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Parity in Age Adjustment for Milk and Fat Yield
Parity in Age Adjustment for Milk and Fat Yield I. L. MAO1, J. W. WILTON, and E. B. BURNSIDE University of Guelph Guelph, Ontario, Canada Abstract
though effects of year should be small in only 2 yr data. More recent studies from the same institute (3, 4) proposed that age adjustments of production records be within lactation number; however, the necessity is questioned. The purpose of this study was to examine whether estimates for maturity rate should be based on both age and lactation number, at least for early lactations in which the increase in production is major and whether age-month adjustments (5, 6) for production records should he within lactation.
Canadian Record of Performance data on the first two conseeutive lactations of 56,959 Holstein cows from 1959 to 1969 were used to examine the joint effects of parity and age prior to 47 mo on 305day yield. This provided 24,479 records in the overlapping period of ages for first and second lactation cows from 31 to 37 mo of age to examine the need for joint adjustment of age and lactation number or age adjustment within lactation. The model had fixed elements of parity, age, parity by age interaction, and herd-year-season. Interaction between parity and age after absorption of herd-year-seasons was nonsignificant for 305-day milk yield but significant at 5% for 305-day milk fat yield. Least squares means for first lactation milk and fat yield were larger than second lactation yield for calving ages of 31 to 33 and 34 to 35 mo hut lower for 36 to 37
Data
mo.
Introduction
In dairy cattle, milk and fat production in~:reases over the first three or four lactations (1, 2, 5, 6). The increase may result from successive parities promoting udder development and from a cow's physiological development in general. Many researchers studying yieldage relationships have ignored the effect of parity (5, 6). Some workers (I, 2, 7), however, have found that for first lactation cows the yield increased with age at freshening up to about 3 yr of age and then dropped markedly. Gravir and Hickman (1) found the age curves for different lactations did not resemble vne another nor did they have the same general slope as the yield-age curve for all lactations. These curves within lactation-seasons were plotted, however, from production averages of consecutive age classes ignoring herd differences, cow selection, and time trend alReceived April 12, 1973. Present address: Department of Dairy Science, Michigan State University, East Lansing 48823.
A total of 696,682 Holstein, 2X, complete lactation (at least 300 days in length) records were available. The data were from the Canadian Record of Performance (R.O.P.) from 1959 to 1969. The lactation numbers on these records were not reliable, so this method identified first and second lactation records: Step 1. If a cow had a record with calving age equal to or less than 47 mo and had a previous lactation record starting when she was 26 mo or less, the records were labeled first and second lactations. Step 2. If the calving age of the previous record was greater than 26 mo and the herd had been on test for at least 10 mo prior to the date of calving, this ensured that a first lactation was not missed due to the herd not being milk recorded. The previous record was labeled first lactation. The date that the herd began its R.O.P. testing program was indicated by the earliest calving date of any cow in that herd. Step 3. The tentatively labeled first lactation was checked to see ff the cow made a record still earlier. If so, the earliest record would be the first lactation record and checked through steps 1, 2, and 3. This screening procedure produced 56,959 pairs of first and second lactation records. The distribution of these records by age in months and lactation number is in Table 1. Average age at first calving was 29.4 mo, which was also the mode of the frequency distribution. The overlapping period of ages for first and second lactation cows was defined by those age classes having 10 or more records for
100
i01
PARITY IN A G E A D J U S T M E N T
TABLV. 1. Number of records in age by lactation subclasses. Age in months
First lactation
Second lactation
18 19 20 21 22 23 24 25 26 27 28 29 30
21 58 96 121 231 481 1,443 3,509 4,579 5,533 6,099 6,685 6,659
0 0 0 0 0 0 0 o 0 0 4 2 7
31 32 33 34
6,540 ) 5,566 l 16,608 4,502 3,137 )
35
1,401 t
36
253 ~
4,538
\
37
Aj
(PA)ij
j,
18 16 I 47 89 ! 291 i
81
380
790 266
13 i
38 39 40 41 42 43 44 45 46 47
g Pi
0 0 2 1 1 0 0 0 0 0
2,606 1,816 2,616 3,867 4,692 5,587 5,944 6,646 6,635 6,393 6,076 5,423
each lactation. This overlapping period from 31 to 37 mo of age is enclosed between the two solid lines in Table 1 and includes 24,479 records. Model and Procedures
