The study used twelve year recorded data’s analysis for milk yield and composition of Holstein Friesian crossbred dairy cows in Holleta agricultural research center dairy farm. Data’s were summarized and categorized based on season, exotic blood level (genotype), parity and lactation stages. The summarized data’s were season (219 wet, 1055 dry), exotic blood level (1, 117 for 50%, 115 for 62.5% and 42 for >75%), Parity (474 for parity-2, 356 parity-3, 270 for parity-4 and 174 for parity-5), Lactation stage (483 for early, 445 for mid and 346 for late). Analysis of means and standard errors of mean for the parameter studied was estimated using SAS. The General Linear Model was utilized for analyses of variance of data on average monthly milk yield and milk composition for the effects of season, parity, Friesian cross blood level and stages of lactation. Season significantly affected milk yield, fat and protein percentage composition. Higher yield and fat % composition record in dry season whereas higher protein% during wet season. Genotype significantly affected milk yield where yield of 62.5% and >75% crossbred cows were significantly higher than that of 50% crossbred cows. Difference in dam Parity significantly affected milk yield and protein content of milk where higher milk yield and protein content was recorded in dam parity five. An increasing trend observed in milk yield and protein content as dam parity advances. Mean monthly milk yield, percent of protein and total solid was varied significantly between different lactation stages where protein and Total solid percentage was significantly higher in late stages of lactation. In overall milk yield was significantly affected by season, genotype, parity and stages of lactation but it is negatively correlated with the percentage of fat, protein and total solid. Season, parity and stages of lactation significantly affected protein content of milk whereas milk composition strongly correlated with each other.

STUDY SITE

The research study used data’s of Holleta Agricultural Research Centre dairy Farm, in the central high lands of Ethiopia. Holleta agriculture Research center Dairy farm was established for research work. The farm has pure boran breed and its cross with different crosses level of Holstein Friesian.

Holleta

It is located in central highland of Oromia special zone surrounding Addis Ababa at latitude of 38°30’ E, 9°3’ N and 29 km west of Addis Ababa on high way to Ambo. It has an altitude of 2400 m above sea level and receives mean annual rain fall of 1100 mm with bimodal distribution 70% of which occurs during the main rainy season (June to September) and 30% during the small rainy season (February to April) and the annual temperature of 11-22°C with relative humidity of 50.4%. The soil type in the area is largely nitosol and major crops grown are teff, wheat, barley, oats, potatoes, oil crops and pulses.

METHODS

Record data analysis of Holeta Agricultural Research Centre Dairy Farm

The data for this study was taken from Holleta Agricultural Research Center dairy farm. Twelve years records of Holstein Friesian crossbred cows were used for analysis. Lactation records of cows having their second up to fifth calving between 2003 and 2014 were used for analysis. The effects of season, parity, genotype (exotic cross blood level) and lactation stage on milk yield and milk composition were evaluated. Data for analysis were classified according to season, genotype (exotic cross blood level), lactation stage and parity. On the basis of main prevailing climatic conditions, the year was classified into two seasons, the wet season from June to September in which the area gets its major rainfall and the dry season from October to May which receives small rainfall. Those data records in these two seasons were used to see the effect of season on milk parameters. To see the effects of lactation stages on milk parameters, data’s were categorized into three stages of lactation (Early stage of lactation: 7-105 days; Mid-stage of lactation: 106 to 210 days; Late stage of lactation: above 211 days). The data’s were further divided into 4 categories viz 2, 3, 4 and 5 parity to study the effect of parity on milk parameters. Cows were also categorized based on their genotype (exotic cross blood level) viz 50%, 62.5% and ≥ 75% exotic blood level to study the effect genotype on milk parameters.

The effects of season, parity, lactation stage and exotic cross level on milk production and compositions will be examined using least square technique of fitting constants.

DATA MANAGEMENT AND STATISTICAL ANALYSIS

Analysis of means and standard errors of mean for the traits studied was estimated using SAS 9.1 of 2008. The General Linear Model (GLM) will be utilized for variance analyses of data on average daily milk yield, including the effects of season, parity, Friesian cross blood level, stages of lactation and the interactions between these effects. Differences will be considered significant at P<0.05. The following model will be used to test:

Y_ijkl=µ + a_i + b_j + c_k + d_l + ab_ij + ac_ik + ad_il + bc_jk + bd_jl + cd_lk + abc_ijkl + e_ijkl

In which,

Y_ijkl=The daily milk yield

μ=The overall mean

a_i=The effect of season (i=dry, rainy season)

b_j=The effect of Friesian blood level (j=50%, 62.5%, 75%)

c_k=The effect of the dams parity (k=2, 3, 4, 5)

d_l=Effect of stage of lactation(l= Early, Mid and Late)

ab_ij, ac_ik, bc_jk, ad_il, bd_jl, cd_lk abc_ijk the respective interactions between the main effects and e_ijkl of the random residual effect. The interaction effect will be considered if the main effects are significant.

