Animal Production, 1966, Vol 8, Part 1

Robertson: A mathematical model of the culling process in dairy cattle

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“….The proportion selected increases by….”

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The occurrence of Hb N in sheep has been investigated by means of starch gel electrophoresis.

In lambs, Hb N occurred in higher relative amounts than in adults, but not until 9 days of age. One-month-old lambs often showed an Hb N content as high as 10–30%.

Haemoglobin N was observed most in sheep of phenotype AA, less in those of type AB and was found to be absent in BB animals. It was also provoked experimentally by bleedings in sheep of types AA and AB but not in those of type BB.

Comparisons between breeds showed differences in the expression of Hb N. It occurred most regularly in Old Norwegian sheep (Spæl), but usually in concentrations of less than 1%. Hb N was, however, also found in other Norwegian breeds, in Cheviots and in Spanish sheep but not in sheep of the Oxford Down breed.

In two flocks of Spæl and Dala sheep where frequent bleedings were made, Hb N was observed at all times of the year.

1. Over a 3-year period 150 farrowings of Large White sows were used to compare a stall farrowing system with a conventional-pen farrowing system.

2. Farrowing in the stall system of management resulted in a significantly lower percentage mortality both at 3 weeks (5·73±2·70%) and at 8 weeks (7·92±2·89%), than in the conventional-pen system.

3. Both total litter and individual pig weights were higher at 3 and 8 weeks in the stall system.

4. Despite higher building and electricity costs the stall system showed a margin of £1, 14s. per litter over the conventional-pen system.

The effects of environmental factors in the pre-weaning period on the subsequent growth of mice were studied. The variance caused by maternal effects, i.e. by environmental factors common to litter-mates, increased from birth to 4 weeks of age (1 week after weaning) and then decreased by about 60% between 4 and 8 weeks. Furthermore, a high litter weight at 4 weeks was followed by low subsequent growth and vice versa. The occurrence of compensatory growth from 4 weeks onwards was thus established. In contrast, the genetically determined variation of body weight did not show any reduction during the period of compensation, but increased up to 8 weeks of age.

The modification of the growth curve, by which compensation was achieved, was then studied. It was found that the inflexion point separating the initial phase of exponential growth from the subsequent phase of asymptotic growth corresponded closely with the attainment of sexual maturity in females, as judged by the opening of the vagina. Sexual maturity was reached at approximately the same weight in all litters, but at widely different ages. Thus, litters with rapid early growth as a result of a good maternal environment reached sexual maturity and consequently entered the asymptotic phase early, while litters with slow early growth entered the asymptotic phase late. Compensatory growth was therefore achieved through curtailment or prolongation of the exponential phase of growth.

When commercial livestock are crossbreds, profit for the total operation including cost of production of the breeding stocks, is a function of the performance of both the crossbred offspring and their parents.

Since the performance level of the offspring is determined by its parents, it means that the parents contribute to profit determination in two ways. Reproduction cost is determined directly by the parents, that is through their own phenotypes, while productive efficiency (food conversion, growth rate, etc.) and quality are determined by the parents indirectly through the genetic determination of their offspring.

In most classes of livestock the number of males in the breeding herds, relative to that of the females, is very small. Consequently, variation in reproductive capacity of the male line contributes only little to the determination of reproductive costs, and this item can be ignored. As a result, the contributions of the parental lines to profit are unequal and of different kinds. Exploitation of this difference may produce ‘profit heterosis’ even when the component traits are genetically additive.

When profit is a non-linear function of one or more component traits another deviation of profit from the arithmetic mean of the parents is created. Unlike the previous one, this heterosis deviation is scale dependent.

Profit was presented graphically, as profit contours, on the two dimensional plane created by the variables productivity and reproductivity. This was found to be a helpful aid to an understanding of the underlying relationships.

Detailed graphic and algebraic procedures were outlined for finding the most profitable parental combination among a group of lines when genetic additivity of component traits is assumed.

Profit was expressed as a linear function of productivity and a reciprocal function of reproductivity. The non-linearity and the unequal contributions to profit of the sire and dam lines were exploited in order to minimise both reproduction and production costs.