Herd environment, calving season, time trend, cow culling, and some of the interactions between them are some of the key factors affecting the relationship of production with age and parity (1, 5, 6). To take herd, year, season, and their interactions into account when examining the joint effect of age and lactation number in the overlapping period of ages of the first two lactations, - - - this model describes a 2X, 305-day, Holstein production record: Xijkn
where
=
P~ "~ Pi -{-{- e i j k n
Aj + (PA)i j
year-season k at j mo of age, in her ith lactation, is a constant, denotes the ith lactation, a first or second lactation, denotes the jth age group, group one being the calving ages of 31, 32, and 33 too, group two 34 and 35 too, and group three 36 and 37 too, is the effect peculiar to the subclass of lactation i and age
+
HYSk
Xijk n is the nth cow calved in herd-
HYSk is the effect of the kth herdyear-season subclass, composed of herd, year, and season effects, three two-way interaction effects, and a three-way interaction effect, and eijkn represents the random sampiing effect within subclass ijk, with the varianee-eovarianee 2
matrix Iae. All elements in the model were fixed except eijkn. Mao et al. (5) found seasonal variation in the same population which can be described by a continuous curve. They concluded that grouping of months into a few seasons is impractical in age adjustments. However, individual months for the model would result in a large number of herd-year-month subclasses which would leave few degrees of freedom for meaningful tests. Therefore, seasonal groupings were necessary. October through December of one year and January through March of the next year were one year-season class; the following April through September was the next year-season class; etc. There were 16,855 herd-year-season subclasses. Generally, the differences in average yield between first and second lactation cows are due to age, parity, and culling. The effect second lactation being high because of culling was negligible in this study due to the fact that only those cows that had both consecutive lactations were studied. Two approaches were applied to investigate the significance of age by parity interaction effect with herd-year-season effects eliminated. To eliminate herd-year-season effects, PA (age, parity, and age by parity) equations were set up during absorption of HYS (herd-year-season) and HYS by PA equations. Records were sorted into coded HYS and as their were processed, all equations were collected. Since each HYS equation was completed before the next one was started, each was absorbed into JOURNAL OF DAIRY SGIENC~ VOL. 57, NO. 1
102
MAO ~ AL
the PA equations as soon as it was completed, and the same computer storage area was used for the next HYS equation. The resulting PA equations were free from herd-year-season effects. The PA equations were then solved for best linear unbiased estimates for parity-age classes. The parity-age constants were really differences between the first age group in the first lactation and other lactation-age subclasses. The other approach was to test for a null hypothesis of no existence of age by parity interaction. The age by parity mean square (after herd-year-season effects were eliminated) was tested against an estimate of a2~, which was computed by pooling variances within parity-age-herd-year-season subclasses.
215 210 205 200 195 190 185 v
~
180
175 170 165 ION
Results and Discussion
260
Gross averages for milk and fat yield for corresponding age-lactation subclasses in Table i were plotted (Fig. 1 and 2). For first lactation cows, the graph shows the yield increased with age at a decreasing rate up to 34 mo of age, after which it dropped drastically. The most striking feature of the graphs was that the two lactation curves intersected each other in a very distinct manner in the overlapping period of ages. This seems to indicate that
155150
5800 5700
d
--o-- SECONDLACTATION
145 140 135
.............................. 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 AGE IN MONTHS
FIC. 2. Age-class means (unadjusted for herdyear-season effects) of 3OS-day fat yield for the first and second lactation. average production of, say, second lactation cows of a particular age is lower than average production of first lactation cows of the same age. However, these gross means were computed, for each age-parity class, over all other effects, which were conceivably confounded with age-parity effect. As a result, these production curves show a combination of age trend and those confounded effects, namely, herd-year-season effects. The primary purpose
5600 5500 5400 5300 5200 5100 5000'
~ 4900 ,:= 4800 ->- 4700'
TABLE 2. Analysis of variance for parity by age interaction.
