Effect of age at first calving and other non-genetic factors on longevity and production traits in Holstein cattle under Vojvodina Province condition, Serbia

The performance of dairy cows, in terms of fertility, milk production and longevity, is a key driver of profitability (Dash et al., 2018). A decrease in age at first calving decreases the cost of raising animals to productive life, increases the annual genetic gain and raises the average productive life of the animal (Singh et al., 2013; Singh et al., 2017). The inclusion of longevity traits in the breeding program increases the profitability of dairy herds (Honarvar et al., 2010). Longevity, as reflected by total lifetime (TL) / lifespan and length of productive life (LPL), is a trait of special interest for breeders. These traits reflect the fertility, health and overall ability of cows, and not just the level of production (Ratwan et al., 2017). Reproductive traits, such as AFC and calving interval affect economically important traits like longevity, productive life and profitability of cows (Thombre et al., 2002; Do et al., 2013; Singh et al., 2017). These authors pointed out that the AFC should range in between 22.5-23.5 months in order to reach the peak of the productive life, while Nilforooshan and Edriss (2004) stated that AFC of 24 months was optimal for the lifetime milk production. A number of countries are included in the models of breeding value functional traits in order to solve the problems associated with unwanted culling (Sewalem et al., 2008). Cooke and Wathes (2014) stated that the increase of AFC over 25 months in Holstein dairy herds in Great Britain, reduced the number of animals that reach the third calving. Mészáros et al., (2008) reported that AFC does not have a major impact on LPL, while Do et al., (2013) stated that with increasing AFC the total lifetime increases linearly. The higher values of AFC are associated with poorer fertility in the first lactation and the shorter LPL of the cow (Zavadilová and Štípková 2013). Pirlo et al., (2000) found that there were small differences in the longevity of cows when the AFC ranged between 671(22.4 months) - 823 days (27.4 months). Haworth et al., (2008) concluded that the first calving should happen in the appropriate mature age of a cow in order to attain good longevity of cows, i.e., too early first calving of heifers affects negatively longevity traits.
 
The aim of the present study was to investigate the effect of age at first calving and various non-genetic factors i.e. season of calving, farm and system of housing on longevity and production traits in Holstein cows of the Vojvodina Province, Serbia.

                          Y_ijklm = μ + A_i + S_j + F_k + H_l + e_ijklm

The mean of AFC, TL, LPL, NL and LMY for Holstein cows were 27.3, 67.5 and 40.2 m, 2.45 and 18798.0 kg, respectively (Table 1).
 
                                                                                                                  
 
The mean age at first calving of the studied population was almost in line with the study of Raguz et al., (2011) in Croatia (27.13 mo) probably because of very similar rearing conditions. The AFC in present study was greater than the estimated mean of 26 m in Italy (Pirlo et al., 2000), 26.84 m in Iran (Nilforooshan and Edriss, 2004), while it was lower than the estimated mean of 32.4 m and 29.81 m in Turkey (Bayram et al., 2009 and Teke and Murat, 2013). On an average, cows in this study had 67.5 m of TL, which was more than the value of 57.2 m obtained in Iranian Holsteins (Nilforooshan and Edriss, 2004) and 50.63 m in Turkey (Teke and Murat, 2013). Average number of lactations obtained from the current study (2.45) was relatively similar (2.64) with the results by Stanojević et al., (2015). The average value for LPL in research was (40.2 m / 3.35 years) higher in relation to the value obtained by Stanojević et al., (2015) in the Serbian group of Black and White cattle improved by Holstein (3.14 years). The values reported by Nilforooshan and Edriss (2004) in Iranian Holsteins was 30.1 m and Haworth et al., (2008) in North Queensland Australia reported 1.8 years. Lower values of LPL in USA Holstein cows with being 2.55 years were obtained by Hare et al., (2006). In population of France Holstein dairy cows,  Beaudeau et al., (2000) established an average duration of LPL as 3.20 years. Phenotypic variability of AFC classes for longevity and production traits are shown in Table 2.
 

        
The longest TL was expressed for cows with age at first calving before 23 months (69.57 mo / 2026 days). Duncan’s test revealed statistically significant differences between the second and all other AFC classes (P<0.05). The TL was significantly shorter for cows calving at 23^rd to 25^th months of age (65.38 m). The maximum values for LPL, NL and LMY were noticed for cows with age at first calving before 23 months of age. The LPL ranged from 47.88 (AFC<23 m) to 35.84 m (AFC>29 mo) and it declined progressively when the AFC was increased, particularly in cows calving at more than 29 m of age. Similar conclusion can be drawn for NL and LMY that with increase in age at first calving continuously increases, LMY is decreased (Fig 1).
 
@figuire1
        
Thereby, cows with lower AFC values (<23 m) significantly showed the highest lifetime production (21064.7 kg), longest productive life (47.88 m) and largest number of realised lactations (3.11).
        
The findings of the present study were in agreement with that of Teke and Murat (2013) who reported that the highest lifetime milk production was obtained when the AFC was 23 months. Similarly, Nilforooshan and Edriss (2004) found that increase in age at first calving from 21 to 24 months was accompanied by increased milk yield with delayed onset of first lactation. However, beyond 24 months of age, milk yield and productive life decreased. In general, they obtained a slight negative phenotypic correlation between age at first calving and first-lactation milk yield. Ettema and Santos (2004) reported that the optimum AFC value was 22 to 24 months to achieve highest first lactation milk yield.
        
Cielava et al., (2017) observed that cows calved for the first time at age of 30 months and older had longer lifespan (total lifetime), but their productive life was significantly shorter than the other AFC groups. The results of survival analysis by Zavadilová and Štípková (2013) indicated that high AFC is related with a shortened LPL. Bewley et al., (2001) reported a negative effect of increasing age at first calving on milk yield. Bayram et al., (2009) stated that the early calving of Holstein increased lifetime milk production. In contrast, Pirlo et al.,  (2000) in their study reported a positive effect of AFC on milk yield. Haworth et al., (2008) concluded that longevity declined progressively as the AFC decreased, particularly in cows calving at less than two years of age in Australian Holsteins in a tropical climate. Brickell et al., (2009) indicated that cows that calved first time at a relatively younger age have a longer lifespan and productive life, while the lifetime productivity was significantly lower.
        
This research showed the highly significant effect of the AFC, calving season and farm, on all studied longevity traits (P<0.01), but had non- significant effect on lifetime production. Also, two types of housing systems showed no significant effect on TL, LPL and NL (P>0.05), except the lifetime milk yield (P<0.01). Results in the present study are consistent with other reports (Singh et al., 2017; Nilforooshan and Edriss, 2004; Teke and Murat, 2013 and Stanojević et al., 2015).

Research was financially supported by the Ministry of Science and Technological Development, the Republic of Serbia, project TR 31086.