Certain aspects of the reproductive performance of zebu cattle in Cameroon

[摘要] English: A study divided into two phases was carried out in order to assess the reproductivecharacteristics of the Ngaoundere Gudali cow of the Adamawa Highlands ofCameroon. In the first part of the study, records from 1966 to 1994 collected at theWakwa Agricultural Research Centre and Animal Production Station were used tostudy productive and reproductive traits such as: the gestation length, thepostpartum period, the age at first calving and calving intervals. The birth andweaning weights as well as the pre- and post-weaning growth rates of Gudali calveswere also evaluated. The second part of the study consisted of a monitoring studyconducted between November 2001 and May 2002 on the length of the oestrouscycle, duration of oestrus, the serum progesterone levels during the oestrous cycle,and the effect of two suckling and two weaning regimes on the postpartum ovarianactivity of Ngaoundere Gudali cows. The General Linear Models procedures of SAS(1991) were used to statistically analyse the data. Due to the unbalanced nature ofthe data, Least Squares Means (LSM) were used and, whenever a significant effectwas detected, the Tukey-Kramer test was used to evaluate the LSM.The overall least square mean for gestation length in Gudali cows was recorded as293.4 ± 0.4 days (N = 697). The sex of the calf significantly (P<0.05) affected thelength of gestation, with male calves being carried in utero approximately 3 dayslonger than their female counterparts (294.1 ± 1.2 and 291.1 ± 1.2 days,respectively). Birth weight tended to increase as the gestation period lengthened.The significant effect (P<0.01) of the month of service on gestation length could notbe explained, as gestation length fluctuated from one year to another. The parityand age of the cow had no significant effect on gestation length.The mean duration of the open period (from calving to conception) was 267.7 ± 7.4days, with a range of 24 to 749 days. Approximately 23.2% of the cows conceivedwithin 90 days of calving and a total of 55.6% had conceived by 360 days. Thedistribution of the open period was bimodal, and could have been influenced by theseasonal availability of feed, or the long (6 months) mating season that allowed cowsto calve during the following mating season. The calving to conception interval wassignificantly (P<0.001) affected by the month of calving and parity, but not by the sexof the calf. Cows calving at the end of the dry season/beginning of the rainy season(March and April) had the longest open period (and consequently the longest calvinginterval), compared to cows calving later (July to September). Cows on their firstparity experienced a significantly longer mean postpartum (open) period (P<0.001)than older cows (320.7 vs. 200.2 days for first and 5th parity cows, respectively). Thesex of the calf did not affect the duration of the postpartum period, although thisperiod was on average 5 days longer following the birth of a male than a female calf.The overall average birth weight was 24.1 ± 2.8 kg (N = 3401), with male calvesweighing approximately 1 kg more than their female counterparts (24.6 ± 0.1 and23.7 ± 0.1 kg, respectively). Calves born during the rainy season were 0.84 kgheavier than those born during the dry season (24.5 ± 0.1 and 23.7 ± 0.2 kg,respectively). No significant effect of parity was recorded on birth weight, althoughthe females calving for the first time tended to have lighter calves than older cows. Ayear effect on birth weight was detected, but no definite trend towards an increase ora decrease was noted over the years. This year effect was attributed to the largevariation in environmental factors (rainfall and nutritive value of the pastures)experienced by the animals.The average weaning weight of Ngaoundere Gudali calves was recorded as 149.4 ±1.4 kg (N = 1827), with male calves weighing 6.1% more than the female calves atweaning (157.0 ± 2.8 and 148.0 ± 2.7 kg, respectively). There was a 20.4% weaningweight advantage of dry season born calves over those born during the rainy season(166.6 ± 4.5 and 138.4 ± 1.9 kg, respectivelv), despite the fact that the former wereborn significantly lighter. There was also a significant (P<0.001) year effect onweaning weight, with no evident trend over the years. This could be due to thevariation in environmental conditions (inconsistent rains, total rainfall, heat stress,solar radiation, seasonal availability of forages, etc). No significant effect ofparity/age of the cow on the weaning weight was detected, but age at weaning wasfound to be a very important determinant of the weaning weight. The earlier thecalves were weaned, the lower their weaning weight, but the higher their adjustedweaning weight to an age of 205 days. The season at which the calves wereweaned did not significantly affect their weaning weight, despite the apparentadvantage that dry season born calves had over their rainy season borncounterparts. A weaner productivity index was used in order to determine the bestcalving season. This index showed that cows calving in the dry season hadsignificantly (P<0.001) higher indexes than those calving during the rainy season(113.1 ± 2.0 and 95.0 ± 1.5 kg, respectively). This index also differed with the sex ofthe calves (107.7 ± 1.6 and 100.4 ± 1.4 kg for male and female calves, respectively)and the month of birth. These results indicate that the best 3 months for calvingspan from November to January, with a corresponding mating season betweenFebruary and April. Pre-weaning weights (1, 3 and 6 months of age) weresignificantly (P<0.001) influenced by the month, season, year of birth, and by the sexof