4600 4500 4400 4300
~
FIRSTLACTATION
4200
~
SEOONDLAUATION
4100 4000
39001 3800 3700 , , , . . . . . . ,, ......... 18 20 22 24 26 28 30 32 34 36
, ...... ,,, 38 40 42 44 46
AGE IN MONTHS
FIG. 1. Age-class means (unadjusted for herdyear-season effects) of 305-day milk yield for the first and second lactation. JOURNAL OF DAIRY SCIENCE VOL. 57, N o . 1
Source of variation
Degree of freedom
Mean squares
305 milk yield Parity and age 3 15,247,025 Parity × age 2 3,274,947 Error for full model 7,619 2,008,771 305 fat yield Parity and age 3 43,025 Parity × age 2 9,657 Error for full mode] 7,619 2,844 **P<.01. *P<.O5.
F 7.6** 1.6
15.1"* 3.4*
103
PARITY I N A G E A D J U S T M E N T
UNADJUSTED MEAN
5400
5300 52O0 51001 v
ADJUSTED MEAN
~
5000
4900 ~- 4800 4700 4600 4500
~ ~ ,
,
FIRSTLACTATION SECONDLACTATION
,
,
,
,
,
AGE GROUPS (MONTHS)
FIG. 3. Comparison of unadjusted 305-day milk yield means and means adjusted for herd-yearseason effects.
of this study was to examine the evidence for parity by age interaction after elimination of herd-year-season effects. Parity by age interaction in the overlapping period from 31 to 37 mo of age for first and second lactation cows was examined for both milk and fat yield by F-tests after fitting the full model (Table 2). The interaction was nonsignificant for milk yield but significant at 5% for fat yield. The F-tests indicate that an all215
UNADJUSTED MEAN
ADJUSTED MEAN
lactations (first and second lactations) yieldage curve for milk would be satisfactory for removing age effects, An all-lactations yield-age curve for fat yield might not be satisfactory for the same purpose. Testing hypotheses by analysis of variances techniques may not be sensitive enough to detect some specific differences that are conceivably important while overall differences among classes are not significant. The contrast of gross averages for each age group (left side of Fig. 3 and 4) with constants adjusted for herdyear-season effects (right side of Fig. 3 and 4) indicated that the elimination of herd-yearseason effects definitely reduced the distinct manner of the two lactation curves intersecting each other. The effect of herd-year-season on the production of the older first calvers was particularly noticeable. The tail end of the first lactation curves leveled to near horizontal after herd-year-season effects was eliminated. However, the beginning of the second lactation curves retained similar slopes after adjustment, except for the very beginning segment of the curve for milk yield. First lactation heifers that freshened late were apparently associated with poor management. Conversely, heifers that were more nearly full grown at first calving were possibly overfat which probably could have a depressing effect on yield. The phenomenon of late first freshening is closely connected with the environment that heifers were raised in which the model described as herd-year-season. There were still moderately large differences with relatively small standard errors in the milk and fat yields of first and second lactation cows at the same
210 TABLE 3. Lactation-age constants for 305-day yield adjusted for herd-year-season effects.
205 3
200
Age-in-month groups
195
Lactation 31, 32, 33
--'~ 190
34, 35
36, 37
5,060.7 (0)" 5,050.8 (151.2)
5,108.9 (20.0) 5,046.7 (69.9)
~,111.6 (72.6) 5,234.2 (39.0)
189.3 (0) 183.9 (5.9)
192.5 (.9) 189.8 (9..7)
191.6 (2.7) 197.9 (1.4)
>-
185 180 175 170 165
Milk
1 2 --x,- FIRST LACTATION SECONDLACTATION
Fat 1 2
AGE GROUPS (MONHS)
Fro. 4. Comparison of unadjusted 305-day fat yield means and means adiusted for herd-yearseason effects.
