relationships of beta-casein genetics with production, fertility, and survival of purebred organic Holstein dairy cows. JDS Commun. 4:458–463, 2023.
Hohmann, L.G. et al. Comparative Effects of Milk Containing A1 versus A2 β-Casein on Health, Growth and β-Casomorphin-7 Level in Plasma of Neonatal Dairy Calves. Animals 11:1–14, 2021.
Kappes, R. et al. Effect of β-casein A1 or A2 milk on body composition, milk intake, and growth in Holstein, Simmental, and crossbred dairy calves of both sexes. J. Dairy Sci. 6:4033-4044, 2024.
Scott, B.A. et al. Evaluating the potential impact of selection for the A2 milk allele on inbreeding and performance in Australian Holstein cattle. Front. Anim. Sci. 4:1–10, 2023.