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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Animal Production

Bradford and Spurlock, 1964, vol. 6, part 3, p. 299, line 7.

When crossbred offspring are used for production of an agricultural commodity profit is a function of three genotypes, that of the sire, the dam and the crossbred offspring. Heterosis in the component traits of profit, in any of the above genotypes, affects profit. Thus only when the final product is a four-way cross can we get crossbred parents plus a crossbred progeny and a full use of the available heterosis.

When the contributions of the sire and dam to profit are unequal, there is a justification for the breeding and use of specialised sire and dam lines, even though the component traits may be genetically additive. Multiple crosses (three- and four-way) may improve profit over two-way crosses only when there is heterosis in one or more of the component traits.

When profitability is presented as a function of productivity and repro-ductivity, five heterosis factors of the component traits may affect profit. These are: (a) Improved efficiency of production in a crossbred ‘commercial offspring’, (b) Improved viability (a component of reproductivity) in a crossbred ‘commercial offspring’, (c) Improved ‘female fertility’ in a crossbred dam. (d) Improved efficiency of the ‘commercial offspring’ through heterosis for maternal effects in the crossbred dam. (e) Improved male fertility through heterosis in the crossbred sire.

When the profit equation is non-linear, the same degree of heterosis as that measured on the component traits may have quite a different effect on profit, depending on the level of performance of the traits. By crosses and backcrosses between the parental lines the location and composition of the heterosis factors may be altered favourably.

Graphic and algebraic procedures were outlined for finding sire-dam combinations that maximise profit in the presence of heterosis when a group of genetic stocks is available.

Selection on an index of two traits was represented graphically by means of a response ellipse or response circle. This procedure was used to find optimal index weights for uncorrelated traits, and to compare the efficiency of alternative indices.

Profit was expressed as a reciprocal function of reproductive performance, from which it was shown that the higher the reproductive performance, the smaller the weight that should be applied to it in a selection index. It was found that in pigs and broilers the average commercial standard of reproductive performance is sufficiently high that selection on production traits alone is almost as efficient as selecting on the optimum index.

Three alternative procedures for selecting from a single original line were compared graphically: A. Maintaining the single line and selecting all animals on the same index. B. Maintaining the single line but selecting males and females on separate indices. C. Splitting the original line into separate sire and dam lines, each selected on a specialised index. Method C was found to be the most efficient of the three; B was more efficient than A for one generation, but less efficient in subsequent generations.

1. Total anatomical dissection data from 62 half carcasses of steers representing 6 breed groups differing in age and nutritional history, were examined with a view to establishing a basis for comparison of carcass composition of breed groups or of cattle from various treatments.

2. The results indicate that total muscular tissue might be used for comparative purposes in certain instances. Muscle weight differences could be examined independently of general size by adjusting to common muscle plus bone weights.

3. It is proposed that relative carcass composition is best assessed by use of two measures: muscle: bone ratio and percentage fat tissue in the carcass.

4. Muscle: bone ratio was shown to increase with carcass weight and after the effects of carcass weight had been statistically removed the influence of percentage fat on this ratio was negligible. The required adjustment to muscle: bone ratio for differences in carcass weight was estimated at 0·03 per 10 kg. increase in carcass weight.

5. Percentage fat in a carcass is proposed as an index of acceptability. The requirements of different markets in terms of this index would need to be established. Adjustment factors for percentage fat based on carcass weight would need to be established on a within group basis.

6. Relative growth coefficients for muscle, bone and fat, based on a logarithmic relationship with total muscle plus bone, are presented and were found to agree closely with those for swine and sheep. The magnitude of differential growth in body tissues indicated by the growth coefficients accentuates the necessity of making carcass composition comparisons at standard weights or following suitable statistical adjustments.

7. The implications of carcass composition assessment in beef breeding programmes are discussed.

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.

1. Herbage was harvested from a single pasture in early June and late August of the same year and was preserved by freeze-drying. The June herbage contained more water-soluble carbohydrate than that cut in August but in other respects, including chemical and botanical composition and the apparent digestibility of organic matter and nitrogen, differences between the herbages were small.

2. The June herbage was given to a mature ewe during eight periods each of about 4 weeks' duration, and the August herbage was given during nine periods. The level of feeding varied between periods from about 0·35 to 1·5 times the maintenance level; in addition the fasting metabolism of the sheep was determined on three occasions. Gross energy intakes and the losses of energy in faeces, urine, methane and as heat were measured in each period.

