I n New Zea l a n d to m aintain a seasonal calving herd, is req uired that c ows m ust c alve on a verage every 1 2 months ( M a l mo, 1 985; Esslemont, 1 993) . Given an average gestation length of 282 days, c a l ving to conception intervals of :::;83 days are required in order to m aintain a 1 2-month produ ction cycle. In order for the New Zea l a n d seasonal calving herds t o maintain a n a n nual calving cycle that consists on 1 0 month period of lactation, a two month dry period a n d a minimum interval o f 3 0 d ays between calvin g a n d PSM (to complete u terine involutio n ) , a l l cows m ust conceive within 53 days of the PSM (equivalent to 2.5 oestrus cycles) (Cavalieri e t al., 2000) . Table 3-9 c o m pares the resu lts of the postpartum intervals in the present study with others similar stud ies carried out with Holstein-Friesia n cows d iffering genetic a l ly for live weight.
Table 3-9. Comparison of the calving intervals in different studies for Holstein-Friesian
c ows differing genetically for live weight
I nterval H L Significance 1 Period
Source CFS (days) Garcia-Munfz, ( 1 998) 78 82 t 1 992- 1 99 7 Present study 80 79 NS 1 998- 1 999 PSMFS (days) Garcia-Munfz, ( 1 998) 1 1 . 7 1 1 .6 NS 1 992- 1 99 7 Laborde, ( 1 998) 1 2 1 1 NS 1 992- 1 99 7 Laborde, ( 1 998) 1 0 1 1 NS 1 996- 1 99 7 Present study 8 1 3 * * 1 998- 1 999 CC (days) Garcia-Murlfz, ( 1 998) 93.5 93.6 NS 1 992- 1 99 7 Present study 9 4 88 NS 1 998- 1 999 PSMC (days) Garcia-Murlfz, ( 1 998) 28 24 t 1 992- 1 99 7 Laborde, ( 1 998) 25 2 1 NS 1 992- 1 99 7 Laborde, ( 1 998) 1 1 1 4 NS 1 996- 1 99 7 Present study 23 22 NS 1 998- 1 999 FSC (days) Garcia-Munfz, ( 1 998) 1 7 1 3 1 992- 1 99 7 Present study 1 5 9 NS 1 998- 1 999 Cl ( days) Garcia-Munfz, ( 1 998) 375 373 NS 1 992- 1 99 7 Present 368 365 NS 1 998- 1 999
1 Significance: NS not significant; t P<O. l ; *P<0.05; **P<O.O l ; ***P<O.OO 1 CFS: Calving to first service interval
PSMFS: Planned start of mating to first service interval
CC: Calving to conception interval
PSMC: Planned start of mating to conception interval FSC: First service to conception interval
Calving to first service i nterval
For all age groups a nd three years & older g roup, H cows had longer C FS interval (80 days a nd 88 days respectively) than L cows ( 79 days and 79 d ays respective ly) . W hile for the two year old g roup, H cows had shorter C FS interval t h a n L cows ( 7 1 days a n d 8 1 days
1 38 Chapter 3
respectively) . The values i n the presen t study are compara ble to those reported by Garcia-Muniz, ( 1 998) a n d La borde, ( 1 998) with H F cows differin g genetically by live weight Table 3-9
S horter C FS i n tervals h a ve been associated with lower c o n ception rate to the first service, especially with high pro d u cing cows ( Dhaliwa l
e t al., 1 996) . H owever, longer C FS intervals are associated wit h i ncreased o p e n days ( Oitenacu e t al., 1 980) , a n d longer calving i ntervals ( M a c millan, 1 979) . However the C FS i nterval observed for the L cows in the present study are a lso compara ble to those reported for N ew Zea l a n d H olstein-Friesia n cows in other studies 77 days ( Ma c millan & Moller, 1 977b) ; 76 days ( Macmilla n et al., 1 987) a n d 76 d ays (Grosshans et al., 1 997) .
O psomer e t. al., ( 1 996) a n d O'Farrell, ( 1 998) a lso indicated that, together with the efficacy of heat detection, the m a n a gement d ecision o n when to start breeding a n d the overall conception rate, t h e early res u m ption of ovarian cyclicity is one of the most i mportant factors determining the length of the calving interval .