the calf. The sex of the calf, month and year of birth but not season of birthsignificantly (P<0.05) affected average daily gain of Ngaoundere Gudali calves. Themean ADG were 1.1 ± 0.4, 0.8 ± 0.2, and 0.7 ± 0.1 kg/day from birth to 1, 3 and 6months of age, respectively. Male calves consistently grew faster than their femalecounterparts at these 3 pre-weaning stages. Post-weaning weight and growth ratewere significantly affected by the sex of the calf for the period considered (from 9 to36 months of age), with bull calves retaining a higher growth rate. However, allcalves experienced negative growth rates between the ages of 8 and 9 months(weaning took place at 8 months ± 2 weeks), due to either the stress of weaning ordue to the new environment to which the weaners were moved. Season of birth nomore significantly affected post-weaning weight and ADG from the age of 18 months.Interestingly, the study showed that the Ngaoundere Gudali heifers reachbodyweights of 222 and 281 kg at 24 and 30 months, respectively, meaning that anearlier age at first breeding could be contemplated.The average length of the oestrous cycle of the Ngaoundere Gudali cow was 21.8 ±0.5 days, ranging from 15 to 35 days. Most oestrous cycles (67.9%) had a lengthcomprised of between 18 and 22 days, while 18.5% lasted between 23 and 28 days.The oestrous cycles were significantly (P<0.05) longer during the dry season (24.1 ±0.7 days), compared to the rainy season (20.6 ± 0.5 days). Historical data from theAI herd showed that out of 1504 services performed at natural oestrus during a 9-year observation period, only 28.9% of the oestrous manifestations were detectedduring the dry season (November to March) and early rainy season (April and May),compared to 71.1% from June to October. The seasonal peak occurrence of oestruscorrelated well with the seasonal concentration of births in the herds to suggest thatoestrous manifestation in the Ngaoundere Gudali cow is more readily expressedduring the rainy season. The most consistent sign of oestrus was the acceptance ofthe female in oestrus to be mounted by a teaser bull or another female, for anoestrous period that averaged 9.8 ± 0.6 hours (range of 5 to 13 hours). However, itwas suspected that in this study, due to the low duration and long intervals betweenobservations, the onset and termination of some oestrous periods could have beenmissed. Thus the shorter duration of the oestrus recorded. Serum progesteroneconcentrations were at the lowest (0.4 ng/ml) between days 1 and 3 of the oestruscycle, before rising from day 5 to reach peak values (5.5 to 6.5 ng/ml) between days15 and 19. These serum progesterone levels declined rapidly thereafter to reachvery low levels by the day of oestrus. Uterine involution was completed inNgaoundere Gudali cows by 45 to 60 days after calving, earlier in a larger proportionof cows with calves under restricted suckling (RS) management, than in theunrestricted suckling (US) group. 26.3% of the cows in the RS group and 0% of theUS group were in oestrus within 75 days postpartum. Between 75 and 90 dayspostpartum, 84.2 and 15.8% had been observed in oestrus and mated in therespective groups. The mean postpartum interval from calving to first oestrus wassignificantly (P<0.05) shorter in the RS (83.4 ± 5.1 days), compared to the US group(126.4 ± 5.1 days). The intercalving period was estimated at 397.9 ± 5.0 days,significantly (P<0.01) longer in cows that were suckled ad libitum (419.4 ± 6.5 days),than in cows in the RS group (376.4 ± 3.2 days). A high correlation was foundbetween the reproductive tract score at 60 and 75 days postpartum (r = 0.6) and theconception rate of the cows. The overall mean weaning weight was 114.8 ± 2.7 kgfor all the calves - significantly (P<0.05) higher for the US than for the RS regimes(134.1 vs. 95.3 kg, respectively). This indicates that restricted suckling had slowedcalf growth and decreased weaning weight. Pre-weaning growth rate wassignificantly higher in the US than in the RS group. Calves that were in the US groupand weaned at 8 months were 34.9, 28.7 and 71.1 kg heavier than the RS calvesweaned at 8 months, US and RS calves weaned at 6 months, respectively. Postweaninggrowth rate to 12 months of age was significantly different (P<0.01) for thetwo weaning regimes and it was found that early weaned calves grew faster (176g/day) than those weaned at 8 months (123.8 g/day) - revealing some compensatorygrowth from weaning up to a yearling age. The level of serum IgG in 35 newbornGudali calves (72 hours following birth) recorded a mean concentration of 7.8 ± 0.5gII (range of 0.04 to 12.7 gII), far below the levels quoted in the literature at 24 to 48hours following birth. These values point either to a low concentration of IgG in thecolostrum of Ngaoundere Gudali dams, leading to an insufficient intake by thecalves, or to a limited absorption of the ingested colostrum.Overall, the present series of studies have contributed to the characterization of themost important aspects of the Ngaoundere Gudali reproductive and productiveparameters. In general, the results obtained are indicative of low reproductiveperformances that seem to be linked to the seasonal availability of feed, which inturn depends on the seasonal and annual variations in rainfall. Certain managementpractices with a potential to improve the reproductive efficiency of the cows wereidentified (shorter breeding season, restricted suckling, early weaning, etc). Animprovement in the reproductive efficiency of Ngaoundere Gudali cows wouldcontribute to improve the overall productivity of the traditional cattle productionsystems in the Adamawa Highlands of Cameroon.

[发布机构] University of the Free State