Least cost rations for specific rates of growth in the growing and finishing periods were given to pigs slaughtered at 200 lb. live-weight. During the growing period pigs grew very close to the planned rates in spite of the fact that some food was rejected. Pigs given rations for a low growth rate in the growing period grew slightly faster than planned in the finishing period. In the finishing period pigs given rations for the high growth rate grew on average very near to this rate. The pigs given the ration for the low growth rate grew slightly faster than planned. Food conversion efficiency was for all pigs very near to, or slightly better than, that planned. The rations had no adverse effect on carcass quality.

1. A statistical analysis has been made of the production records in all lactations, up to the third, of 10,967 British Friesian cows. The work was done in two sections, for those animals by home-bred sires and for those by sires imported from Holland and Canada. Within each section, three parallel sets of calculations were made, based on all animals for lactation 1, on those with at least two completed lactations for lactations 1 and 2, and on those with three completed lactations. The analysis was done in a hierarchical manner, taking variables in the order sire, herd, year, month and age at calving. The production measurements were 305-day yield, fat content and lactation length, the latter having an upper limit of 305 days.

2. It was found that in all three characters the variance between animals identical for all environmental variables was approximately two-thirds of that between animals commencing the same lactation in the same herd in the same year. This difference could not be attributed to any particular variable and appeared to be an effect of exact contemporaneity.

3. Culling produced appreciable selection differentials in yield and lactation length but not in fat content. The regression coefficient, calculated within sire groups, of selection differential for heifer yield within sires on the heifer progeny test was 0·11 for culling at the end of the first lactation and —0·07 for culling at the end of the second. In the analysis of British sires for yield, selection in the first lactation reduced both sire and within sire components by about 20%, so that the heritability was little changed. In the Imported sires, the sire variance was reduced by nearly 40 %, so that the heritability fell from 0·26 to 0·22. Because of the truncated distribution of lactation length, selection reduced the within sire variance for the Imported sires by about 60% and the heritability from 0·13 to 0·09.

4. The heritability of yield was lower in second and third lactations than in first. This seemed to be due to an increase of about 50 % in the within-sire component whereas that between sires remained constant. Fat content showed a similar trend and the heritability was slightly less in later lactations.

5. The repeatability of the characters was in general agreement with the results of other workers. The genetic correlation of first yield with later ones averaged 0·80 (significantly different from unity), but that between second and third yields averaged 0·91. The genetic correlations between fat content in different lactations were all close to unity, as were those between length in different lactations.

6. In agreement with other results, the negative genetic and phenotypic correlations between yield and fat content appeared to approach zero as the animals aged. This was due to an above average increase of fat with age in the daughters of bulls high for yield.

7. As would be expected from the genetic correlations between yield in first and later lactations, the increase of yield with age had a heritability significantly different from zero. First lactation yield was negatively correlated (—0·27) with increase from first to second but positively (0·03) with increase from second to third. The heritability of the average yield and fat content increased little with age.

8. The consequences of these results for the optimum combination of different records in selection decisions are discussed.

1. An analysis has been made of the relationship between first lactation progeny test for milk yield and the survival of a bull's daughters to different ages using data for British Friesian, Ayrshire and Holstein-Friesian cows. It was found that the relative effect on survival of differences between bulls in milk yield increased by about 10% per 100 gallons for each lactation survived, e.g. for two bulls differing in heifer progeny test by 100 gallons, that with the higher figure will have 10% more daughters surviving to have a second lactation, 20% more having a third and so on. The probable effect on total length of life is 1·2 lactations per 100 gallons.

2. No evidence was found that selection on first lactation progeny test would reduce productive life in the dairy herd or even that it would increase the proportion culled for reasons other than yield in later lactations.

3. The genetic effects of culling in the first lactation on yield were in agreement with those expected from multiplying the phenotypic selection differential by the heritability. On the other hand, the selection differential in heifer yield produced by culling in later life considerably under-estimated its genetic effect. The effect of culling was to increase the genetic potential of animals for yield in the first lactation by roughly 10 gallons for each lactation survived, so that the average genetic superiority over their heifer contemporaries of cows in the herd at any time would be 30 gallons.

