Early spring forage, if not growing among old forage so they must be grazed together, is high in protein and water content, and fairly low in energy. Runny manure in many herds every spring testifies to the effects of washy forage.
Patrick Gunn, Iowa State University Extension cow-calf specialist, recently noted that although water content of a feed is not a limiting factor for intake, there is a limit to the number of bites a cow can and will take in a day.
He said experienced, mature grazing animals may take as many as 60 bites per minute, eight hours per day, totaling perhaps 130 pounds of forage eaten. Young cows and heifers, on the other hand, may graze 20% to 40% less than mature cows and commonly have a lower rumen capacity.
Because dry matter content of that washy grass may be only 15% to 30%, together with the higher protein content, the ability of these younger cows and heifers to have a positive energy balance is difficult in most environments in early spring. It can be even worse when transitioning from a drylot to fresh pasture.
Here’s the problem: If these are bred heifers or young cows, this adds another dimension to the problem and could decrease pregnancy success rates.
Research at South Dakota State University showed heifers moved from drylot onto pasture immediately following timed AI lost nearly 1.5 pounds per day during the first week on grass, whereas heifers that had been on pasture for 44 days prior to AI gained more than a pound per day during that same week.
Julie Walker, SDSU Extension beef specialist, said the researchers also noted an improvement in AI pregnancy rates of 15 percentage points (76% vs. 61%) for heifers that were supplemented, compared with those that were not supplemented when turned out on grass immediately following AI.
Gunn noted that research at other universities supports the SDSU data, with reports that heifers that do not maintain a positive average daily gain for the first 21 days after AI have compromised pregnancy rates to that insemination.
Cows affected, too
Cows also are affected by this early-spring forage problem.
Data from the University of Minnesota have shown that cows consuming only 80% of their dietary requirements for the first seven days post-AI had reduced embryo quality and fewer live cells in those embryos at uterine flush.
A research project in Illinois showed spring-calving cows supplemented with a low-quality energy ration in early spring actually achieved higher first-service conception rates by 67%, vs. 45% for unsupplemented cows.
The supplement was 4 pounds per head of a mix containing 45% soybean hulls, 45% ground corncobs and 10% molasses.
Interestingly, body condition did not vary between the two cow groups, and neither did overall rebreeding rate over the entire breeding season. But there was an early difference in reproductive performance, even in cows.
Walker and George Perry, SDSU Extension beef reproductive management specialist, recently wrote that research outcomes such as these make it clear: Postbreeding management can affect reproductive performance in early spring.
They suggested a grazing adaption period for heifers prior to the breeding season, or supplementation when bred heifers are moved to lush pasture.
Another option is forage management that “stockpiles” enough standing old forage to graze through early spring, which will be increasingly mixed with young, high-quality forage as summer nears.
Here’s why spring forage is a problem
Finding data on the nutritional content of early-spring forage is nigh impossible, but a small amount of data exist.
Some of the earliest data on high crude protein in relationship to energy in physiologically immature forage came from early rotation-grazing trials in Europe, in which cattle actually died from too much protein and not enough energy.
More recently, some authors in the U.S. have begun to address the energy deficiency in beef diets.
Physiologically immature forage refers mainly to the stage of growth or regrowth, and whether the plant has developed adequate aboveground leaf material to be fully operating on photosynthesis, and whether composition of the plant has changed to more energy and less protein.
This is controlled by weather and grazing pressure or mowing pressure. For example, fresh regrowth in a recently mowed hay field would be classified as “young” forage.
Here are three situations in which forage should be unusually high-protein and low-energy.
1. early spring growth of any forage with no old grass or hay to supplement it
2. too-rapid return to grazing paddocks which are not fully recovered in a rotation grazing system
3. wheat pasture or other high-quality and lush forage that is young and immature