Importance of dam BW change and calf birth weight in double-muscled Belgian Blue cattle and its relationship with parity and calving interval.

Factors affecting calving interval (CI) in double-muscled Belgian Blue (DMBB) beef cows were investigated with regard to the BW yield (BWY) of the cow-calf pair, using 834 CI records from 386 females with parities 1 to 6. The effect of parity and CI on BWY was also studied. Cow-calf pair BWY was defined as calf birth weight plus dam BWY per CI. CI (mean±s.e.: 404±1.9 days) was affected by parity, calving season, suckling and calf birth weight/dam weight. Primiparous cows had a shorter CI than cows with three or more calvings (P<0.05), with an intermediate CI for second-calf cows. Spring calvings resulted in a shorter CI than summer and autumn calvings, with intermediate values for winter calvings. Suckling dams had longer CIs than non-suckling dams. There were interactions (P<0.05) between calving season and suckling, and between calving season and mating system. Shortest CIs were observed for spring calvings in case of non-suckling and for summer calvings in case of suckling. Longest CIs were observed for autumn calvings in case of natural service (NS) and for winter calvings in case of artificial insemination (AI). Calf birth weight/dam weight of 6% to 10% resulted in shorter CI than a ratio of <6% (P<0.05). Body condition and mating system (NS v. AI) did not affect CI. Daily cow-calf pair BWY was affected by parity (P<0.001) and CI (P=0.013), with a tendency for an interaction (P=0.094). Daily cow-calf pair BWY did not differ for CIs of <12 to 16 months in primiparous cows and was lowest for a CI of 13 to 15 months in second-calf cows, whereas the effect of CI was more variable in older cows. Dam contribution to cow-calf pair BWY was larger than calf birth weight in first- and second-calf cows, and increased with increasing CI. Dam contribution to cow-calf pair BWY was smaller than calf birth weight in older cows, varying from 0.2 to 1.0 depending on CI. A short CI is advised for DMBB cows because of a larger BWY and more efficient nutrient utilisation.