1
Q
is probably the single most important factor affecting the economics and
profitability of beef cattle breeding operations. For bulls, reproduction is all about the capacity and ability
to sire a large number of viable offspring in each mating year.
A
Reproduction
2
Q
one generation to the next and can greatly influence genetic gainforms the basis of livestock improvement as it allows the transfer of genetic
material from
A
Reproduction
3
Q
Improved management of
reproduction can increase economic returns to cattle producers. In severe environments where nutrition
is a major stress factor, improvements of 5–10% in weaning rates are possible through improving
nutrition and management.
A
Notes
4
Q
/ Measuring reproductive performance
/
Reproductive performance is influenced by a number of independent traits.
A
Measures for the bulls
Measures for the cow
Measures for the breeder herd
5
Q
Measures for the bulls include:
A
• physical and structural soundness
• scrotal size and sperm production capacity
• semen quality, including morphology
• serving ability/serving capacity.
6
Q
Measures for the cow include:
A
• weight and age at first estrous cycle
• the inter-calving interval
• lactation status at subsequent pregnancy diagnosis.
7
Q
Measures for the breeder herd include:
A
• branding and/or weaning rates
• kilograms of calf weaned per 100 kg of cow mated.
• conception rates determined by pregnancy diagnosis (PD)
8
Q
Limitations to the above measures include:
A
- fertility cannot be assessed directly for bulls.
- pregnancy status for cows requires skills in pregnancy diagnosis, which should be coupled with an
assessment of lactation status. - branding and weaning rates (in isolation) do not provide indications of when or where reproductive
losses may be occurring. - unless birth dates are known and a restricted joining period used, a slight ‘creep’ in inter-calving
interval can often be overlooked. - it is important to consider that these rates must be calculated for a standard or specific period of time
(e.g., 12 months) to allow accurate analysis and comparison.
- This is difficult in a herd where controlled mating is not carried out, as distinctions between
calving periods become blurred.
9
Q
Calculating a number of reproductive rates can
A
help to identify areas of loss.
10
Q
A combination of
pregnancy diagnosis, lactation status, branding rates and weaning rates will help to determine stages and
possible causes of low herd fertility and assist in the following:
A
- identifying the importance of age, weight, body condition and lactation status impacts on herd fertility.
- identifying how management, nutrition and breeding practices can be modified to optimize fertility.
- making an assessment of whether disease status of both cows and bulls may be interfering with herd
fertility.
11
Q
Bull costs per calf weaned will depend on:
A
1.) purchase price
5.) number of cows per bull
2.) bull salvage value at ultimate sale
6.) weaning percentage achieved;
3.) number of breeding seasons that involve the bull
7.) bull mortality rates;
4.) whether bulls are checked annually for soundness and fertility.
12
Q
can be used to distinguish infertile or sub-fertile
bulls from those that meet satisfactory fertility standards.
A
bull breeding soundness evaluation (BBSE)
13
Q
Key components of bull fertility:
A
- Bull breeding soundness evaluation (BBSE)
- Physical (structural) soundness
- Sheath, prepuce and penis.
- Semen and spermatozoa.
- Libido and serving ability.
14
Q
involves assessing four key components—(i) physical
(structural) soundness, (ii) scrotal size (functional testicles), (iii) semen assessment (including
morphology where appropriate) and (iv) serving ability—to provide an indication of likely fertility.
A
Bull breeding soundness evaluation (BBSE)
15
Q
Bull breeding soundness evaluation (BBSE) involves assessing four key components:
A
i) physical
(structural) soundness,
(ii) scrotal size (functional testicles), (iii) semen assessment (including
morphology where appropriate) and (iv) serving ability—to provide an indication of likely fertility
16
Q
Assessing a bull’s structural soundness should include a systematic
examination of eyes and surrounding areas, jaws, mouth and teeth, limb structure, joints, stance, gait (at
trot), hoof and claw structure. Common problems (Figure 3.1) include post legs (excessively straight),
sickle hocks (standing under), swollen and puffy hocks, bow legs and cow hocks.
A
Physical (structural) soundness
17
Q
structures are moderately
heritable. Likewise, any structural defects are passed
on to offspring.
A
Leg and foot
18
Q
The penis is housed
within the
A
Prepuce and sheath
19
Q
is the inner
lining of the sheath and is the pink mucosa, seen at the
end of the sheath. Figure 3.2 shows some structural
problems that can occur.
