There has been a 30% increase in cow mature size over the last 30 years. From 1975 to 2015, cow numbers have decreased by 35%, but beef production has been maintained at a level similar to 1975. In response to the low cow numbers, carcass weights have increased. These relationships suggest that the progeny of small cows, similar to the weights observed in the 1950s and 1960s, would not have the potential to produce the carcasses necessary to maintain beef production at the current level with the number of cows currently in the national beef cow herd, unless they take part in a postweaning growing period.
This phenomenon is explained by the increased productivity per calf in the progeny of the United States cow herd. The average hot carcass weight in 2014 was 38% greater than 1975, averaging 870 pounds in 2014 compared with an average hot carcass weight in 1975 of 630 pounds.
Use of growth promoting technology has contributed to a portion of the increase in carcass weight. Anabolic implants increase ADG by 21%, body weight at slaughter by 128 pounds, and hot carcass weight by 55 pounds. Additionally, β-adrenergic agonists increase body weight at slaughter by 18 to 24 pounds but increase hot carcass weight by 35 to 42 pounds. These growth promoting technologies only account for about 97 pounds of the 240 pounds increase in hot carcass weight, thus much of the increase in beef production per calf is likely linked to the use of EPDs and genetic selection for greater yearling weights, which in turn increases mature size. Beef production is more efficient because of technology advances in growth promotants, feed milling, and feed additives in the last 30 years.
The average cow slaughter weights have increased from 1,047 pounds in 1975 to 1,369 pounds in 2005. This increase in cow bodyweight is tied to the genetic changes of the cow herd. For instance in the Angus breed the average yearling bodyweight of bulls and heifers has increased by 7.9 and 5.7 pounds per year since 1972 and selection for yearling body weight and weaning body weight have strong correlation with mature size.
A 30% larger cow requires 22% more daily maintenance energy and will consume 22 to 28% more forage dry matter daily, decreasing cow carrying capacity of the farm or increasing input costs associated with pasture management, supplementation, and stored forages.
Increased hay production and grazing strategies
An indication of increased intensification of cow-calf production is displayed by the increased hay production over the last 40 years. The increased use of the round baler and other hay production technologies since the early and mid-1970s has lowered the labor requirement and increased the convenience of hay production and thus the total amount of hay produced. The OARDC-EARS research station at Caldwell was a location for much of this research (Check out Page 83 of this 1972 OARDC Research Summary). At the same time, forage management strategies (stockpiling and strip grazing) were being developed to reduce reliance on stored forages for wintering beef cows (Check out Page 1 of this 1970 OARDC Research Summary).
Environment and cow size
Research from the 1960s and 1970s indicates that in limited resource environments (Western Plains States for instance), the reduced efficiency of larger cows may be a limiting factor to the economics of production, whereas in less restricted environments (higher rainfall environments such as the humid states or in Dry Lot Feeding Systems.) mature cow size may not be a significantly limiting factor.
Intensification has occurred through increased feeding of hay and stored forages, which increases the expense of maintaining the cow herd due to machinery, fuel, labor, and fertilizer costs. There are forage management strategies that can be used to reduce or replace the need for stored forages. For example, rotational grazing increases harvest efficiency of grazing livestock and can help maintain plant populations of clovers or other desirable forage species that lack persistence under continuous grazing management. Stockpiling of perennial pastures during the late summer for use during the fall and early winter can be utilized. Readers can checkout other grazing strategies at the OSU Forage Team Website.