a Estimated standard error of the difference between this age group-parity subclass mean from the youngest first parity subclass mean. JOURNAL OF DMRY SCIENCE VOL. 57, N o . 1
104
MAO I~T AL
age as measured with herd-year-season effects eliminated (Table 3). Johnson and Corley (2) suggested that late first-ealver records should not be considered in estimating age-yield relationships because late first calvings cannot be considered normal nor economical for production; however, lactation number is presently ignored in age adjustment. Our results suggest that parity probably is unimportant in adjusting first and second lactation records for age. The most serious disadvantage of an all-lactations adjustment would be that it would penalize early calving heifers in second lactation. However, one might consider tolerating some biases by using an all-lactation yield-age curve in place of additional computations and refinement in record keeping that would be involved if age adjustments were to be within lactation.
(2)
(3)
(4) (5)
(6)
References
(1) Gravir, K., and C. G. Hickrnan. 1966. Importance of lactation number, age and season of calving for dairy cattle breed ira-
JOURNAL OF DAIRY SCIENCE VOL. 57, No. !
(7)
provement (based on Canadian Record of Performance). Can. Dep. Agr. Pub. 1239. Johnson, L. A., and E. L. Corley. 1961. Heritability and repeatability of first, second, third, and fourth records of varying duration in Brown Swiss cattle. J. Dairy Sci. 44:535. Lee, A. J., and C. (3. Hiekman. 1970. Effectiveness of an age herd-level adjustanent procedure for milk and fat yield. J. Dairy Sci. 53:913. Lee, A. J., and C. G. Hickman. 1972. Age and herd adjustment of first lactation milk yield. ]. Dairy Sei. 55:432. Mao, I. L., E. B. Burnside, J. W. Wilton, and M. G. Freeman. 1972. Age-month adjustments of Canadian dairy cattle records. Can. J. Anita. Sei. 52:577. (Abstr.) Miller, IL D., W. E. Lentz, and C. R. Henderson. 1970. Joint influence of month and age of calving on milk yield of Holstein cows in the northeastern United States. ]. Dairy Sci. 53:351. Skjervold, H. 1949. Genetic and environmental factors in milk yield of dairy cows. Agr. Coll. Norway Sci. Rep. 28:141.
though effects of year should be small in only 2 yr data. More recent studies from the same institute (3, 4) proposed that age adjustments of production records be within lactation number; however, the necessity is questioned. The purpose of this study was to examine whether estimates for maturity rate should be based on both age and lactation number, at least for early lactations in which the increase in production is major and whether age-month adjustments (5, 6) for production records should he within lactation.
Canadian Record of Performance data on the first two conseeutive lactations of 56,959 Holstein cows from 1959 to 1969 were used to examine the joint effects of parity and age prior to 47 mo on 305day yield. This provided 24,479 records in the overlapping period of ages for first and second lactation cows from 31 to 37 mo of age to examine the need for joint adjustment of age and lactation number or age adjustment within lactation. The model had fixed elements of parity, age, parity by age interaction, and herd-year-season. Interaction between parity and age after absorption of herd-year-seasons was nonsignificant for 305-day milk yield but significant at 5% for 305-day milk fat yield. Least squares means for first lactation milk and fat yield were larger than second lactation yield for calving ages of 31 to 33 and 34 to 35 mo hut lower for 36 to 37
Data
mo.
Introduction
In dairy cattle, milk and fat production in~:reases over the first three or four lactations (1, 2, 5, 6). The increase may result from successive parities promoting udder development and from a cow's physiological development in general. Many researchers studying yieldage relationships have ignored the effect of parity (5, 6). Some workers (I, 2, 7), however, have found that for first lactation cows the yield increased with age at freshening up to about 3 yr of age and then dropped markedly. Gravir and Hickman (1) found the age curves for different lactations did not resemble vne another nor did they have the same general slope as the yield-age curve for all lactations. These curves within lactation-seasons were plotted, however, from production averages of consecutive age classes ignoring herd differences, cow selection, and time trend alReceived April 12, 1973. Present address: Department of Dairy Science, Michigan State University, East Lansing 48823.