A
Prepuce
20
Q
Scrotal size (Table 3.2) is a useful criterion for assessing bull fertility for the
following reasons:
A
1.) It is easily measured and is repeatable, and can take place at a younger age than most other male traits;
2.) It is related to a bull’s own fertility, and is a highly heritable trait that influences the scrotal size of
male offspring.
3.) Finally, it is genetically correlated—scrotal size in a sire is positively related to scrotal size of his
male progeny, and also earlier puberty and enhanced fertility of his female progeny.
21
Q
The major factors influencing scrotal size include:
A
(i) genotype, (ii) age, (iii) liveweight, (iv) nutrition,
and (v) timing of nutritional stress
22
Q
Common conditions that interfere with fertility and can be diagnosed by palpation of testicles include:
A
- unequal size of testicles
- hardness of one or both testicles
- thickened scrotal skin.
- softness and flabbiness of testicles and tail of epididymis
23
Q
sample is often
collected as part of a routine BBSE conducted by a
veterinarian and examined crush-side under a
microscope to assess factors such as volume, color,
density and motility.
A
Semen
24
Q
is the sexual
desire of a male to serve a receptive female. Number of mounts, mounts plus serves and libido score of
bulls that had passed a reproductive examination were found to be positively related to calf output in
multiple sire mating.
A
Libido
25
The incorporation of serving-capacity testing as part of the breeding soundness examination
enables the farmers to first identify bulls that are able to serve while reducing the risk of bull failure in
single sire mating.
Notes
26
provides an indication of the bull’s ability to serve.
serving-capacity test
27
is a
measure of the sex drive (libido), or eagerness of the male to seek out a female on heat, and provides
information about one of the many traits that influence calf output.
Serving capacity
28
he presence of pheromones in
the females or estrus leads to the
flehmen response
29
is defined as the time when a bull is capable of producing 50 million live sperm
per milliliters.
Puberty
30
The testicles usually grow steadily between about
7-10 months of age
31
The testicles grow more rapidly after
7-10 months
32
The testicles grow more rapidly after 7–10 months of age until
18-24 months of age
33
Puberty in bulls is usually reached by between
14-18 months
34
Factors influencing when bulls reach puberty include:
1.) genetic effects (later in Bos indicus)
2.) nutritional influences (poor nutrition delays the onset of puberty, conversely over-fatness may also
influence pubertal development).
35
may be transmitted through sexual intercourse to females, affecting female
fertility, or may indirectly affect male fertility.
Diseases
36
regime that bulls are subject to throughout their life affects their fertility, and
also their ability and desire to serve females. Bulls fed on grain diets or those having surplus protein and
energy often have fat deposits in the neck of the scrotum.
Nutrition
37
can increase bull cost per calf, as it decreases the
number of calves produced over the lifetime of the bull.
Bull wastage
38
Such wastage can be due to a large number of factors, including:
1. structural defects resulting in lameness and sometimes inability to serve
2. degenerative conditions (such as arthritis) limiting serving ability
3. age structure in the bull team, with increased proportion of physically unsound older bulls
4. reproductive abnormalities, particularly in the testes, penis and prepuce
5. infertility due to testicular degeneration, the incidence of which increases with age
6. infertility due to disease effects
7. traumatic injuries due to fighting
39
It is therefore important to keep the age of the bull team as young as possible, not only to gain
benefits from genetically superior sires, but also to ensure the highest level of fertility in the team by
eliminating older and unsound bulls.
Notes
40
A veterinarian should carry out a BBSE to examine bulls for soundness— prior to sale or
purchase, and annually for existing bulls. Such an examination involves:
1. checking that the bull’s overall structure including legs, feet and external reproductive organs are
free from defects
2. assessing the sheath, scrotum and testicles
3. measuring and recording scrotal circumference to ensure it is within the acceptable limits
4. palpating of the testicles to check for normal tone, size and function.
5. collecting a semen sample and a microscopic examination to assess the quality and percentage of
normal sperm.
41
Reproduction in the female is hormonally driven and influenced by
Genetics
Nutrition
Management factors
42
Unlike the bull, the female is born with her lifetime quota of ‘ova’, which she sheds
during estrus cycles throughout her reproductive life.
