The body size of dairy cows has garnered much interest over the years. Breeders who participate in shows often find that larger cows have a better chance of being crowned champions. Selection pressure for higher milk production levels has also led to bigger cows being bred.

In addition, dairy farmers are complaining that cows are becoming too big, and these observations are based on cull cows. Research shows that body mass, which is an indirect indicator of cow size, has a negative economic value. This is because cows’ maintenance requirements increase as they grow bigger (or heavier).

These days, however, dairy farmers are more cognisant of the fact that a dairy’s profit margin is determined by milk production efficiency. Milk quantity and quality determine the herd’s income. The biggest cost factor relating to milk production is the amount of feed that cows consume over the lactation and dry period.

Efficiency is usually expressed as kilograms of milk produced per kilogram of feed consumed. For many years selection was based predominantly on higher milk production levels, with little attention paid to body size.

The feed intake of cows

Because of the difficulty in measuring the feed intake of cows, body mass is commonly used in dairy cattle nutrition as an indirect indicator of feed intake. But other factors also affect feed intake. The National Research Council’s (NRC, 2001) intake model for example uses 4% fat-corrected milk (FCM) production, metabolic body mass (BM0,75) and lactation stage (weeks in milk) to estimate feed intake.

More complex models have since been developed. It was found that the digestibility of the complete diet also affects feed intake. For example, low-grade roughage and diets high in moisture impair feed intake.

Figure 1: Change in bodyweight (BW), fat-corrected milk (FCM) production, and estimated feed intake (DMI) according to the NRC model for a Holstein cow over a lactation period.

Figure 1 illustrates how milk production increases after calving and reaches a peak roughly six to eight weeks later, after which it drops until the end of the lactation period. In contrast, the cow’s body mass decreases during early lactation due to the utilisation of body reserves (fat). This is because the increase in feed intake is slower than the increase in energy demand due to milk production. Cows experience a negative energy balance during this period.

Estimating efficiency

The NRC model indicates that cows’ feed intake follows the milk production curve rather than the body mass curve. Older cows are heavier and larger, implying an increase in their milk production, feed intake and maintenance requirements. A standard lactation stage or age must therefore be used to calculate the effectiveness of individual cows for selection purposes.

Cows’ body mass is also influenced by factors such as their rumen capacity, feed and water intake (before weighing), milk production (weighed before or after milking), body condition (fat reserves) and muscle and skeletal size.

Jersey cows have more delicate bones and muscles. Their rumen capacity is therefore proportionally greater than that of Holstein cows. Jersey cows’ intake is therefore proportionately greater than that of Holstein cows – 4% versus 3,4% of their body mass. Cows should preferably be weighed according to a standard procedure to limit variation in body mass.

Should cows be smaller?

Where Holsteins are concerned, it is increasingly recommended that cow size be limited and that producers should select for medium-framed cows. This goal is probably aimed at reducing the maintenance requirements of cows. The trough space and sleeping pen dimensions in buildings cannot be changed. Management problems and injuries can occur in buildings that are more than 20 years old. Buildings therefore limit cow size.

Producers who milk from grazing believe that larger cows find it more difficult to trek over long distances and uneven terrain between grazing and the milking parlour. Little research has been done in this regard. However, according to Irish researchers, larger cows do not trample grazing more than smaller cows, as they have larger hooves and pressure is spread over a larger area.

Care must be taken when selecting for smaller cows due to the positive genetic correlation of about 20% between cow size (or body mass) and milk production. The opposite is also true, namely that smaller cows produce less milk.

Value of bull selection

Bull selection is the most important way of influencing traits, but it requires patience. The degree of heritability in terms of body mass or size is at a medium level (45%); however, several generations are needed to achieve results and there is a sizeable variation between cows.

Due to the expected slow change, producers are considering crossbreeding with Jersey bulls. However, a crossbreeding programme is much more complex and can impair milk production. There is enough genetic variation in the Holstein breed when looking for bulls to breed smaller cows, while limiting the drop in milk production.