A total of 696,682 Holstein, 2X, complete lactation (at least 300 days in length) records were available. The data were from the Canadian Record of Performance (R.O.P.) from 1959 to 1969. The lactation numbers on these records were not reliable, so this method identified first and second lactation records: Step 1. If a cow had a record with calving age equal to or less than 47 mo and had a previous lactation record starting when she was 26 mo or less, the records were labeled first and second lactations. Step 2. If the calving age of the previous record was greater than 26 mo and the herd had been on test for at least 10 mo prior to the date of calving, this ensured that a first lactation was not missed due to the herd not being milk recorded. The previous record was labeled first lactation. The date that the herd began its R.O.P. testing program was indicated by the earliest calving date of any cow in that herd. Step 3. The tentatively labeled first lactation was checked to see ff the cow made a record still earlier. If so, the earliest record would be the first lactation record and checked through steps 1, 2, and 3. This screening procedure produced 56,959 pairs of first and second lactation records. The distribution of these records by age in months and lactation number is in Table 1. Average age at first calving was 29.4 mo, which was also the mode of the frequency distribution. The overlapping period of ages for first and second lactation cows was defined by those age classes having 10 or more records for
100
i01
PARITY IN A G E A D J U S T M E N T
TABLV. 1. Number of records in age by lactation subclasses. Age in months
First lactation
Second lactation
18 19 20 21 22 23 24 25 26 27 28 29 30
21 58 96 121 231 481 1,443 3,509 4,579 5,533 6,099 6,685 6,659
0 0 0 0 0 0 0 o 0 0 4 2 7
31 32 33 34
6,540 ) 5,566 l 16,608 4,502 3,137 )
35
1,401 t
36
253 ~
4,538
\
37
Aj
(PA)ij
j,
18 16 I 47 89 ! 291 i
81
380
790 266
13 i
38 39 40 41 42 43 44 45 46 47
g Pi
0 0 2 1 1 0 0 0 0 0
2,606 1,816 2,616 3,867 4,692 5,587 5,944 6,646 6,635 6,393 6,076 5,423
each lactation. This overlapping period from 31 to 37 mo of age is enclosed between the two solid lines in Table 1 and includes 24,479 records. Model and Procedures
Herd environment, calving season, time trend, cow culling, and some of the interactions between them are some of the key factors affecting the relationship of production with age and parity (1, 5, 6). To take herd, year, season, and their interactions into account when examining the joint effect of age and lactation number in the overlapping period of ages of the first two lactations, - - - this model describes a 2X, 305-day, Holstein production record: Xijkn
where
=
P~ "~ Pi -{-{- e i j k n
Aj + (PA)i j
year-season k at j mo of age, in her ith lactation, is a constant, denotes the ith lactation, a first or second lactation, denotes the jth age group, group one being the calving ages of 31, 32, and 33 too, group two 34 and 35 too, and group three 36 and 37 too, is the effect peculiar to the subclass of lactation i and age
+
HYSk
Xijk n is the nth cow calved in herd-
HYSk is the effect of the kth herdyear-season subclass, composed of herd, year, and season effects, three two-way interaction effects, and a three-way interaction effect, and eijkn represents the random sampiing effect within subclass ijk, with the varianee-eovarianee 2
matrix Iae. All elements in the model were fixed except eijkn. Mao et al. (5) found seasonal variation in the same population which can be described by a continuous curve. They concluded that grouping of months into a few seasons is impractical in age adjustments. However, individual months for the model would result in a large number of herd-year-month subclasses which would leave few degrees of freedom for meaningful tests. Therefore, seasonal groupings were necessary. October through December of one year and January through March of the next year were one year-season class; the following April through September was the next year-season class; etc. There were 16,855 herd-year-season subclasses. Generally, the differences in average yield between first and second lactation cows are due to age, parity, and culling. The effect second lactation being high because of culling was negligible in this study due to the fact that only those cows that had both consecutive lactations were studied. Two approaches were applied to investigate the significance of age by parity interaction effect with herd-year-season effects eliminated. To eliminate herd-year-season effects, PA (age, parity, and age by parity) equations were set up during absorption of HYS (herd-year-season) and HYS by PA equations. Records were sorted into coded HYS and as their were processed, all equations were collected. Since each HYS equation was completed before the next one was started, each was absorbed into JOURNAL OF DAIRY SGIENC~ VOL. 57, NO. 1
102
MAO ~ AL
the PA equations as soon as it was completed, and the same computer storage area was used for the next HYS equation. The resulting PA equations were free from herd-year-season effects. The PA equations were then solved for best linear unbiased estimates for parity-age classes. The parity-age constants were really differences between the first age group in the first lactation and other lactation-age subclasses. The other approach was to test for a null hypothesis of no existence of age by parity interaction. The age by parity mean square (after herd-year-season effects were eliminated) was tested against an estimate of a2~, which was computed by pooling variances within parity-age-herd-year-season subclasses.
