Recovery times tended to be longer for R. crispus compared to other weed species, particularly when fed to growing heifers .All main effects and their interactions were significant for the seed viability and recovered viable seed measured over the four consecutive dates . The highest value of total recovered viable seed was observed with S. halepense when fed to lactating cows but was not significantly different from P. aviculare in this group and growing heifers. It was minimal in R. crispus when fed to dry cows . For all weed species , recovered viable seeds showed a consistent decreasing trend with feedlot lactating cows > growing heifers > feedlot male calves > dry cows. Averaged over the four cattle groups, the most persistent seeds were P. aviculare with 52% viability of fed seeds, the least persistent fed seeds were those of R. crispus with 32% viability . The power model provided adequate fits to viability data over time . Seed viability declined with time after seed intake, however, the rate and magnitude of reductions varied across weed species and cattle groups . There were few changes in seed viability during the initial times for growing heifers and dry cows, whilst viability loss was more rapid in other cattle groups . We also estimated the half-life of seeds fed to different cattle groups using Eq 5. The half-time, tV50, varied from 62 h for R. crispus in lactating cows and feedlot male calves, vertical grow rack to 82 h for P. aviculare in dry cows . All weed species had a longer half-life when fed to dry cows and growing heifers, as shown by larger tV50 values .
Seed mortality was fastest in feedlot male calves, where seeds had a half-life of 64 h. Although this parameter indicates higher seed mortality in feedlot male calves than growing heifers, but the total recovered viable seed was greater in growing heifers . The relationship between seed recovery and viability was curvilinear and showed a predictable pattern over time. Recovery was highly variable for the first two sampling days, ranging from 2% to 50%, and these seeds had viabilities as high as 70% to 100%. Conversely, for sampling 3 and 4 days after intake, the daily seed recovery was less variable but did not exceed 20%, while seed viability was highly variable and ranged from 0% to 70%. By four days after seed intake very few seeds were recovered and the majority were dead . A significant positive correlation was found between seed viability and ruminal pH with r = 0.86 . The pH was higher in dry cows and growing heifers than in feedlot male calves and lactating cows, and was also associated with higher seed viability.This study showed that seed recovery and viability, as well as passage time through the digestive tract, can differ markedly between cattle types of the same livestock species. However, not all weed species showed a similar response. The passage time and ruminal retention time of feed are determined principally by the frequency and amount of feed consumed, forage physical form, concentrate/forage ratio and forage fiber content. Bodmer & Ward found a positive linear relationship between seed survival and animal body size, however, our higher seed recovery observed with growing heifers and feedlot male calves with small body sizes, rather than with dry cows with large body sizes. It seems that the amount of feed intake is more important than the body size.
The amount of feed intake for lactating cows was approximately twice as much as that of the other cattle groups . Increasing the amount of feed reduced the retention time while accelerating the flow through the reticulo-rumen, which in turn resulted in high seed output rate as observed in lactating cows . Furthermore, diets with a high digestibility, i.e. with higher concentrate/forage ratios and lower levels of neutral detergent fiber , and acid detergent fiber pass more quickly through the digestive tract of ruminants. For example, in lactating cows, such higher digestibility can result in higher seed recoveries than in other cattle groups . In our study, although concentrate/forage ratio was much lower in growing heifers, seed recovery in this group was higher than in feedlot male calves . Only 10% of ingested seed was recovered with low-digestibility diets compared to 28% with high-digestibility diets in sheep. Wheat straw, which constituted 34% of the diet in dry cows , can encourage chewing and increase ruminal retention time due to its high fiber content. The recovery of seeds also varied among weed species, which can be attributed to their differences in physical characteristics . Gardener et al. found a strong positive correlation between the specific gravity of seed and the rate of passage through the digestive tract of cattle, however, seed size was only weakly positively correlated with the passage time. We found lower seed recovery in R. crispus than S. halepense despite the two species having seeds of the same size . However, this difference in recovery can be explained by the differences in specific gravity between the two seed types, in that S. halepense seeds have a higher specific gravity than those of R. crispus and thus were recovered in higher numbers. Small seeds are expected to have a pattern similar of rate passage to that of the liquid fraction in a fermentational bag, whereas large seeds are expected to have the pattern similar to that of particulate matter. Thus, specific gravity has a greater influence on the rate of passage of small particles than that of the sieving effects of the mass of reticulo-rumen. In all cattle groups, the highest seed recovery occurred two days after seed intake. Gökbulak also reported a similar peak time in Holstein heifers for seed recovery from three perennial species and two forbs species. The length of time for 50% recovery of tropical pasture seeds after intake average over the ruminants has been measured to be about 51–71 h and in cattle it was 34–51 h. The recovery rate for undamaged seed depends on the chewing style, which varies between ruminants, with sheep and goats causing more damage to seeds than cattle. These results demonstrate that a wide range of seed excretion rates is likely to happen because of differences in animal diet and seed characteristics. Several studies have demonstrated that the viability of excreted seeds declines with the length of time seeds spend in the digestive tract. The seed coat and degree of seed hardness and dormancy are important factors in determining the viability of seeds passing through the digestive tract. Initial seed germination was lower in the three species with higher viability than in R. crispus, which had an initial germination as high as 87%. Impermeable seed coat of C. campestris prevents germination leading to physical dormancy in this species, which may help it to survive the passage. These results suggest that seeds with higher dormancy could probably be more resistant to digestion. However, commercial vertical hydroponic systems to test this hypothesis one needs to use seeds that vary in the degree of dormancy only but no other traits e.g. seeds from the same species but with different dormancy levels.Feedlot male calves and lactating cows caused higher seed mortality than dry cows and growing heifers , whereas total recovered viable seed was highest in lactating cows followed by growing heifers . Ruminal pH varied from 6.2 for feedlot male calves to 7.4 for dry cows at 0700 h before the morning meal . A high feed intake as ad libitum, especially with a high level of concentrate, can cause fluctuation in ruminal pH, ammonia and volatile fatty acids concentrates. Furthermore, a high proportion of wheat straw in dry cow diets can increase total chewing time, which in turn can lead to an increase in buffering conditions in the rumen. The level of NDF has a positive effect on increasing chewing activity and rumen buffering. These factors might have led to more seed loss observed in feedlot male calves and lactating cows over the third and fourth days after the seed intake.
