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Lush pastures can lead to poor reproductive performance

guy-ozenne/iStock/Thinkstock beef cows and calves grazing on lush pasture
UNBALANCED DIET: Lush spring pastures can be high in protein and moderate in energy.
Beef Column: Supplementing cows' diets can boost reproduction.

After a long winter, most of us relish those first warm and sunny days of spring. For beef farmers, those first spring days also signal that pastures will be greening up soon.

Making the transition from winter feeding to spring pasture often represents the most economical and labor-efficient feeding of the year. While both farmers and their cows may be excited to get on the first grass of the year, recent research shows that easing the transition from stored feeds to pasture can impact the herd’s reproductive performance. 

At the 2017 Driftless Beef Conference, Travis Meteer, University of Illinois Extension, outlined three challenges for the beef cow herd on lush spring pastures. 

Low dry matter. Lush spring growth can be as low as 20% dry matter. This makes it challenging for a cow to physically consume enough pasture forage to meet her nutritional needs. The rumen simply runs out of physical capacity. 

High protein. Lush spring pastures can be high in protein and moderate in energy. When protein intake exceeds nutritional need, energy is required for the body to excrete it. If ration energy is low or moderate to begin with, the additional energy required to excrete excess protein can cause a negative energy balance. 

Low fiber. Low fiber in lush spring growth results in high passages rates through the digestive tract. Low fiber also results in an unsatisfied appetite. 

In a University of Illinois experiment, cow-calf pairs were divided into supplemented and nonsupplemented groups at the beginning of the grazing season. The supplement was a mix of soybean hulls, ground corn cobs and dry molasses. The feeding rate for the supplement averaged 4 pounds per head per day for 70 days. These feedstuffs were chosen because they are high in energy or fiber while low in protein, and are economically accessible in the local area. 

At the end of the 70-day supplementation period, there was no significant difference in cow body weight or body condition score. There was a numerical difference in reproductive performance. In the first year of the experiment, the supplemented cows averaged a 67% conception rate to timed AI while nonsupplemented cows averaged 45%. In the second year, the supplemented group’s timed AI conception rate averaged 52%, while the nonsupplemented group averaged 38%. 

Further work is needed to replicate these results regarding conception rates before broader recommendations can be made, especially in regard to timing of pasture turnout in relation to timing of AI.  However, the experiment serves as a reminder that while we may consider first pasture growth to be the lushest and most nutritious, it often is not balanced to the cows' needs. 

Different conception rates
This experiment was not able to conclusively identify the biology behind the difference in conception rates.  Possible explanations include that excessive protein in combination with low dry-matter intake causes a temporary negative energy balance, which has been linked to poor reproductive performance. Also, excessive protein levels in the diet can cause physical stress. When excessively high, ammonia from protein will attached to red blood cells instead of oxygen. Cows will pant, similar to when they are heat-stressed, to get rid of the ammonia and breathe more oxygen.

While this has not been tested, it can be hypothesized that a shorter supplementation period could be effective if timed around the peak in pasture forage protein content. Practical options that were not tested in the experiment but may produce similar results include allowing pastures to mature a little longer before initial grazing, providing access to dry hay that is low in protein (i.e., not prime alfalfa) or supplementing with a high-energy grain.

While the focus of this project was lush spring growth, in rotational grazing systems, allowing adequate regrowth between grazing periods is recommended to avoid similar situations throughout the season. 

More details from Meteer’s presentation can be found in the conference proceedings available from the Iowa State Extension Store.

Sterry is the St. Croix County Extension agriculture agent.

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