4. The heritability of survival to different ages was significantly different from zero in all analyses. The theoretical prediction that this would have a maximum at the fourth lactation was borne out in one sample but not in the other.

5. The genetic correlation of first lactation yield with survival was lowest in all samples for survival to have a second lactation. For survival to the third or later lactations it was higher than 0·7 in all analyses. As the genetic correlation between first and later yields in these data averaged about 0·8, this can only mean that milk yield is by far the most important genetic determinant of survival at all ages.

Freshly-cut forages and lucerne chaff were given to penned sheep fistulated at the oesophagus. The feed given and the samples obtained through the fistula were analysed for in vitro digestibility, nitrogen, ash, phosphorus, sodium, potassium and calcium. Organic matter digestibility was predicted from the in vitro determination.

Three regression equations were calculated relating predicted organic matter digestibility of the feed to the digestibility of the extrusa sample. The most precise of these equations had a residual standard deviation of ±3·4 units.

The relationship between the nitrogen contents of the feed and sample was calculated. The equation did not pass through the origin but the differences in nitrogen content between feed and extrusa were small over the range 4·69 to 1·28% N included in this study. The nitrogen and calcium contents of the feed could be estimated from the sample with errors of ±7·5 and 9% respectively. The sodium, potassium and ash contents of the diet could not be reliably estimated from an oesophageal sample.

Four Scottish Blackface rams were mated with a group of their own daughters and a similar number of non-related ewes. The matings were made in three successive years. All ewes were maintained under normal hill conditions. Female progeny from the matings entered the ewe flock unculled. Records of birth type, birth weight, fleece and live-weights throughout life were obtained.

Inbred lambs had lower mean values for all measured characters than did their non-inbred half-sibs and there was a greater number of apparently barren ewes amongst the flock ewes which were expected to produce inbred lambs. The inbred progeny, also maintained in the normal hill flock, showed a higher incidence of barrenness and lower lamb survival rates at all ages than did their non-inbred contemporaries in the same environment. In the final year of the observations, when the standard of nutrition during late pregnancy was raised, the neo-natal mortality rate in both groups was reduced.

Samples of the longissimus dorsi muscle were removed by biopsy from 4 pigs at intervals during the period from weaning to 125 lb. live-weight. In parallel studies involving 127 pigs attempts were made to relate mean fibre diameter to indexes of lean meat content of a range of bacon pig carcasses but only poor correlations were obtained. It was concluded that fundamental knowledge of the structure of the longissimus dorsi is not yet sufficiently advanced to allow accurate predictions of this type to be made.

The mean concentrations of total serum protein, γ-globulin and antibody were almost identical in groups of Finnish Landrace lambs from litters in which one, two or three, respectively, survived till the 3rd day. Lambs from two litters each with four survivors had considerably lower mean concentrations. Merino × Cheviot and Scottish Blackface lambs examined previously had significantly lower serum protein concentrations than the Finnish lambs and higher antibody levels in single than in twin lambs.

1. Because dressing percentage increases as body weight increases, an adjustment for body weight is necessary when the dressing percentages of animals of different body weights are being compared. Instead of comparing animals on this basis, it is suggested that the power function y = axb, relating carcass weight and body weight, when computed in the logarithmic form, provides a suitable method for making adjustments of this kind.

2. Using this function, differences in yield of carcass have been shown, between breeds and between seasons, in a study involving Hereford, Aberdeen Angus and Beef Shorthorn cattle.

3. A similar function has been used to describe the relationship between fat content of carcass and carcass weight and body weight, using data from the same breed study.

Monthly coat and skin samples were taken from three goats for one year in Australia, and from six others for two years in Scotland. The coat appeared to have a simple cycle of active growth in summer followed by inactivity in winter.

The kemp fibres of the outer coat formed brush ends at about the time of the autumn equinox and remained dormant until late spring. Follicles developed first in the foetus formed a brush first. No moult was obvious because new kemps grew before all the old ones had shed, the only manifestation of shedding being a shortening of the coat. The old wool tended to be lost before growth of the new, so that wool was completely lacking in the coat during summer.