It seems that the timing of seed excretion is the preliminary factor affecting the seed survival whilst other factors such as pH and NDF became important once seeds persist in the digestive tract for a longer period. For example, lactating cows and growing heifers excreted a high amount of seeds within the first two days after feeding: this rapid excretion rate resulted in high total survival rate . As lactating cows exhibited the highest recovered viable seeds , this group of Holstein cattle is more likely to infest cropland with manure rich in weed seeds than other cattle types. However, this hypothesis is based on the assumptions that weed seeds are distributed uniformly across all the feed types offered to cattle. Common practice in formulating cattle diets is based solely on the nutrient requirements of the herd and on production goals. However, if the manure of the cattle is to be used on farmlands, the risks associated with the spread of weed seeds from that manure also need to be considered.Horseweed and hairy fleabane are summer annuals belonging to the Asteraceae family. The temperature and light requirements for germination, soil type preference, and depth of soil emergence of these two species are fairly similar. The optimal temperature for germination of both species ranges from 65ºF to 75ºF, and they can germinate under moderate water stress. However, hairy fleabane can germinate at lower temperatures than horse weed . Some germination of hairy fleabane has been recorded at temperatures as low as 39.5ºF . Although both these species can germinate under complete darkness, greater germination occurs under light . Nanudula et al. found greatest germination of horseweed under a 13-hour day length. Based on these characteristics, conditions are ideal for horseweed and hairy fleabane germination in mid to late fall and late winter in the Central Valley. Therefore, we often see seedlings emerge multiple times during the year in the Central Valley . Under some conditions, horseweed seeds can germinate year-round .Seedlings that germinate and emerge in late fall can overwinter as a rosette and then bolt in spring. Emergence of these two species is greater and more rapid in coarser than in finer soils , and germination is greater in soils with a neutral to alkaline pH range than in acidic soils . However, horseweed has been observed in great numbers in the acidic to neutral soils of the southeast Central Valley. Seedling emergence is primarily from the soil surface, and no seedlings emerge from depths greater than 0.2 inch . The seeds and seedlings of these two species look very similar and are very difficult to distinguish. The characteristic features, at the seedling stage, that may help in distinguishing the two are that the leaves of horseweed are dull green, oval, and have fine, short hairs, whereas the leaves of hairy fleabane are gray-green, narrower, and more crinkled . Seedlings of these two species can also be confused with common winter annual weeds such as common chickweed and shepherd’s-purse . Horseweed and hairy fleabane plants are difficult to distinguish from each other until about the 12-leaf stage. Once the plants bolt it is easy to differentiate the two species. Horseweed bolts, sending up a single, or primary, vegetative stem that is erect with dark green leaves that are up to 4 inches long and are crowded together with an alternate arrangement on the stem . The stem is smooth or covered with shaggy hairs. Hairy fleabane, unlikehorseweed, develops multiple lateral branching without a central stem and has leaves that are much narrower with stiff hairs. The distance between the leaves is greater in hairy fleabane than in horseweed . At maturity, horseweed is usually much taller than hairy fleabane. Horseweed can be 10 feet tall, whereas hairy fleabane is usually 1.5 to 3 feet tall . Both species usually start flowering in July and produce small, yellowish flower heads at the terminal end of branched stems . Occasionally, in Tulare County, hairy fleabane has been observed to flower as early as mid-March. However, the factors responsible for this earliness are not known. Horseweed plants can produce more than 200,000 tiny, narrow tan-colored seeds . In some cases, horseweed seeds have been found up to 300 miles away from where they were produced .