The genetics of beef cattle reproduction

Paul L. Charteris
Institute of Veterinary, Animal & Biomedical Sciences,
Massey University, Palmerston North, New Zealand

The reproductive rate of cattle influences the number of animals available for sale or slaughter and the whole herd efficiency of beef production. This issue of Beef Breeding Matters examines the genetics of beef cattle reproduction, its importance and some of the new reproductive traits introduced in Group Breedplan. Reproductive success obtained by cost effective measures is the most important factor affecting profitability of cow-calf enterprises. Management practices such as timing of mating and length of the breeding season greatly influence these reproductive success and genetic differences among animals may often be masked by these management and other environmental effects. For example, cows which are genetically capable of rebreeding successfully each year, may fail to rebreed due to nutritional constraints following calving. Most male and female reproductive traits have been shown to be lowly heritable (Table One). In contrast, most growth traits and carcass traits have a higher heritability range 0.3-0.4 and 0.4-0.5 respectively.

Table One: Average heritability of some beef cattle reproduction traits (number of studies shown in parentheses)

Genetic progress is achieved by selecting superior cattle and culling inferior individuals. Response from selection for growth traits will be moderately rapid since these traits are readily measured and are moderately to highly heritable. Reproductive traits can sometimes only be measured late in the animals lifecycle (for example, number of calves weaned per cow lifetime), are lowly heritable and respond poorly to selection. Those traits which respond poorly to selection also show the greatest heterosis (hybrid vigour) response in a crossbreeding programme. Due to the low heritability of reproductive traits, culling for poor reproductive performance is essentially a management decision, not a genetic one. Table Two shows the effect of maternal heterosis (the benefit from using a crossbred dam compared to straightbred dams of either breed) for some growth and reproductive traits. Heterosis % is the increase in crossbred performance relative to the average of the two straightbreds. From this New Zealand study, a larger heterosis response was achieved for reproduction traits (calves weaned per cow joined) than for growth traits (calf weaning weight). Productivity (total calf weight per cow joined) and efficiency (productivity divided by average cow weight) showed greater heterosis response than either weight or fertility alone.

Table Two: Heterosis of growth and fertility traits for Angus x Hereford cows compared with straightbred Angus and Hereford. (Morris et al. 1995).

The bottom line- commercial farmers managing a beef breeding cow herd should select for growth and carcass traits and crossbreed for fertility.

Breeding for fertility - when is it important?

The importance of fertility to farm income is beyond dispute. From a genetic stand-point, should we select bulls on the basis of their potential for increasing fertility of their daughters? The benefits from selection for fertility are slow to accrue since they rely on purchasing bulls with above average fertility which will then breed daughters with an improved reproductive capacity. Cattle with a high genetic potential for growth and and milk production also have a genetic potential for high fertility. However, in practical farming circumstances high growth rate-high milk production cattle may show prolonged post-partum anoestrus intervals, especially if underfed following calving. Genetically improving growth or milking ability of cattle without improving the level of feeding at critical times of the year may have a detrimental effect on fertility. Any gains resulting from selection for fertility may not be visible if the nutritional environment is limiting, even though genetic progress is being made for fertility. The benefits from selection for fertility will be dependant on the time horizon for improvement and the current performance level of the herd. Research from Australian circumstances (Barwick et al. 1995) suggests that the relative economic importance of fertility compared to growth increased from 0.6:1 to 1.4:1 for a time horizon of 5 to 20 years and was 1:1 (fertility equally important to growth) at a time horizon of 13 years and cow weaning rate of 80%. Fertility was expressed as the economic value of cow weaning rate (number of calves weaned per cow mated) and growth was the economic value of live weight at slaughter. When cow weaning rate was 60% the economic importance of fertility to growth was 2.5:1, when calving rate had increased to 92% the economic importance of fertility to growth had decreased to 1:1 (at a time horizon, 20 years). These results are based on a situation where bulls are purchased from a registered herd and some heifer progeny are retained for rebreeding. When a terminal sire is used within commercial herds, cow fertility is not part of the selection objective. When bulls are used to breed a specialist maternal line and these commercial cows are mated to a terminal sire, the bulls are responsible for half of the genes for cow fertility but only quarter of the genes for growth of progeny of those cows. In such a maternal case, the relative importance of reproduction to growth would be approximately twice that shown above. In summary, selection for fertility is important when: 1. Feeding levels are sufficient to avoid increases in milk production or growth rate affecting cow fertility 2. The benefits from selection are likely to accrue over a long time frame (10 to 20 years) 3. Initial herd fertility is low 4. Developing a specialist maternal line.

New Traits in Group Breedplan 1996The importance of reproductive traits on beef cattle profitability is evident, less apparent are what characters to measure and include in a genetic evaluation. New Zealand beef cattle breeders enrolled on Group Breedplan will notice some new traits in their sire and dam summary. It is important to note that these selection criteria are themselves seldom economically important (commercial farmers are not rewarded for larger scrotal circumference or shorter calving interval) but they can help identify genetically superior cattle for some financially important traits such as cow weaning rate or potential number of cows pregnant per bull.

