As we move into late summer, spring-calving cows are bred, hopefully, haying is complete, and our mind starts thinking about weaning and fall grazing of the cows. A non-lactating cow in mid-gestation has the lowest nutrient requirements at any time during the production cycle (Figure 1). This allows the cow to regain body condition after lactation if needed or maintain body condition with low-quality forages and feedstuffs. As the cow moves into late gestion, fetal growth increases and nutrient requirements increase. Thus, mid-gestation is the best time for the cow to regain condition and can be the most economically viable time if inexpensive forages and feedstuffs are available.
Fall forages options vary considerably across the U.S. and many are cost effective for feeding dry cows in mid-gestation. Tallgrass prairie can be 40 to 50% TDN in the fall, but with protein supplementation digestibility is easily increased 5 to 10 percentage units. Mixed-grass prairie is typically 45 to 50% TDN in the fall but can occasionally be below 45% TDN. Stock-piled bermudagrass is usually 45 to 55% TDN between November and February. Cool-season forages such as smooth bromegrass and tall fescue are usually 55+% TDN due to regrowth with cooler temperatures.
Additionally, crop residues are viable options to meet nutrient requirements of mid-gestation beef cows. Corn stalks can be 50 to 55% TDN, but decline rapidly with wet fall weather, and grain sorghum stalks and wheat straw are 40 to 45% TDN. Soybean stubble has a high concentration of lignin and thus low TDN (35-40%), and typically will not meet the TDN requirements of mid-gestation cows – supplemental energy would be required.
Most of these fall forages are low in crude protein, less than 6%, except for the cool season forages. This level of protein will not meet the requirements of the cow and will not allow maximum digestion of the low-quality forage, and may hinder the development of the fetus.
As the fetus develops, body tissues develop at different rates and stages of gestation (Figure 2). During early gestation, nutrient use is primarily for development of internal organs, but during mid-gestation muscle and fat tissue develop. Thus, maternal nutrition can affect the development of fetal tissues through supply of nutrients to the fetus.
Research evaluating the effect of maternal nutrition during mid-gestation on fetal development indicates that protein deficiency of the dam can have negative consequences on development of muscle tissue and intramuscular fat. These changes affect the offspring later in life resulting in lesser growth pre- and post-weaning, lesser ribeye area and marbling at slaughter, and higher yield grades in carcasses. Protein appears to be the most important maternal nutrient affecting fetal development, although energy restriction and mineral deficiency have also affected fetal development. The fetus uses amino acids for both energy, along with glucose, and building of new tissue, and thus has a high protein requirement relative to body weight.
Does this mean that low-quality forages should not be used for mid-gestation cows? The answer depends on the ability to supply adequate protein to cows grazing low-quality forages through fall and early winter. For most fall forage options, TDN is adequate for cows to meet energy requirements, especially with the increased digestibility that comes with protein supplementation. If protein can be supplemented in such a way for all cows to consume their protein requirement daily or every other day, then the use of these forages is a viable option. This generally means that the protein supplement needs to be hand-fed.
Methionine is one of the key amino acids involved in achieving the changes to DNA that are necessary for optimum fetal development and future offspring performance. Methionine is one of the first limiting amino acids from microbial protein of cattle on forage diets. Thus, ensuring adequate maternal methionine supply is necessary to achieve optimum fetal development. However, supplementation of synthetic methionine is not necessary to meet methionine requirements, but using high protein feedstuffs with greater rumen bypass protein and methionine concentration are beneficial. This includes feedstuffs like dried distillers grains, fishmeal, porcine bloodmeal, and porcine/poultry meat and bone meal, but feedstuffs like soybean meal and cottonseed meal have lower concentrations of methionine and are highly degraded in the rumen. However, high rumen bypass protein might limit forage digestion; thus, a combination of low and high rumen bypass protein feedstuffs would work best in this situation.
In conclusion, dry cows in mid-gestation have low nutrient requirements and many forages can be used as feedstuffs for these cows. Most fall forages will meet the TDN requirements of dry cows in mid-gestation, especially with protein supplementation. Protein, particularly the amino acid methionine, is a critical nutrient for fetal development and programming the offspring for optimal performance. Thus, protein supplements need to be formulated to meet the rumen degradable protein needs of the cow and supply enough rumen bypass methionine to meet the needs of the developing fetus.
Figure 1. Total digestible nutrient (TDN) and crude protein (CP) requirements throughout the beef cow production cycle for 1200-lb cow producing 20 lb peak milk. The period between the red lines is the time where low-quality forages can meet the nutrient requirements of the cow.
Figure 2. Changes in proportion of body tissues with fetal age illustrating the primary use of nutrients at that time.