Genome-wide association studies for citric and lactic acids in dairy sheep milk in a New Zealand flock.

The objectives of this study were to estimate genetic parameters for citric acid content (CA) and lactic acid content (LA) in sheep milk and to identify the associated candidate genes in a New Zealand dairy sheep flock. Records from 165 ewes were used. Heritability estimates based on pedigree records for CA and LA were 0.65 and 0.33, respectively. The genetic and phenotypic correlations between CA and LA were strong-moderate and negative. Estimates of genomic heritability for CA and LA were also high (0.85, 0.51) and the genomic correlation between CA and LA was strongly negative (-0.96 ± 0.11). No significant associations were found at the Bonferroni level. However, one intragenic SNP in C1QTNF1 (chromosome 11) was associated with CA, at the chromosomal significance threshold. Another SNP associated with CA was intergenic (chromosome 15). For LA, the most notable SNP was intragenic in CYTH1 (chromosome 11), the other two SNPs were intragenic in MGAT5B and TIMP2 (chromosome 11), and four SNPs were intergenic (chromosomes 1 and 24). The functions of candidate genes indicate that CA and LA could potentially be used as biomarkers for energy balance and clinical mastitis. Further research is recommended to validate the present results.

Heritabilities and genetic and phenotypic correlations for milk production and fertility traits of spring-calved once-daily or twice-daily milking cows in New Zealand.

The objectives of this study were to estimate the genetic and phenotypic correlations and heritabilities for milk production and fertility traits in spring-calved once-daily (OAD) milking cows for the whole season in New Zealand and compare those estimates with twice-daily (TAD) milking cows. Data used in the study consisted of 69,252 first parity cows from the calving seasons 2015-2016 to 2017-2018 in 113 OAD and 531 TAD milking herds. Heritability estimates for production and fertility traits were obtained through single-trait animal models, and estimates of genetic and phenotypic correlations were obtained through bivariate animal models. Heritability estimates of production traits varied from 0.26 to 0.61 in OAD and from 0.13 to 0.63 in TAD. Heritability estimates for fertility traits were low in both OAD and TAD milking cow populations, and estimates were consistent (OAD: 0.01 to 0.10 and TAD: 0.01 to 0.08) across milking regimens. Estimates of phenotypic and genetic correlations among production traits were consistent across populations. In both populations, phenotypic correlations between milk production and fertility traits were close to zero, and most of the genetic correlations were antagonistic. In OAD milking cows, genetic correlations of milk and lactose yields with the start of mating to conception, 6-wk in-calf, not-in-calf, and 6-wk calving rate were close to zero. Interval from first service to conception was negatively genetically correlated with milk and lactose yields in OAD milking cows. Protein percentage was positively genetically correlated with 3-wk and 6-wk submission, 3-wk in-calf, 6-wk in-calf, first service to conception, 3-wk calving, and 6-wk calving rate in the TAD milking cow population, but these correlations were low in the OAD milking cow population. Further studies are needed to understand the relationship of protein percentage and fertility traits in the OAD milking system. The phenotypic correlations between fertility traits were similar in OAD and TAD milking populations. Genetic correlations between fertility traits were strong (≥0.70) in cows milked TAD, but genetic correlations varied from weak to strong in cows milked OAD. Further research is required to evaluate the interaction between genotype by milking regimen for fertility traits in terms of sire selection in the OAD milking cow population.

Estimation of genetic parameters and individual and maternal breed, heterosis, and recombination loss effects for production and fertility traits of spring-calved cows milked once daily or twice daily in New Zealand.

The objectives of this study were to estimate genetic parameters and individual and maternal breed, heterosis, and recombination loss effects for milk production and fertility traits of Holstein Friesian (F), Jersey (J), and crossbred Holstein Friesian and Jersey (F × J) cows milked once daily (OAD) or twice daily (TAD) in New Zealand. Data on 278,776 lactations from 30,217 OAD and 170,680 TAD milking cows across 644 spring-calving herds were available. Genetic parameters and individual and maternal breed, heterosis, and recombination loss estimates were obtained from univariate animal models. Heritability and repeatability estimates for milk production, milk composition, and fertility traits were consistent across the milking frequencies. Heritability estimates for yields of milk, fat, protein, and lactose varied between 0.21 and 0.29 in OAD and TAD. Heritability estimates for fertility traits ranged from 0.01 to 0.08 in both populations, and estimates were slightly greater in TAD than OAD milking cows. In both milking populations, individual breed effects for yields were in favor of F cows; however, maternal breed effects for yields were in favor of J dams. Jersey cows were more fertile than the F cows in both milking populations, but maternal breed effects for fertility traits were in favor of F dams. Individual heterosis effects were favorable for all traits and were consistent across milking regimens. Crossbred F × J cows had significantly shorter intervals from start of mating to first service and from start of mating to conception, and a higher proportion of 3-wk submission, 3-wk in calf, and 3-wk calving relative to the average of purebred F and J cows. Recombination loss effects were not always unfavorable for production and fertility traits, but most estimates were small with larger standard errors. Favorable maternal heterosis effects were associated with production traits in both milking systems, but maternal heterosis effects were less likely to influence reproductive performance.

Fertility of dairy cows milked once daily or twice daily in New Zealand.