Notes
43
Female born with her lifetime quota of ‘ova’, which she sheds
during estrus cycles throughout her reproductive life. The female has limited opportunities to pass on her
genetics to the progeny, with a maximum of
8–12 calves per breeder.
44
Reproductive events in the female are marked by:
1. expression of estrus (heat) in the presence of a fertile male
2. ovulation and fertilization of the ovum (egg)
3. implantation, growth and survival of the embryo and fetus
4. maintenance of pregnancy
5. birth and survival of a viable calf
6. adequate lactation for calf survival and good calf growth through to weaning
7. early return to reproductive activity after calving to achieve the next pregnancy.
45
is associated with increased longevity,
decreased breakdown, and the production of turnoff cattle and replacements that are less likely to break
down.
Structural soundness.
46
Successful reproduction involves
cycling (regular heat periods), mating, conception (fertilization
of the ovum), gestation (pregnancy), parturition (birth) and lactation (suckling the calf).
47
s defined as the age (or weight) at first estrus when ovulation
also occurs. This is the age/weight when reproduction can occur—in other words, the onset of sexual
maturity.
Puberty in female
48
Signs of estrus include:
1. behavioral changes (i.e., chin resting, mounting and being mounted by other animals, congregation
into groups of estrus cows, restlessness)
2. swelling and reddening of the vulva
3. vaginal mucus discharge adhering to tail and legs
4. abrasion of skin and rub-marks on tail heads and pin bones
5. ruffling of tail hair
6. increased tone in the uterus and cervix on palpation.
49
The average duration of estrus is 6–18 hours; being slightly shorter for heifers. The length of
estrus is affected by:
• breed (shorter in Bos indicus)
• nutrition (higher incidence of short cycles in poorly fed cows which
eventually become anestrus.
• temperature stress
• age (shorter and less variable in heifers)
• transport stress (increase in number of short cycles)
• ovarian abnormalities (either increased or decreased cycle length)
• uterine infections (increased cycle length).
50
n the progressive stages of pregnancy, the embryo leaves the fallopian tubes (oviduct) and
enters the uterus at about day six or seven, attaching to the uterine wall a few days later.
Pregnancy
51
examination of the reproductive tract from about 21 days after mating will detect
the presence of fluid in the uterus. Over the next 10–15 days fluid increases and the developing fetus
can be identified.
Ultrasound
52
Pregnancy testing may be carried out for a number of other reasons, including:
• identification of animals that are not pregnant at a certain time for culling and disposal
• estimating stage of pregnancy for purposes of breeder herd segregation or selling of stock
• assessing nutritional requirements of animals in relation to the season
• estimating future management requirements, stock numbers and times for different classes of stock.
53
The options for retaining or culling will range from:
• retaining all pregnant females
• retaining only lactating pregnant females
• retaining only those pregnant or lactating females that are expected to wean a calf within a defined period
54
Depending on the location and climatic variation, the percentage of females categorized as
being
Wet and pregnant
55
About 35–45 days after calving the reproductive tract and ovaries of a cow should return
to normal and reproductive cycles recommence.
Post calving
56
In some breeds it is even longer. The condition is known as
post-partum anestrus (PPA)
57
is the most important non-genetic
factor influencing conception in beef cows.
Nutrition
58
Options to achieve this include:
1) supplementation; 2) grazing management and adjustment of
stocking rates; 3) paddock rotation; and 4) weaning to reduce nutritional demand of the breeder.
59
Bone structure of
shoulder, ribs, back, hooks and pins
sharp to touch and easily visible. Little
evidence of fat deposits or muscling.
Emaciated
60
Beginning of fat cover over the
loin, back and fore ribs. Backbone
still highly visible. Processes of the
spine can be identified individually by
touch and may still be visible. Spaces
between the processes are less
pronounced.
Thin
61
2th and 13th ribs are
not visible to the eye unless animal
has been shrunk. The transverse
spinous processes can only be felt
with firm pressure to feel rounded –
not noticeable to the eye. Spaces
between the processes not visible and
only distinguishable with firm pressure.
Areas on each side of the tailhead are
Fairly well filled but not mounded
Moderate
62
Cow appears fleshy and
obviously carries considerable fat. Very
spongy fat cover over ribs and round
tailhead. 'Rounds' or 'pones' becoming
obvious. Some fat around vulva and in
pelvis.