215 210 205 200 195 190 185 v
~
180
175 170 165 ION
Results and Discussion
260
Gross averages for milk and fat yield for corresponding age-lactation subclasses in Table i were plotted (Fig. 1 and 2). For first lactation cows, the graph shows the yield increased with age at a decreasing rate up to 34 mo of age, after which it dropped drastically. The most striking feature of the graphs was that the two lactation curves intersected each other in a very distinct manner in the overlapping period of ages. This seems to indicate that
155150
5800 5700
d
--o-- SECONDLACTATION
145 140 135
.............................. 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 AGE IN MONTHS
FIC. 2. Age-class means (unadjusted for herdyear-season effects) of 3OS-day fat yield for the first and second lactation. average production of, say, second lactation cows of a particular age is lower than average production of first lactation cows of the same age. However, these gross means were computed, for each age-parity class, over all other effects, which were conceivably confounded with age-parity effect. As a result, these production curves show a combination of age trend and those confounded effects, namely, herd-year-season effects. The primary purpose
5600 5500 5400 5300 5200 5100 5000'
~ 4900 ,:= 4800 ->- 4700'
TABLE 2. Analysis of variance for parity by age interaction.
4600 4500 4400 4300
~
FIRSTLACTATION
4200
~
SEOONDLAUATION
4100 4000
39001 3800 3700 , , , . . . . . . ,, ......... 18 20 22 24 26 28 30 32 34 36
, ...... ,,, 38 40 42 44 46
AGE IN MONTHS
FIG. 1. Age-class means (unadjusted for herdyear-season effects) of 305-day milk yield for the first and second lactation. JOURNAL OF DAIRY SCIENCE VOL. 57, N o . 1
Source of variation
Degree of freedom
Mean squares
305 milk yield Parity and age 3 15,247,025 Parity × age 2 3,274,947 Error for full model 7,619 2,008,771 305 fat yield Parity and age 3 43,025 Parity × age 2 9,657 Error for full mode] 7,619 2,844 **P<.01. *P<.O5.
F 7.6** 1.6
15.1"* 3.4*
103
PARITY I N A G E A D J U S T M E N T
UNADJUSTED MEAN
5400
5300 52O0 51001 v
ADJUSTED MEAN
~
5000
4900 ~- 4800 4700 4600 4500
~ ~ ,
,
FIRSTLACTATION SECONDLACTATION
,
,
,
,
,
AGE GROUPS (MONTHS)
FIG. 3. Comparison of unadjusted 305-day milk yield means and means adjusted for herd-yearseason effects.
