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AAES Research Series 488
allowed to clot, and were then centrifuged at 2,000 x g.
Serum was decanted and stored frozen at –20°C until analyzed
for progesterone by radioimmunoassay.
Fertile bulls were placed with cows for 60 days beginning
on May 15. Pregnancy status was checked by ultrasound
at the end of the breeding season and non-pregnant animals
and animals of less than 30 days of pregnancy were checked
again by ultrasound at 30 days after the end of the breeding
season.
Initial analysis of data indicated that the performance of
heifers and their calves differed from that of cows for most
variables measured; therefore, data for heifers and cows were
analyzed separately with treatment as the only effect in the
model. Differences between least squares treatment means
were determined by multiple LSD tests. Percent pregnant and
calving percentage were compared by Chi-Square test.
Results and Discussion
Heifers: Pre- and post-partum data for heifers by treatment
is presented in Table 2. There were no differences (P >
0.10) in days pre-partum at the start of the experiment, initial
body weight, or initial BCS for heifers among treatments.
Body weight and BCS at the start of breeding and the change
in BCS from the start of the experiment to the start of the
breeding season did not differ (P > 0.10) among treatments.
However, the decrease in body weight between the start of the
experiment and the start of the breeding season was greater in
the non-supplemented Control group than in the Corn or
Corn-Soy groups. The reduction in body weight loss in the
Corn and Corn-Soy treatments suggests that heifers benefited
from the additional energy and protein provided by the supplements.
The number of heifers cycling at the start of the
breeding season, pregnancy rates at the end of the breeding
season, pregnancy rates at 30 days after the end of the breeding
season, and calving percentage were low in all treatment
groups, but differences among treatments were not significant
(P > 0.10). Calving interval was also not different (P > 0.10)
among treatments. It appears that the amount of supplement
fed prevented some weight loss in heifers, but was not adequate
for acceptable reproductive performance.
Calf birth weight was not affected (P > 0.10) by treatment;
however, calf weaning weight (P < 0.10) and ADG (P
< 0.08) of calves from birth to weaning was greater in Corn
and Corn-Soy treatments than in Controls. It is likely that the
additional energy in the Corn treatment and the energy and
protein in the Corn-Soy treatment increased milk production
in heifers and therefore increased gains in calves in the supplemented
groups.
Cows: Pre- and post-partum data for cows by treatment
is presented in Table 3. There were no differences (P > 0.10)
in days pre-partum at the start of the experiment, initial body
weight, or initial BCS for cows among treatments. Body
weight at the start of the breeding season and change in body
weight between the start of the experiment and the start of the
breeding season were not different (P > 0.10) among treatments.
However, BCS at the start of the breeding season was
higher (P < 0.05) in the Corn treatment compared to the
Control or Corn-Soy treatments. Similarly, the change in
BCS between the start of the experiment and the start of the
breeding season was less (P < 0.05) for the Corn treatment,
than for the Control and Corn-Soy treatments.
Similar to results in heifers, the number of cows
cycling at the start of the breeding season, pregnancy rates at
the end of the breeding season and at 30 days after the end of
the breeding season, and calving percentage were low in all
treatment groups, but were not different (P > 0.10) among
treatments. Mean calving interval in days tended to be shorter
in the Corn-Soy treatments compared to non-supplemented
Controls (P = 0.10) Average calf birth weight, calf weaning
and ADG from birth to weaning did not differ (P > 0.10)
among treatments.
Forage intake was not monitored in this trial. However,
the poor reproductive performance and low calf weaning
weights in all treatment groups indicate that intake of forage
may have been limited to such an extent that heifers and cows
were not able to meet their nutritional requirements, even
with the Corn or Corn-Soy supplements. Pregnancy rates in
this cow herd in previous years have ranged from 75 to 90%.
In previous years, forage was not analyzed, and cows were
fed 4 lb per day of a supplement similar to the Corn - Soy
treatment in this study. Crude protein and TDN supplied by
the supplement in each treatment plus estimated forage intake
and contribution of forage to CP and TDN is presented in
Table 4 for heifers and Table 5 for cows, along with the NRC
(1996) requirements. Forage intake for both cows and heifers
was estimated by dividing 120 by the NDF percentage in the
forage and expressing the result as a percent of body weight
(1.6%). This estimate of intake was well below the NRC
(1996) guide for both cows and heifers. Based on the analysis
of hay (Table 1) used in this trial, cows without supplement
(Control) should have been able to meet their CP and
TDN requirements by consuming forage alone. The degree of
weight loss observed in all treatment groups and the poor
post-partum reproductive performance suggest that intake
was well below NRC (1996) estimates and that nutrient
requirements were not supplied in the available diet. Intake
may have been limited by unknown factors. The forage may
have contained fescue toxins which can limit intake; however,
problems associated with fescue toxicosis have not been
previously observed on this farm.
Implications
The poor reproductive performance of cows and heifers
and poor post-partum performance of calves observed in all
treatments in this study suggest that intake of forage can be a
limiting factor in the ability of animals to meet their nutritional
requirements when consuming relatively high quality
harvested forage.
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