Good
63
Cow has lost definition. Contours
disappear across back and sides as
cow takes on a smooth, block-like
appearance. Tailhead and hips buried
in fatty tissue and look blocky; 'rounds'
or 'pones' of fat are protruding. Bone
structure no longer visible and barely
palpable. Large fatty deposits may even
impair animal's mobility.
Fat
64
There are two practical techniques
available.
One is (i) to reduce the nutritional
trough by the use of supplements.
(ii) weaning
65
Protein
supplements, and especially non-protein nitrogen
(NPN) supplements such as urea-based mixes, are
the most economical means available.
to reduce the nutritional
trough by the use of supplements.
66
The second form of ‘supplementation’ is
Weaning
67
Is the most
practical form of supplementation of the breeder
cow.
Weaning
68
involves limiting the mating period by removing bulls from the
breeding herd. It is often used in more favored areas with more reliable seasonal conditions and 100%
bull control.
Seasonal or controlled mating
69
If it can be incorporated into a breeder management system it allows:
• timing of calving and weaning to seasons where conditions are most likely to be favorable.
• more even lines of weaners to be produced.
• easier management, including better scheduling of mustering times
• pregnancy testing to be carried out at a time which affords maximum benefit in terms of accuracy.
• comparison of growth rates of weaners without the need to allow for out-of-season calves.
70
Controlled mating is really the ultimate goal in breeder cow management, if it can be achieved
economically. The following hints may help.
1.) Removal of bulls on a particular day is not essential IF pregnancy testing with fetal aging is performed
at the appropriate time of the year.
This will provide a safety net in case of bull failure, and the producer doesn’t have the worry of bull
security for such a long period of the year.
2.) A good heifer management program to ensure heifers are aligned with the main breeder herd right
from the start will help ensure a compact calving.
3.) Implement a good bull testing (BBSE) program.
4.) Gradually change from a continuous calving pattern to a compact joining period over a number of
years.
5.) Early weaning and breeder supplementation may be required in drought years
71
that calves late initially will always calve late or will miss out on getting back in calf
Heifer
72
Heifers that are to calve at 2 years of age (yearling mating) should be:
(1.) mated to calve 2–3 weeks ahead of the mature cows. Heifers with a calf at foot are always the hardest
to get back in calf and mating a little earlier as maidens gives them an extra cycle in which to get pregnant
again.
(2.) mated for only 6–9 weeks. If they are above their critical mating weight, in good condition, on
reasonable feed and with bulls of satisfactory mating potential, then over 80 per cent should conceive
during that period.
(3.) pregnancy-tested 12 weeks after the start of mating. If replacement numbers are sufficient, then cull
those that do not become pregnant in the first six weeks.
(4.) fed for good growth through to calving, but watch that they do not become too fat—Condition Score
3 (forward score) is ideal.
(5.) Calving problems are always worse in maiden heifers—they can be minimized by choosing low birth
weight bulls, taking pelvic measurements of the heifers and monitoring the plane of nutrition during
pregnancy.
(6.) ‘Spike-feeding’ maiden heifers with additional protein in the later stages of their pregnancy will
improve conception rates after they calve.
(7.) Yearlings’ mating is not always practical or economical where pasture conditions are not good
enough to attain critical mating weights by 15 months of age.
73
First calvers and mature breeders. Once the calving pattern has been set, there are techniques available
to help maintain it. Some of these include the following:
(1) first-calf heifers are the most difficult breeders to get back in calf as they are still growing as well as
producing milk etc.
(2) aim to have cows calve out in Condition Score 3 or above.
In some years early weaning can be used to help maintain the condition on the back of a pregnant cow.
(1) if the spring is dry, then a protein supplement can be fed to help improve conception rates
(2) have in place an adequate vibriosis/leptospirosis program
(3) ensure adequate ‘bull power’.
74
The advantages of this strategy are:
(1) Easier management, since segregated mobs have similar age groups of calves, making mustering
easier, with less likelihood of mismothering during mustering of big paddocks.
(2) Enhancing overall herd fertility by concentrating on culling of nonproductive groups rather than just
older groups.
(3) Decreasing calf losses and breeder mortality rates through better nutrition of the most productive
cows
(4) Better use of pasture resources, by concentrating more productive groups on better country
(5) Targeting supplements to herds that have the most need and where responses are likely to be best,
e.g., first-calf cows
(6) Segregation and seasonal mating of heifers, enabling some concentration of calving patterns
throughout life, regardless of the type of environment.
