No abstract

SUMMARYAn estimate of the frequency of twin calvings was obtained from the analysis of over 38 000 calvings of Friesian cows in M.M.B. milk-recorded herds. The incidence of twinning increased from 0·54 % in the first parity to 3·37% in the fifth parity. A significant excess of cows above expected produced either more than one set of twins or no twins at all. Repeatability ranged from 0·0 for cows with three calvings to 0·063 for cows with five calvings.Data on lactation yield and duration, butterfat percentage and calving interval showed no significant effect of twinning. However, consistent trends were noted showing that dams which calve twins have longer subsequent lactations, higher whole lactation yields and lower fat percentages than contemporary dams with single calves. The possible causes for these trends are discussed and are considered to be the lower breeding efficiency and longer calving intervals of dams of twins.

The relationship between the date of first oestrus of the breeding season and litter size when mating took place at the first, second, third, fourth or fifth oestrus was studied in 160 Polled Dorset Horn ewes which conceived to their planned mating. The ewes were maintained in one flock with vasectomized`teaser' rams at a ratio of less than thirty ewes to one ram. Checks were made twice daily for ewes in oestrus. At first oestrus, ewes were allocated to be mated at their first, second, third, fourth or fifth oestrus. Ewes to be mated were withdrawn from the flock and penned with a ram for 2 days. All livebirths, stillbirths and premature stillbirths were recorded. The previously reported pattern of low litter size from matings at the first oestrus rising to a peak at the third oestrus was found. There was an indication that ewes exhibiting first oestrus late relative to other ewes in the flock had lower litter sizes, irrespective of the oestrus at which they were mated. Ewes which exhibited first oestrus early showed no seasonal rise in litter size. Ewes with a later onset of oestrus showed an increase in litter size with successive oestrous periods.

Data on growth rate and carcasses of pedigree Polled Dorset Horn castrated male lambs have been collected. The lambs were the halfsib progeny of 18 rams mated to groups of 20 ewes in 1968 and 1969. The lambs were slaughtered at between 35 and 45 kg live weight and when considered 'fit' on the basis of a handling appraisal. The whole carcasses were classified by a retail organization as either 'accepted' or 'rejected' and subsequently all carcasses were cut into commercial joints ready for sale. Analyses indicated that classification of carcasses was largely but not solely based on backfat thickness. Accepted carcasses were from older and heavier lambs which had larger eye-muscle areas. There was no difference in value per unit weight between accepted and rejected carcasses. Heritability for growth rate to 16 weeks of age was zero but for growth rate and age at slaughter was 0-26 ± 0-22 and 0-28 + 0-22 respectively. The heritability estimates for percentage best end, loin and backfat thickness were greater than 0-30. There were insufficient data to estimate genetic correlations accurately, but the estimates of phenotypic correlations and heritabilities were very similar to those obtained in a previous study of carcasses of Down cross lambs. It is suggested that the method of choosing lambs for slaughter accounts for the large difference in the heritabilities for growth to 16 weeks and to slaughter. Selection to change backfat thickness would be effective and would probably result in a reduction in age at slaughter and an increase in growth rate to slaughter. Selection to change proportions of the carcass is not considered to be worthwhile.

Thirty-two N'Dama bulls aged 3 to 4 years were used to study the interactions between work, trypanosomosis and nutrition. The bulls were randomly allocated to two treatments, working (W) and non-working (N). Half in each treatment were placed on an andropogon hay basal diet (B), the other half on a better quality groundnut hay diet (H). Five days a week, four pairs of animals in the BW group and and four pairs in the HW group pulled weighted sledges four times around a 2056-m track. Loads were set to ensure energy expenditure was equivalent to 1·4 times maintenance. After 4 weeks all 32 bulls were injected intradermally with 104 Trypanosoma congolense organisms. The trial continued for a further 8 weeks.Trypanosome infection caused a significant (P < 0·001) decline in packed cell volume (PCV), and the anaemia was more severe (P < 0·05) in working animals; three pairs in the HW group and two pairs in the BW group were withdrawn because PCV fell below 17%. Diet had no effect on PCV or parasitaemia. Infection caused a decline in food intake (P < 0·001) but with significant interactions between diet and work. Intake patterns were similar in the BN and BW groups whilst the HW animals consumed significantly more groundnut hay compared with the HN group (P < 0·01). However, nutrition had no significant effect on lap times or the team's ability to work under trypanosomosis challenge. Post-infection, diet was the dominant factor determining weight change, HN and HW animals weighed significantly more than BN and BW animals (P < 0·01) and the interaction between period, diet and work demonstrated that BWhad the lowest weights in the latter stages of the trial (P < 0·05).The results suggest that supplementation with better quality forages confers no benefit to an animal infected with trypanosomes. Nor can trypanotolerant cattle sustain long periods of work if subiected to a primary challenge of trypanosomes.