3. The metabolisable energy of the June herbage was utilised more efficiently than that of the August herbage for fattening, and probably for maintenance also. It is suggested that this result stemmed from the difference in soluble carbohydrate content which was lower in the August material. The cellulose of that herbage was digested to a significantly greater extent, a larger proportion of the gross energy was lost as methane, and the ruminai volatile fatty acids, included a significantly lower proportion of propionic acid.

1. Twelve gilts, six pregnant and six non-pregnant, were each given a daily ration of 2.2 kg. for a 110-day period.

2. Live-weight gain during this period was 53 kg. for the pregnant animals compared with 23 kg. for the non-pregnant animals.

3. Apparent digestibility of nitrogen and dry matter and the energy of digested feed did not differ significantly between the pregnant and non-pregnant animals.

4. Daily nitrogen retention was on average 9·2% higher for the pregnant animals, the difference between the two groups becoming progressively greater during the trial.

5. Highly significant differences were found in pre-slaughter live-weight, weight of reproductive tract and weight of mammary region. The difference in the weight of free draining blood was also significant. No difference was found in the weight of alimentary tract, visceral organs, head or decapitated carcass.

6. The carcasses of the pregnant animals contained less subcutaneous fat plus skin and more intermuscular fat plus muscle than those of the non-pregnant animals. The weights of bone did not differ.

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.

An analysis has been made of the reproductive performance of a breeding flock of Clun Forest ewes in Northumberland. Data are based upon 941 breeding females and 75 males. The average lambing percentages were 43·9 115·4 and 137·7 for ewes aged 1, 2 and over 2 years. Total lamb mortality to 3 or 6 weeks of age was 23·7% for lambs from 1-year-old dams, 16·6% from 2-year-olds, and 12·9% from older ewes. Mortality for all ages of dams was 12·1 % for single lambs, 15·0% for twins and 34·0% for triplets. Birth weight was strongly associated with survival within all birth types and maternal ages.

Analyses of weights at birth, 21, 42 and 112 days showed that much of the observed variation in lamb weight was associated with age of dam and type of birth and rearing. Single lambs and those from mature dams were significantly the heaviest. The effect of age of dam on lamb weight decreased sharply from birth to weaning, whereas the type of birth and rearing was the major influence at 21 and 42 days, and was still important at 112 days.

The mean weights of all lambs adjusted by least squares analysis to the basis of male twins from 3-year-old dams were 9·2, 19·7, 30·6 and 62·7 Ib. at the four ages. Four- and 5-year-old ewes reared the heaviest lambs. Interactions between the effects studied were significant both within and between years and accounted for from 7 to 10% of the total variation. A single set of general correction factors at each age was shown, on average, to account for 80–90 % of the variation between subclasses within years and may have applicability to comparable flocks.

1. Twelve sets of 3 litter-sister Large White gilts were mated at first oestrus after reaching 250 lb. live-weight and given daily during 3 successive pregnancies either 6 lb. (A), 3 lb. (B) or 3 lb. for 76 days then 6 lb. until parturition (C) of the same meal mixture. During an 8-week lactation all were given 4 lb. meal plus 0·8 lb. per piglet suckled.

2. There was a significant linear increase in numbers born with successive parities (P<0·05) but no significant differences between treatment or sister groups. There was a highly significant difference between treatment groups in mean piglet birth weight (P< 0·001) but no parity effect; mean birth weights were 2·76, 2·40 and 2·58 Ib. for Groups A, B and C respectively. Differences between sister groups in mean piglet birth weight were also significant (P<0·05).

3. Post-natal litter performance was poor for all groups, probably because of the presence of E. coli, but there was no evidence of treatment effects other than on 3-week weight of third litters, which was significantly greater in Group C than in A or B (P<0·05). There were no significant effects on either number or weight of piglets at 8 weeks.

1. Nitrogen balance studies were conducted on 16 steers in two separate trials. The steers were fed on four diets containing respectively dry rolled barley; dried, steamed and rolled barley; dried, crimped barley and barley that had been stored in a sealed silo and rolled.

2. There were no significant differences in the dry-matter intake of any of the diets.

3. No consistent differences between the diets in dry-matter digestibility or nitrogen retention were observed.

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.

An experiment was conducted on 13 commercial barley beef units in Scotland, using a total of 301 early-weaned Friesian male calves. On each unit the trial batch of calves was divided into two groups, which were then allocated to a control diet (85% barley + 15% protein supplement) or the same diet supplemented with 20 mg. chlortetracycline per kg. of feed. The experiment was conducted over the live-weight range 115 to 360 kg.

