The greatest nutrient demand of beef cows is during lactation, a time when cows need special consideration in meeting their energy and protein requirements.

In most Northern Plains locations, the primary grasses available for grazing are warm-season grasses, which become available in late May to mid June and gradually decrease thereafter. But if cows calve in March, it means they must be fed a lactation diet of high-quality hay, and supplemented, for about 90 days before summer grass arrives.

Sequencing calving closer to the time when the grazed resource will meet the nutrient demands of the lactating female reduces feed costs and increases profit potential. Basically, this means moving early-spring calving to early-summer calving.

The key components to changing calving time using a "systems" approach include:

• Cows have access to vegetative forage for a short period of time prior to calving.
  
• Cows meet their energy and protein needs from the pasture resource.
  
• Hay and supplement costs are reduced because peak lactation occurs when vegetative, high-quality forage is available.
  
• Reduced calf losses and sickness because calving occurs when the weather is warmer.
  
• Less labor is needed at calving because calves weigh less at birth for June calving cows compared to February/March born calves.
  
• Labor is reduced because less harvested feeds are fed.
  
• Different market alternatives are available for the calves and cull cows and bulls.

In 1993, the University of Nebraska's Gudmundsen Sandhills Laboratory developed a summer-calving herd to compare spring- and summer-calving systems. In the spring herd, cows began calving in March and the breeding season began in June, with calves weaned in October. For the summer-calving herd, calving began in June, breeding in September, and weaning occurred in November or January. Data were collected from 1994 to 1996.

Results show summer-calving cows were fed 327 lbs. of hay/cow/year compared to 3,947 lbs. of hay/cow/year for the spring calvers. Similar amounts of protein supplement were fed -- summer-calving cows were fed 154 lbs./cow/year, while spring calvers consumed 96 lbs. The length of the grazing season went from 233 days to 357 days by adjusting the calving time from March to June. Cow reproductive performance wasn't different between groups.

When calves were weaned at similar days of age, summer-born calves were about 35 lbs. lighter. However, January calf prices tend to be higher for the same weight of calf as sold in October. Therefore, summer-born calves generate similar gross income as spring-born calves. Due to costs savings in the summer-calving system, primarily due to less labor and less hay fed, the summer-calving system was more profitable, even at weaning time.

Among the concerns with summer calving is that breeding season occurs at a time when temperatures are hot. Depending on local climate conditions -- high humidity and no night cooling, for instance -- a breeding season that occurs during this time period could result in lower pregnancy rates.

Rebreeding rates for mature cows is high in a summer-calving system, but lower rebreeding performance for young females trying to get pregnant with their second calf can be an issue. Because warm-season grass quality is decreasing at this time, such females may need additional management to achieve acceptable rebreeding rates.

-- Rick Rasby, University of Nebraska-Lincoln