75
Possible disadvantages of this strategy are:
(1) Logistical difficulties in application
(2) Lack of appreciation of the aims of the strategy
(3) The need for pregnancy diagnosis as a tool for segregation
(4) Additional initial work in putting the practice into place --- this is a useful strategy to enhance fertility
and reduce breeder mortalities.
76
Investing through time and money in the genetics of the herd is worthwhile because genetic
improvement is:
1. Cheap
2. Permanent
3. Cumulative
77
because it involves buying a sire that will add genetic improvement and the only extra is the
marginal cost of the superior sire over the average
Cheap
78
passed on from one generation to the next
Permanent
79
the improvement made this year will be in addition to the improvements made last year.
Cumulative
80
When obtaining a bull, the buyer is acquiring a
Package of genes
81
Genes occur in pairs, one obtained from each parent.
Dominance
82
In an animal breeding context, variation usually refers to the difference among individuals
within a population.
Variation
83
is defined as the proportion of superiority in a trait that is transmitted to
the offspring.
Heritability
84
are the two characteristics or traits that influence the potential for
genetic improvement. Genetic improvement will be most rapid for a trait with high heritability and high
variation.
Heritability and variation
85
This is the difference between the mean of those selected to be parents and
the mean of all potential parents. The smaller the number of animals selected, the higher the selection
differential and the greater the difference between average of the animals selected and the average of the
entire group. selection differential
86
This is the average age of the parents when their offspring are born or in simple
terms, the time interval between the same stage in the life cycle of two successive generations.
Generation interval
87
This process has limited use in achieving improvement in most production traits and it
has minimal value in selecting for carcass traits. I
Visual selection
88
These are measures of genetic differences between animals,
expressed as the unit for each trait in positive or negative terms.
Estimated Breeding values
89
These are measures of genetic differences between animals,
expressed as the unit for each trait in positive or negative terms. Dedicated software called
best linear
unbiased prediction (BLUP
90
uses computer technology to find the best fit for all the pieces of
information, for all the traits in question and all the animals in the analysis simultaneously.
best linear
unbiased prediction (BLUP)
91
This is a program that provides a genetic description of cattle for a range of traits in the
major areas of growth, carcass performance and fertility. Where common sire linkages occur with other
groups, comparisons can be made across groups.
Breedplan
92
allows for comparison of animals across herds—most reported EBVs are
Group BREEDPLAN
BREEDPLAN
93
In recent years researchers have integrated genetics and economics to
define genetic improvement in economic terms.
INDEX SELECTION
94
These provide a means of assessing the true genetic merit of an animal, by ‘marking’ the
presence of an individual gene. They give the industry the potential to identify and select animals for
specific traits at an early age.
Gene markers
95
play a significant role in the livestock production depending upon the purpose.
Breeding systems
96
there are two
systems of breeding.
Pure breeding and cross breeding
97
is used for registry in the breed associations and these animals will perform better
than non-pure breeds.
Pure breeding
98
is used mostly by many commercial producer and animals also
have good performance because of hybrid vigor.
Cross breeding
99
Systems of Breeding
1. Inbreeding
2. Outbreeding
3. Crossbreeding
4. Grading up
5. Outcrossing
6. Backcrossing
7. Rotational crossing
100
The method of mating between individuals which are more closely related, such as brothers and
sisters and of sires with daughters is called
Inbreeding
101
increases the genetic purity of the stock produced, but reduces performance.
Inbreeding
102
It is a practice of breeding the farm animals which are not closely related within the same breed.
Outbreeding
103
It is the mating of the animals belonging to two different breeds. It is done to take advantage of
good qualities of two or more breeds.
Crossbreeding
104
It is mating of pure-bred males of established breed with nondescript females successively over
several generations to produce a progeny that resembles and performs similar to the pure breed.
Grading up
105
Mating of the unrelated pure breed animals, within the same breed is called out crossing.
Outbreeding
106
The cross bred females obtained by crossing two breeds are mated to males from one of the two
parental breeds.
Backcrossing
107
males from one of the pure breeds are used in alternate generations to breed
the cross bred females. This rotational crossing could be between two breeds is called crisscrossing and
among three breeds is called triple crossing.
Rotational crossing
108
g is the system of breeding mostly used for genetic improvement of cattle whereas,
Grading up is the method used for genetic improvement of the buffalos.