The objective of this study was to evaluate the reproductive performance of New Zealand dairy cows with different milking regimens. A total of 2,562 herds calving in 2017 met the criteria for inclusion in this study. The herds were classified into 5 different milking regimens: 260 herds with cows milked once daily (OAD) during the entire lactation, 1,206 herds with cows milked twice daily (TAD) during the entire lactation, 94 herds that were switched to OAD milking from TAD milking during the mating period (OAD-M), 700 herds that were switched to OAD milking from TAD milking after peak lactation (OAD-P), and 302 herds that switched to OAD milking from TAD milking at end of the lactation (OAD-E). Time from the start of mating to first service (SMFS), start of mating to conception (SMCO) and first service to conception (FSCO) were analyzed using survival analysis. Time from SMFS, SMCO and FSCO was significantly shorter in cows milked OAD compared with cows milked TAD. Also, cows milked OAD had fewer services per conception and higher mean 3-wk submission (SR21), in calf by 3 wk (PR21), in calf by 6 wk (PR42), conception to the first service (PRFS), 3-wk calving (CR21) and 6-wk calving (CR42), and lower not in calf (NIC) than herds with TAD, OAD-M, OAD-P, OAD-E milking cows. Fertility performance differed with parity; first-parity cows had lower SR21, 6-wk submission (SR42), PR21, PR42, PRFS, CR21, and CR42 values, and higher NIC values than second-parity cows. Third parity cows had the highest values for SR42, PR21, PR42, PRFS, CR21, and CR42, and lowest value for NIC compared with cows of other parities. Significant but minor interactions between milking regimen and parity existed for SMFS, SMCO, FSCO, SR21, SR42, PR21, PR42, PRFS, NIC, and CR21.

A review of once-a-day milking in dairy cow grazing systems.

Twice-a-day (TAD) milking during the whole lactation is practiced in about 55% of New Zealand herds and once-a-day (OAD) milking during the whole lactation in about 10% of herds. The remainder of farmers use a mixture of TAD and OAD. Results from long-term comparisons show that over a full-lactation cows milked OAD, on average, had lower yields of milk (27%), fat (23%), and protein (24%) and higher percentages of fat and protein than cows milked TAD, but cows milked OAD for the entire lactation had better reproductive performance. Herds of cows milked OAD have higher mean 3-wk submission rate, 6-wk in-calf rate, and conception to the first service, and lower not-in-calf rate than the herds of cows milked TAD for the entire lactation. Farmers that have adopted OAD milking have culled cows that are unsuitable for OAD, and used sires selected on a OAD selection index to produce cow replacements that are more suitable for OAD milking. This OAD index includes the same traits that are included in the New Zealand national selection index with different relative economic weights on these traits plus including udder support, front teat placement, milking speed, and body capacity. A pasture-based milk production system based on OAD can be an alternative for many dairy farmers to maintain or increase farm profitability, with the additional benefits of better cow fertility and flexibility of using labor.

Genomic Regions Associated with Milk Composition and Fertility Traits in Spring-Calved Dairy Cows in New Zealand.

The objective of this study was to identify genomic regions and genes that are associated with the milk composition and fertility traits of spring-calved dairy cows in New Zealand. Phenotypic data from the 2014-2015 and 2021-2022 calving seasons in two Massey University dairy herds were used. We identified 73 SNPs that were significantly associated with 58 potential candidate genes for milk composition and fertility traits. Four SNPs on chromosome 14 were highly significant for both fat and protein percentages, and the associated genes were DGAT1, SLC52A2, CPSF1, and MROH1. For fertility traits, significant associations were detected for intervals from the start of mating to first service, the start of mating to conception, first service to conception, calving to first service, and 6-wk submission, 6-wk in-calf, conception to first service in the first 3 weeks of the breeding season, and not in calf and 6-wk calving rates. Gene Ontology revealed 10 candidate genes (KCNH5, HS6ST3, GLS, ENSBTAG00000051479, STAT1, STAT4, GPD2, SH3PXD2A, EVA1C, and ARMH3) that were significantly associated with fertility traits. The biological functions of these genes are related to reducing the metabolic stress of cows and increasing insulin secretion during the mating period, early embryonic development, foetal growth, and maternal lipid metabolism during the pregnancy period.

Genome-Wide Association Studies of Live Weight at First Breeding at Eight Months of Age and Pregnancy Status of Ewe Lambs.

This study estimated genetic parameters and identified candidate genes associated with live weight, and the occurrence of pregnancy in 1327 Romney ewe lambs using genome-wide association studies. Phenotypic traits considered were the occurrence of pregnancy in ewe lambs and live weight at eight months of age. Genetic parameters were estimated, and genomic variation was assessed using 13,500 single-nucleotide polymorphic markers (SNPs). Ewe lamb live weight had medium genomic heritability and was positively genetically correlated with occurrence of pregnancy. This suggests that selection for heavier ewe lambs is possible and would likely improve the occurrence of pregnancy in ewe lambs. No SNPs were associated with the occurrence of pregnancy; however, three candidate genes were associated with ewe lamb live weight. Tenascin C (TNC), TNF superfamily member 8 (TNFSF8) and Collagen type XXVIII alpha 1 chain (COL28A1) are involved in extracellular matrix organization and regulation of cell fate in the immune system. TNC may be involved in ewe lamb growth, and therefore, could be of interest for selection of ewe lamb replacements. The association between ewe lamb live weight and TNFSF8 and COL28A1 is unclear. Further research is needed using a larger population to determine whether the genes identified can be used for genomic selection of replacement ewe lambs.