on the heritability and economic values of genetically correlated traits . Currently in the USA , net merit ( NM ), an index comprising 13 traits , is being used as an aid in sire selection . Live weight is included as a negative value although contributing only 6 % to the index . In comparison , fat and protein yield and productive life contribute 18.3 , 23.7 and 13.4 %, respectively to the index . This seems to suggest that putting a negative value on LW in the index would prevent an increase in live weight in Holstein cows .
The heritability of LW in dairy cows is high , about 45 %, which means that this trait , together with body size , could genetically be changed easily . However , some researchers warn against the careless use of LW as a measure of animal size . A small cow at a high condition score would be regarded as a larger cow . The LW of dairy cows is in a constant flux because of a change in body condition during the lactation . It has been suggested that LW should be corrected to a standard body condition score . A standard lactation period for LW recordings should also be used , i . e . 7 days after calving .
Reducing the LW of cows may have negative effects on traits not considered in simulation studies . Genetic parameters for LW and milk yield should be combined in a selection index for dairy cows . As LW is not routinely recorded , body size traits should probably be used as indicator traits for LW and feed intake .
Research in the USA on cow size A study , started in 1966 , compared large and small cows in a total mixed ration ( TMR ) feeding system . Sires were selected on predicted transmitting ability ( PTA ) for stature , strength and body depth using an index consisting of ( 0.5 x stature ) + ( 0.25 x strength ) + ( 0.25 x body depth ). Three sires with the most extreme PTA index for small and large body size were selected each year . Cows were randomly mated by sires from each sire selection group .
As expected , the live weight and body measurements of cows differed significantly between sire groups . The post-partum LW of first parity cows was 558 and 609 kg for small and large lines , respectively . However , the LW of cows ranged widely within lines
AgriKultuur | AgriCulture being 416-720 and 450-822 kg for first parity cows . Across years , the LW of cows did not change for the small line although increasing significantly for the large line .
Reproduction parameters favoured the small line ; however , only differing significantly for services per conception for first lactation cows . Although the birth weights of calves were higher for the large line , calving ease did not differ between the two lines .
However , sire selection protocols used in this study indicate that little emphasis was put on improving the milk yield of cows while aiming to change their size . This resulted in milk , fat and protein production of cows in the small and large line not differing , being 8535 vs . 8492 kg for first parity lactation cows , respectively . As expected , this resulted in a higher income above feed cost for cows in the small line . Selecting sires according to both production and LW would have resulted in a different outcome regarding production performance and profit over feed costs . However , based on these results and from practical experience of housing requirements , the continuing emphasis on large cows in the US Holstein breed is currently being questioned .
South African dairy farmers Local dairy farmers follow similar breeding trends as in the major dairy countries overseas . This is because a large proportion of semen used in dairy herds are imported from the USA , Canada , Europe , New Zealand and Australia . The local semen industry has declined and is currently co-owned by an overseas company . In South Africa pasture- and TMR-based dairy farmers tend to use sires from countries with similar production systems . Local pasture-based dairy farmers claim to breed towards smaller cows while also adopting corrective breeding programmes by crossbreeding using Jersey sires on Holstein cows . Some TMR-based dairy farmers have only recently adopted some corrective measures to reduce the increase in body size . This stems from an increasing number of dairy farmers investing in intensive free stall housing systems which essentially limits the size of cows as free stall space is fixed at construction and structures are generally used for 20 + years comprising about
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