Cow selection can be used in the short to medium term to identify and cull larger cows in their first lactation, if their milk production is also lower than the group average. Dividing milk production by body mass is not necessarily the best way of identifying such cows. It is used to identify larger cows but will also include cows with high milk production.

Local research

Breeding projects to determine the effect of bull selection on specific selection goals is a long-term undertaking. However, continuity is usually a problem as successive researchers do not always share the same interests or knowledge. Producers’ research requirements are focused on current problems. Few producers are willing to provide their data sets with cows’ milk production and body mass information for analyses.

Milk recordings in South Africa have stagnated, with fewer herds and cows participating. Few new traits have been developed to improve especially milk production efficiency. This is attributed chiefly to a lack of herd records at national level.

Nevertheless, a study was undertaken on the effects of bull selection. Two groups of bulls were selected annually according to the following selection goals:

  • The highest breeding values for fat and protein production.
  • Bulls with the highest negative deviation for shoulder height in the group of bulls with the highest breeding values for fat and protein production.

Two groups of cows

The cows in the herd were divided into two groups, and a commercial breeding programme was used to allocate bulls to cows. The heifers of the two bull groups were reared in the same way. The cows were taken up in a grazing system post-calving. The cows received a concentrate of 7kg per cow per day, regardless of their milk production levels and lactation stage. The milk recording scheme was used to determine cows’ milk production and composition.

Table 1: Test day milk production and estimated efficiency norms of first-lactation cows that are the offspring of bulls selected for production and cow size (statistics).

Production traits Bull groups Efficiency norms Bull groups
Production Statistics Production Statistics
Number of cows 36 24 Body weight (kg) 546a 520b
Milk (kg/day) 23,0a 20,8b CV (kg/day) 7,51a 6,61b
Fat (%) 4,01a 3,80b KR (kg/day) 0,153a 0,139b
Protein (%) 3,19 3,16 FCM/BW (kg/day) 0,042a 0,039b
Lactose (%) 4,90a 4,78b Milk/BW (kg/day) 0,042 0,040
Fat (kg/day) 0,92a 0,79b DMI (kg/day) 18,0a 16,9b
Protein (kg/day) 0,73a 0,65b Milk/DMI (kg/day) 1,29 1,24

a, bValues differ significantly (P<0,05); CV: Component value: ((Fat x 6) + (Protein x 13))/2; KR: Kleiber ratio: (NEL/BW0,75); FCM/BW: 4% fat-corrected milk production/body weight; Milk/BW: milk production/body weight; DMI: dry matter intake; Milk/DMI: milk production/dry matter intake.

Although cows from the bulls in the statistical group are smaller, there is a large overlap in body mass between cows in the two groups. Cows in the statistical group have lower milk production, estimated dry matter intake and efficiency norms, such as CV and KR. Reduced milk, fat and protein production eliminate the ‘advantage’ of smaller cows. Based on the genetic merit of bulls, the productive life of the offspring of the statistical group is expected to be longer.

Automatic milk recording

New computer programs that collect information on cows’ milk production and composition during milking and that weigh cows post-milking, facilitate the formulation of alternative efficiency benchmarks for cows. When based on these different benchmarks, the effectiveness of selection can be compared to milk production as a control.

The value of the popular farmer’s trait of milk production divided by body mass, can then be determined. However, this requires a great number of records from different dairy herds as research herds are too small to yield this type of information.

In conclusion

Producers who are considering selecting for smaller cows, should take the proven positive correlation between milk production and body mass into account. A novel recommendation is to use a selection index that includes body size as a negative value.

Higher milk production potential and smaller cows will improve production efficiency on two levels. Shoulder height can be used as an indirect indicator of cow size. Cows’ lifetime production has greater financial value than their body size. – Dr Carel Muller, research associate, Stellenbosch University

For more information, send an email to Dr Carel Muller at