Observations on skin sections showed that the majority of both kemp (primary) and under-wool (secondary) follicles become active immediately after the longest day, although there was a tendency for re-growth of wool fibres to be delayed, particularly in females.

Whereas the proportion of kemps with brush ends during winter reached 100%, the proportion among wool fibres was as low as 50%. A complete explanation for this is not yet evident. The bad winter of 1963 apparently caused a greater proportion of secondaries to form brush-ends earlier and to remain inactive longer.

Details are given of some of the histological features of brush formation.

Sixty-four yearling Friesian steers, in eight groups with eight animals per treatment, were fed on pelleted diets containing ground barley straw, maize and soya bean meal. The percentage of straw in the diets varied from 0% to 50% and the crude protein was adjusted in all diets to 12% on an air dried basis. Levels of 0, 10, 20 and 30% ground straw were tested in 1963–4 and 20, 30, 40 and 50% in 1964–5.

Growth rates in the range of 8·28–9·49 kg. per week were obtained, with growth rate falling significantly (P<0·05) by 0·36±0·15 kg. per week for each 10% increase in straw in the 1964–5 trial; the differences between treatments in the 1963–4 trial were non-significant. The efficiency of food conversion varied from 6·5:1 for 0% straw to 7·7:1 for 50% ground barley straw. Feed intake increased as the percentage of straw in the diet rose above 20%. Regressions relating those carcass measurements in which differences between groups were significant to the percentage of straw in the diet were not significant except that for eye muscle area (P<0·05). Neither carcass nor meat quality differed significantly in animals given 20 % and 50 % ground straw.

The results of digestibility trials with male hoggets showed a linear depression in the digestibility of diets as the percentage of straw increased.

The results are discussed in relation to previous knowledge of the factors affecting the voluntary feed intake of ruminants.

Agricultural Research Council, 1965. Recommended procedures used in the measurement of beef cattle and carcasses. H.M.S.O., London.

Fleece characteristics of three distinct strains of Scottish Blackface sheep were studied by sampling eighteen ewes of the Lanark, Galloway and Lewis strains on three occasions in a single year. During the period of maximum growth the Lewis strain, which is largely of historical interest, differed significantly from the others in S/P ratio and mean weight, diameter and degree of medullation in fine and coarse fibres. The differences between the other two strains were small and although a considerable range in fleece characteristics was found, most of the whole fleeces would be classed as ‘medium’ wool.

All three strains showed a considerable reduction in follicle output during the winter and spring months when differences amongst them were minimised.

Data are presented concerning the efficiency of growth of Specific-Pathogen-Free Large White boars on individual performance test. Various aspects of growth rate, backfat thickness and food conversion are compared.

It is suggested that tests should be shortened by starting later, rather than finishing earlier.

Some support is given to the theory that one type of pig may not be suitable, from the point of view of efficiency of growth, for supplying the full range of carcass weights in current demand.

The variations that occur in saliva flow in cattle at rest may affect digestive processes in the reticulo-rumen (Bailey and Balch, 1961). To learn more about the effects of variations in saliva flow on digestion, it would be of value to have a reliable method of measuring saliva flow in intact animals. Stevens and Sellers (1959) showed that emptying the rumens of cows altered the rate at which saliva was swallowed and suggested that this was a result of changes in the rate of secretion. The rate of swallowing was measured during the course of experiments on saliva secretion described earlier (Bailey and Balch, 1961). Those earlier values are here analysed to study the relationship between rate of secretion of saliva and rate of swallowing.

Open front pole barns have proved to be very economical for the housing of feeder cattle, dry stock and, in certain areas, milking cows in Canada. In California, Heitman and Hughes (1949), Heitman, Kelly and Bond (1958) and Heitman, Kelly and Hughes (1949) investigated the effect of ambient temperatures (40 to 110° F.) and humidity on rate of gain in swine. Optimum temperatures for rate of gain were 73·5° F. and 61° F., for pigs of 100 lb. and 300 lb. live-weight, respectively. Hazen, Currie and Giese (1950), at Iowa State College, showed that growing swine weighing under 100 lb. are less severely affected by cold weather (34 to 40° F.), when protected from wind and wet, than heavier animals are by high temperatures.