of this study was to examine the evidence for parity by age interaction after elimination of herd-year-season effects. Parity by age interaction in the overlapping period from 31 to 37 mo of age for first and second lactation cows was examined for both milk and fat yield by F-tests after fitting the full model (Table 2). The interaction was nonsignificant for milk yield but significant at 5% for fat yield. The F-tests indicate that an all215
UNADJUSTED MEAN
ADJUSTED MEAN
lactations (first and second lactations) yieldage curve for milk would be satisfactory for removing age effects, An all-lactations yield-age curve for fat yield might not be satisfactory for the same purpose. Testing hypotheses by analysis of variances techniques may not be sensitive enough to detect some specific differences that are conceivably important while overall differences among classes are not significant. The contrast of gross averages for each age group (left side of Fig. 3 and 4) with constants adjusted for herdyear-season effects (right side of Fig. 3 and 4) indicated that the elimination of herd-yearseason effects definitely reduced the distinct manner of the two lactation curves intersecting each other. The effect of herd-year-season on the production of the older first calvers was particularly noticeable. The tail end of the first lactation curves leveled to near horizontal after herd-year-season effects was eliminated. However, the beginning of the second lactation curves retained similar slopes after adjustment, except for the very beginning segment of the curve for milk yield. First lactation heifers that freshened late were apparently associated with poor management. Conversely, heifers that were more nearly full grown at first calving were possibly overfat which probably could have a depressing effect on yield. The phenomenon of late first freshening is closely connected with the environment that heifers were raised in which the model described as herd-year-season. There were still moderately large differences with relatively small standard errors in the milk and fat yields of first and second lactation cows at the same
210 TABLE 3. Lactation-age constants for 305-day yield adjusted for herd-year-season effects.
205 3
200
Age-in-month groups
195
Lactation 31, 32, 33
--'~ 190
34, 35
36, 37
5,060.7 (0)" 5,050.8 (151.2)
5,108.9 (20.0) 5,046.7 (69.9)
~,111.6 (72.6) 5,234.2 (39.0)
189.3 (0) 183.9 (5.9)
192.5 (.9) 189.8 (9..7)
191.6 (2.7) 197.9 (1.4)
>-
185 180 175 170 165
Milk
1 2 --x,- FIRST LACTATION SECONDLACTATION
Fat 1 2
AGE GROUPS (MONHS)
Fro. 4. Comparison of unadjusted 305-day fat yield means and means adiusted for herd-yearseason effects.
a Estimated standard error of the difference between this age group-parity subclass mean from the youngest first parity subclass mean. JOURNAL OF DMRY SCIENCE VOL. 57, N o . 1
104
MAO I~T AL
age as measured with herd-year-season effects eliminated (Table 3). Johnson and Corley (2) suggested that late first-ealver records should not be considered in estimating age-yield relationships because late first calvings cannot be considered normal nor economical for production; however, lactation number is presently ignored in age adjustment. Our results suggest that parity probably is unimportant in adjusting first and second lactation records for age. The most serious disadvantage of an all-lactations adjustment would be that it would penalize early calving heifers in second lactation. However, one might consider tolerating some biases by using an all-lactation yield-age curve in place of additional computations and refinement in record keeping that would be involved if age adjustments were to be within lactation.
(2)
(3)
(4) (5)
(6)
References
(1) Gravir, K., and C. G. Hickrnan. 1966. Importance of lactation number, age and season of calving for dairy cattle breed ira-
JOURNAL OF DAIRY SCIENCE VOL. 57, No. !
(7)
provement (based on Canadian Record of Performance). Can. Dep. Agr. Pub. 1239. Johnson, L. A., and E. L. Corley. 1961. Heritability and repeatability of first, second, third, and fourth records of varying duration in Brown Swiss cattle. J. Dairy Sci. 44:535. Lee, A. J., and C. (3. Hiekman. 1970. Effectiveness of an age herd-level adjustanent procedure for milk and fat yield. J. Dairy Sci. 53:913. Lee, A. J., and C. G. Hickman. 1972. Age and herd adjustment of first lactation milk yield. ]. Dairy Sei. 55:432. Mao, I. L., E. B. Burnside, J. W. Wilton, and M. G. Freeman. 1972. Age-month adjustments of Canadian dairy cattle records. Can. J. Anita. Sei. 52:577. (Abstr.) Miller, IL D., W. E. Lentz, and C. R. Henderson. 1970. Joint influence of month and age of calving on milk yield of Holstein cows in the northeastern United States. ]. Dairy Sci. 53:351. Skjervold, H. 1949. Genetic and environmental factors in milk yield of dairy cows. Agr. Coll. Norway Sci. Rep. 28:141.