T h e c a lving to first service i nterval depends on t h e re-establish ment of t h e ovarian a ctivity after calving, the occurrence a n d detection of oestrus a n d the farmer's pla n ned start of mati n g date, if this occurs later t h a n the previous factors ( Malmo, 1 985; Peters & Ball, 1 987) .
O uweltjes e t. al., ( 1 996) showed a sig nifica nt effect of herd on C FS i n terval i ndicating the stro n g influence that a farmer has on the timing of first i nsemination .
Plan ned start of mating to first service interva l
For the all age grou ps, L cows had longer PSM FS interval ( 1 3 d ays) t h a n the H c ows (8 d ays) , confirming the tight calving pattern a chieved by the latter g rou p . The three year olds gro u p had very similar PSMFS between two strains. However, i n the two year olds g rou p, H cows had m u c h shorter PSMFS i ntervals than the L c ows ( 5 d ays a n d 1 4 days respectively) , because o f their slig htly more s prea d c a lving pattern .
The resu lts in the present study are i n a similar ra nge to those reported by Garcia-Mu n fz, ( 1 998) and La borde, ( 1 998) Table 3-9 . In other studies with Holstein-Friesia n cows, Macmilla n , ( 1 987) reported the overall mean of PSMFS i nterva ls was 1 3 days, h owever Grosshans e t al.,
( 1 997) reported o n average a m u c h longer PSMFS interva l ( 1 9 days) .
Mac milla n , ( 1 985) pointed out that the accuracy of oestrus detection a nd the proportio n a l incidence of postpartum a noestrus wou l d i n fluence t h e PSMFS i nterva l . For example, i f detection rate were 90%, then the PSM FS interval would be 1 3.5 days, a nd if it were only 80%, then the PSM FS wou l d be 1 6.2 days.
C a lving to conception i nterval (open days)
The calving to conception i nterval determ i n ed in this study for t h e combined g ro u p w a s longer for H cows t h a n for L cows (94 days a n d 8 8 days res pectively) . These resu lts were s lig htly different from those reported by Garcia-Mu nfz, ( 1 998) (Table 3-9 ) . However, the two year old group had shorter CC interval for H cows tha n for L cows (86 d a ys a n d 90 days respectively) ; while the three year & old er gro u p h a d considera ble longer CC interval for t h e H c ows tha n t h e L cows ( 1 02
140 Chapter 3
d ays and 87 d ays respectively) . These results a re also in a greement with other studies reported by Macmillan et al., ( 1 987) and Grosshans
et al., ( 1 997) , for which the overall mean for the Holstein-Friesia n cows were 88 days a nd 90 days respectively.
Plan ned start of mating to conception interval
T h e PSMC interval for t h e all age g roups was similar for the H cows and for the L cows ( 23 days a n d 22 days respectively) . These difference a n d the lower C R FS for the heavy strain indicates that H cows were comparatively less likely to conceive than L cows. However, in the two year old g roup L cows had slig htly longer PSM C interval than the H cows (23 days a n d 22 days respectively) .
T h e present resu lts agreed with the results presented by Garcia-Munfz, ( 1 998) and La borde, ( 1 998) , from the analysis of the 1 992- 1 997 period (Ta ble 3-9 ) . These results are also in the range of other studies in Holstein-Friesia n cows reported by different a uthors: 28 days ( Ma c millan & C layton, 1 980) ; 24 days ( Macmilla n e t al., 1 987) ; 2 1 days ( X u e t al., 1 996) , a n d Grosshans e t al., ( 1 997) reported a higher mean for PSMC interva l 33 days .
M a c m illan ( 1 985) pointed o u t that conception rate and distribution of return intervals, as well as t h e submission rate will influence the interval PSM C . In combination with C R FS or su bseq uent inseminations, the oestrus detection rate will a lso influence the intervals PSMC and FSC ( Ma cmillan, 1 985) . Therefore, the longer PSMC interval for the H group is compared to the other L g roup, which is consistent with its lower C R FS .