Scrotal size (SS) EBV is a measure of the animal’s genetic merit for scrotal size. Scrotal size is measured as the circumference of the widest part of the scrotum and is measured at a bull age of 13-14 months. This trait is positively associated with increased semen production and decreased age at puberty in heifer progeny. The results of several studies show that an increase in scrotal size is associated with an increase in sperm motility, percent normal sperm, semen concentration, total sperm and a decrease in the percentage of abnormal sperm. A genetic correlation of -0.71 between scrotal size and heifer age at first oestrus indicaces that selection for increased scrotal size would result in heifer progeny being younger at puberty - a favourable relationship. Additionally, the genetic correlation between SS and growth traits are positive suggesting that selecting heavier cattle at any age will increase scrotal circumference.

Days to Calving (DC) EBV reflects the genetic merit for the time from the start of mating until the actual calving date (measured in days). The primary use for this EBV is to identify sires whose daughters are more fertile and tend to calve earlier in the season. Days to calving incorporates the interval from the start of mating until successful conception and the gestation length. New Zealand research has found that the heritability of days to calving was low, averaging 0.11 suggesting that slow selection response can be expected when culling females based purely on days to calving EBV. There is a small favourable genetic correlation between days to calving in heifers and scrotal size in bulls. New Zealand research suggests that pregnancy rate should increase in response to any selection pressure applied to reduce days to calving. For each one day phenotypic increase in days to calving, pregnancy rate decreased by approximately 2%.

Gestation length (GL) EBV is a prediction of genetic merit of the number of days from mating until birth of the calf. A positive genetic correlation between days to calving and gestation length suggests that a decrease in gestation length would be associated with a decrease in intercalving interval. The genetic correlation between gestation length and birth weight is positive. Selection for a decrease in gestation length is would result in heifers with shorter gestation lengths, lowered calf birth weights and a lesser likelihood of difficult births. Gestation length is used in the calculation of calving ease EBV. In summary, selection for growth or fertility alone is rarely the most profitable genetic improvement strategy, a balanced selection objective should comprise fertility, growth and carcass traits.

What the processors say To maintain a sustainable beef industry, animals must be produced that secure profit for breeders, commercial beef cattle farmers and processors. Selection objectives implemented within bull breeding herds should also aim to anticipate processor (and their customers) requirements. A survey was sent to representatives of each major beef processing company to determine traits important for improving their profitability. Replies were received from eight companies. Processor representatives were asked to rank traits (from 1, not important to 10, extremely important) in order of financial importance from the perspective of their company. Rankings of traits important to processors for both Asian Table Beef and North American manufacturing markets are shown in Table Three. Meat quality traits (marbling, pH, meat and fat colour) and consumer important traits (meat tenderness, taste, juiciness and flavour), ranked higher when the beef product was destined for Asian table beef markets than for North American manufacturing grade markets. The high ranking placed on meat quality traits suggests the inclusion of these traits within selection objectives is warranted when the breeding goal is to meet Asian market requirements. In practice, such traits will only be included in selection objectives when producers are financially rewarded for superior meat quality. Traits associated with carcass yield (dressing out %, lean meat yield %), ranked higher when the beef product is destined for North American manufacturing markets than Asian table beef markets. A maximum yield of lean meat per animal appears desirable to improve processor profitability when the aim is meeting North American manufacturing market requirements. Variable responses were received for some traits (for example, processors assigned an importance for marbling that ranged from 1 to 10) when the beef product is aimed at Asian market requirements. Variable responses maybe due to: 1. a highly differentiated Asian beef market with changing importance of meat quality traits within different niche markets or 2. some processors may have experienced previous difficulties in supplying beef products within market guide-lines (due to cattle supply, processing or transportation factors) which will affect their perception of the importance of a trait. Consumer requirements and pricing signals are being sought from N.Z. Meat Producer’s Board representatives within our important Asian beef markets.

Table Three: Carcass and meat quality traits that processors consider important for improving profitability and meeting customer requirements. Responses are shown for Asian table beef and North American manufacturing grade markets. Left-hand tip of triangle = minimum response, right hand tip = maximum response. Apex of triangle = average response.





The following sources are acknowledged in preparing this publication:

Barwick, S.A.; Henzell, A.L.; Goddard, M.E. 1995. Beef breeding for cow fertility: when is it important?. Proceedings of the Australian Association of Animal Breeding and Genetics, 11: 443-446.

BIAA Technotes. Understanding Group Breedplan fertility EBV’s. BIAA Technote No. 95/1.

Brinks, J.S.; Genetic studies of reproduction in beef cattle in the USA. Proceedings of the Australian Association of Animal Breeding and Genetics, 11: 329-339.

Koots, K.R.; Gibson, J.P.; Smith, C.; Wilton, J.W. 1994. Analyses of published genetic parameters estimates for beef production traits. 1. Heritability. Animal Breeding abstracts vol 62, 5: 309-338.

Morris, C.A.; Cullen, N.G. Genetic studies of days to calving in beef cattle. 1995. Proceedings of the Australian Associa tion of Animal Breeding and Genetics, 11: 350-355.

New Zealand Angus Association. The 1996 New Zealand Angus Genetic Evaluation Report.