Cross breeding
109
is the increase in production achieved when mating one or more
pure breeds.
Heterosis and hybrid vigor
110
Calves are
identified to
(1) provide positive identity for recording performance and family relationships
and (2)
serve as a means to establishing legal title.
111
A number of methods provide good permanent identification for a record program. These
methods include
(1) ear tattoo, (2) hot iron brand, (3) freeze brands, (4) chemical brands and (5) ear
notching.
112
may also be used for easy
identification.
s chains, nylon cords or ear tags,
113
Steer calves are preferred over bull calves in the feeder market. Bull calves should be
castrated if not intended to be retained for breeding purposes. Castration can best be done when the calf
is quite young. Older calves are more difficult to restrain and suffer a greater set back. Many cattlemen
castrate newborn calves at the same time they tattoo them for permanent identification.
Castration
114
is the most positive method of castration and is preferred by many stockmen.
Surgical castration
115
is a bloodless method of castration
Emasculating, or clamping
116
is another bloodless method
of castration.
Elastration
117
Cattle without horns attract some preference over horned cattle in the market
Dehorning
118
Crossing cows with a bull that is homozygous for
the polled trait is an often-over-looked solution to
dehorning
Genetic
119
A caustic paste or stick can be used
on very young calves (up to two or three weeks of age)
where only a button can be felt.
Chemical Dehorning
120
Horn buttons or small horns
just emerging can be readily removed with spoon or tube
dehorners.
Spoon or Tube Dehorners
121
may be used to dehorn calves with
buttons or small horns.
Hot irons
122
can be used on horns too large for tube or hot irons
but small enough for the instrument to fit properly and permit cutting a ring of skin and hair. Barnes type
dehorners are available in calf and yearling sizes.
Barnes type dehorners
123
Cattle with horns too large to remove with the above methods can be dehorned
with a saw, clippers or obstetrical wire.
Removing Large Horns –
124
is the practice of providing supplemental feed, usually grain, to nursing calves in
a facility that prohibits the brood cow from having access to the feed.
Creep feeding
125
Creep feeding may be worthwhile in the following situations:
1. periods of drought
2. poor milking cows
3. calves from first-calf heifers and old cows
4. cows on poor pasture or toxic fescue
5. just before weaning to teach calves to eat
6. calves being fed for slaughter at weaning.
126
Situations under which creep feeding is probably not profitable are:
1. cows with good milking ability
2. pastures high in quality and abundant, such as clover-grasses
3. calves to be grazed or backgrounded at relatively low rates of gain after weaning
4. heifer calves being raised for replacements
127
is a distress period for both calf and cow.
Weaning
128
is a weaning process in which the calves are removed from
their dams but are allowed to see, hear and smell their dams.
Fence line Weaning.
129
A growth implant is implanted into the
ear. Research data strongly indicate a positive and economical response
to implants in all segments of the beef industry.
Growth-Stimulating Implants
130
can be generally described as a component of the diet that does not fall
within the basic nutrient
Feed additives
131
A feed additive can be generally described as a component of the diet that does not fall
within the basic nutrient categories:
protein, fat, carbohydrates, mineral or vitamin.
132
Feed additives may include
antibiotics, anthelmintics, surfactants, estrus suppressors,
coccidiostats, yeast cultures and direct fed microbials
133
feed additives are often used on cow-calf operations.
Antibiotics
134
is approved for treatment of bovine respiratory disease complex and
prevention of anaplasmosis.
Chlortetracycline
135
Monensin and lasalocid are special types of medicated feed additives called
ionophores
136
delivered through feed have been around for years.
Anthelmintics
137
when added to feedlot heifer rations stops the normal hormone production
sequence that produces estrus. Because feeding the exact recommended level of MGA prevents estrus
but allows the optimum level of estrogen production, feedlot heifers gain at an increased rate.
Melengestrol acetate
138
are relatively new to feedlot cattle. These are utilized toward the final days on feed and
increase lean deposition and weight gain. Direct-fed microbials, yeast cultures and yeast cell wall are
specialized feed additives.
Beta-agonists
139
are being studied to improve health such as sustaining a higher ruminal pH under
concentrate feeding to prevent sub-acute acidosis, yeast cultures are being studied as specialized nutrients
for the rumen microbes, and yeast cell wall is being studied for binding properties to toxins.
Direct-fed microbials