RESULTS AND DISCUSSION

This paper was tried to show the effect of season, blood level of Holstein Friesian cross, parity and stages of lactation on milk yield and composition. These were:

Effects of season on milk yield and composition of dairy cows

The effects of season on milk yield and milk composition of dairy cows are presented in Table 1. A significant mean difference (p<0.05) was observed in milk yield, percent of fat and protein due to season. Higher yield and percent fat was recorded during dry season but for protein during wet season. As result of this study season didn’t significantly affect milk percentage composition of total solid. Opposing to this finding, Baset et al. [12] reported that season didn’t affect milk yield. Similar finding were reported by Sharma et al. [10] that the overall mean for fat content of milk were 4.53 percent which was almost similar(4.66) with this study. Contrary to this Sharma et al. [10] also reported that TS content varied among seasons being highest in winter. In contrast to this study in the same location, Mesfin and Getachew [13] reported higher milk fat content from milk sampled in July to September. The same study also reported finding which agrees with this finding that Friesian crossbred dairy cows have shown high protein content in milk during rainy season. Similar results were reported by Cheruiyot et al. [14] that fat content was significantly higher (3.97 + 0.24%) in the dry season than in the wet season (2.59 + 0.24%)whereas Casein (protein) content was higher in milk sampled in the wet season (3.27 + 0.06%) than in the dry season (2.88 + 0.06%). Similar study also reported that TS contents were not affected by the month of sampling. Similar results were reported by that the milk protein level in the hot month was less than wet season due to the decrease in the casein. Contrary to this earlier study in the same location by Mesfin and Getachew [13] reported that milk total solids content of Boran-Friesian crossbred dairy cows was highest in July to September and the lowest in January to March. Effects of genotype on milk yield and composition of dairy cows

The effects of genotype on milk yield and composition of dairy cows are presented in Table 1.

Milk yield of 62.5% and >75% crossbred cows were significantly higher than that of 50% crossbred cows. Average monthly milk yield of cows of 50%, 62.5% and >75% crossbred were 215.59, 231.09 and 232.81 liters, respectively, which differed significantly (p<0.05). Though there was no significant differences in milk components, milk protein of 62.5% crossbred cows was lower in figure than the other. Mesfin and Getachew [13] and Turki et al. [15] finding agrees with this study on milk yield that high merit cows had the highest yield of milk, whereas the low merit cows had the lowest yield of milk fat, protein, and lactose concentrations. Similar finding were reported by that in the highland climatic zone, the mean MYL for cows with 50 percent B. taurus genes was 2.6 times higher than that of the indigenous cows and cows with exotic inheritance of 75 percent B. taurus genes showed almost a similar performance, with an MYL 2.7 times higher than that of local cows. Similar finding was reported Nantapo [16] where milk yield and fat content of milk differ in in different Genotypic. On contrary to this study Haile et al. [17] and Islam et al. [18] reported that increasing the proportion of exotic genes in a cow leads to decreased milk component levels.

Effects of parity on milk yield and composition of dairy cows

The effects of parity on milk yield and composition of dairy cows are presented in Table 1.

Difference in Parity of cow significantly (p<0.05) affected milk yield and protein content of milk. Significantly higher milk yield and protein content was recorded on dam parity five. An increasing trend observed in milk yield and protein content as dam parity advances. Similarly Bath et al. [19] finding justified this, where an increase in milk yield with the increasing age was partially attributed to higher body weight, whereas the remaining 20% is the result of increased development of the udder during recurring pregnancies which results in larger mass of digestive system and mammary glands for synthesis of milk. Almost similar results were reported by Niraj et al. [20] where mean lactation milk yield was found to be 2503.6 ± 76.8 L (242.9 ± 2.6 L/month). In agreement with this study Afzal et al. [21] reported that parity significantly affected milk production where more milk in cows with greater parities than those with lesser parities. Lee and Kim [22] also reported finding in agreement with this results that there is an increase in milk yield towards 5^th parity and decline thereafter.

Effects of stage of lactation on milk yield and composition of dairy cows

The least squares means (LSM) of milk yield and composition of the different stages of lactation crossbred dairy cows in Holleta agricultural research center of dairy farms is indicated in Table 1. Mean monthly milk yield was varied significantly (p<0.01) between different lactation stages. As lactation stage advances there was decreasing in milk yield and increasing states of total solid % and protein. In agreement with this Baset et al. [12] study reported highest milk yield was observed in the early lactation stage while the lowest yield was recorded in late stages of lactation. Although, the fat percentage composition was not significantly affected by advancing lactation stages there is an increasing trend of figures. Similar result was reported by Natapo [16] that record of highest milk yield in the early lactation stage and lowest yield in late lactation. Mahmoud et al. [23] reported similar results higher milk yield in the early lactation stage then it decreased gradually until the end of lactation. Mushtaq and Subhan [24] finding reported similarly for higher yield in early lactation but differently vise-versa for mid and late lactation in milk yield.

Interaction effect of seasons, genotype, parity and lactation stage on milk yield and composition of dairy cows

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