The chlortetracycline significantly (P<0·01) improved live-weight gain ( + 4·9%) and feed utilisation ( + 4·8%). This overall effect was largely due to a high response during the first 12 weeks. From 13 to 28 weeks the chlortetracycline had only a marginal effect, which was not significant. There was no significant difference in the feed intakes of the two groups. There was a significant (P<0·01) negative correlation between the effect of the chlortetracycline on live-weight gain at each unit and the growth rate of the control animals at that unit; the response was greatest on farms with poor hygiene. The animals given chlortetracycline showed a significantly (P<0·05) lower incidence of liver abscesses (11·8%) than the controls (28·2%). Macroscopic examination of the rumens showed that rumenitis is common among intensively fed cattle, but no correlation could be found between incidence of liver abscesses and rumen lesions.

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.

Thirteen Blackface and Merino × Cheviot female sheep were shorn and each exposed for a maximum of 10 hours to intense cold (—18° C, 4 m.p.h. wind). Individual exposures were terminated earlier if rectal temperatures fell by 4·5° C. In the first year of the experiment (1963) 11 out of 13 of the sheep were in late pregnancy. There were also 12 control sheep, 9 of them pregnant, which were not subjected to cold treatment. In 1964, 12 of the previously treated sheep were subjected again to similar cold exposures. This time they were unmated and there were no controls.

The cold treatment apparently caused increased variation in gestation length, but the effects on neo-natal lamb mortality were inconclusive.

In 1963 and 1964 there was wide variation between individual sheep in cold tolerance—defined as the ability to maintain rectal temperature during cold exposure. Repeatability for this character was approximately 0·73, suggesting the existence of a genetic component. The average decline in rectal temperature was similar in both years: 3·9·2° C. in 1964. Pregnancy and breed had no detectable effects on cold tolerance.

Skin temperatures on the midside, ear and hind foot (pastern) were measured in 1964. They all declined during cold exposure. Midside skin temperatures typically fell to about 10° C., ear temperatures to about 3° C. and foot temperatures to zero. There were temperature fluctuations due to periodic vasodilation on the ears but the feet generally showed sustained vasoconstriction.

Superior cold tolerance tended to be associated with low mean body skin temperatures and a previous history of live-weight gain. It is suggested that subcutaneous fat deposition may be one factor responsible for improving skin insulation and thereby reducing heat loss in shorn sheep, but direct evidence for this was lacking.

1. Nitrogen balance studies were conducted on 8 early-weaned calves fed on four diets containing respectively Peruvian fish meal, soya bean meal, groundnut meal and dried distillers grains as the major sources of protein.

2. Nitrogen retention differed significantly between diets, being highest on the fish meal diet, and lowest on the groundnut diet.

The possibility that restriction of pasture and controlled supplementation of the diet of milking cows with concentrates might improve the efficiency of utilisation of grazing was examined in a double reversal grazing experiment with 9 Ayrshire cows per treatment which extended over 4 months.

Cows received either pasture alone allocated daily by electric fence (treatment B), or half the pasture area plus one-third of their expected DOM intake offered as concentrates (treatment A).

In the first two periods, treatment A resulted in closer defoliation of the pasture.

Over the whole experiment there was no significant difference in yield of fat-corrected milk between treatments, although treatment A depressed butterfat percentage and raised milk yield significantly. Treatments did not influence live-weight gain significantly and toth groups gained over the season.

Feed intake estimated by chromic oxide-faecal index methods was similar for both treatments except in period 1. Intake estimates by clipping methods or by faecal index methods were in close agreement in period 1 for treatment B and in reasonable agreement in periods 2 and 3.

The results are discussed particularly with a view to the possibilities of replacing some grazing with barley or other concentrated feeds. On the available evidence this is economically feasible only where very profitable uses of the grassland saved are possible.

Curves of growth in live-weight, from birth to 16 weeks of age, of 362 Suffolk × Welsh lambs were analysed for the effect of sex, birth type, sire and test centre environment and related to variation in age at slaughter.

The relative pre-natal disadvantages of twin lambs resulted in their curves of growth differing from those of single-born animals, particularly in males. Test centre environment exerted a marked effect in such cases. At one centre, male twins showed marked compensatory growth.

Differences in growth existed between the 14 sire progeny groups. As the linear component of the growth curves decreased, the quadratic values became increasingly negative. Deceleration of growth was most marked in slowest growing progeny groups and this was accentuated by a poor test centre environment.

These factors combined to influence age at slaughter, which was also related to birth weight of the lamb. Lambs at the best test centre reached slaughter weight 10 and 16 days earlier than lambs at the two remaining centres. Sires produced differences in age at slaughter of 10 to 14 days, within centres.