Animal manures are used to enrich soil and encourage strong crop and flower growth. The safest animal manure comes from herbivores like cows, horses and rabbits. Manure from cats, dogs and other carnivorous animals poses the danger of pathogens that can be transmitted to humans. But even the beneficial manures are extremely strong when fresh and could damage seedlings or create too much microbial activity to grow healthy plants. Let manure age before using it in the garden.
Garden Soil Food - Cow manure contains the three main elements of garden soil fertilizer: nitrogen, potassium and phosphorus. But it may contain other things, too. Undigested seeds, bits of soiled barn bedding and even some pathogens like salmonella. Allowing the manure to age lowers the risk of planting weeds or transmitting diseases through garden crops or contact with the soil.
Manure Strength - Manure has abundant amounts of nitrogen, which improves the soil and boost plant foliage production. Nitrogen stimulates good bacteria in the soil and speeds up the decomposition of organic materials. But unaged manure is powerful stuff and can cause so much microbial activity that healthy nutrients heat up too much to be useful to nurture plants. Tender plants are susceptible to too much nitrogen in the soil and can be damaged by it. Plants that absorb too much nitrogen are attractive to pests who will make a meal of them.
Aged Manure - Let cow manure mellow for a while before using it. One method is to spread fresh manure in the fall after plants are harvested and work into the soil the following spring before planting. Another is to compost the manure in a pile that heats up enough to kill any pathogens or viable undigested seeds. It's safe to wait six months before adding cow manure to garden soil. Even after waiting six months, don't plant in the enriched soil for one month after working in the manure. That way you lower the chance that the manure will burn developing roots or heat up the soil too much to prevent seed germination.
Alternatives - Use aged cow manure to enrich established garden plots by lightly digging the fertilizer in to the side of plants. Irrigation or rainfall will cause the nutrients to seep into the soil around the plant, feeding the roots. Or make compost tea. Soak a muslin or cheesecloth bag of composted or aged cow manure in non-chlorinated water and use the liquid to water the plants. Diluted nutrients enter the soil and will not burn roots or young shoots.