Milk yield and quality at udder quarter level are influenced by quarter position, pathogen, and somatic cell score

Figures & data

Table 1. Descriptive statistics of quarter milk yield and quality traits.

Trait^1	N	Mean	Standard deviation	Coefficient of variation, %	Minimum	Maximum
Milk yield, kg/quarter/milking	120	3.14	1.14	36.40	1.32	5.79
Milk quality traits
Fat, %	120	2.98	0.70	23.33	1.29	4.95
Protein, %	113	3.32	0.30	9.10	2.85	4.16
Casein, %	113	2.60	0.25	9.65	2.17	3.30
Lactose, %	117	4.80	0.15	3.15	4.34	5.22
Urea, mg/dL	120	21.91	3.87	17.64	14.00	29.10
SCS, units	120	1.54	1.97	127.82	−1.32	6.76
DSCC, %	120	56.93	15.87	27.88	45.00	89.40
pH	120	6.59	0.07	1.02	6.46	6.73

¹SCS: somatic cell score, calculated as SCS = 3 + log₂(SCC/100,000), where SCC is somatic cell count (cells/mL); DSCC: differential somatic cell count, calculated as the ratio of polymorphonuclear neutrophils plus lymphocytes to total milk SCC.

Table 2. Average somatic cell score in positive (pos) and negative (neg) udder quarter milk samples.

	Right front (n = 30)		Left front (n = 30)		Right rear (n = 30)		Left rear (n = 30)
Trait	Pos (n = 9)	Neg (n = 21)	Pos (n = 2)	Neg (n = 28)	Pos (n = 4)	Neg (n = 26)	Pos (n = 4)	Neg (n = 26)
SCS^1, units	3.04	1.86	1.05	0.95	4.80	1.26	3.50	0.60

¹ Somatic cell score, calculated as SCS = 3 + log₂(SCC/100,000), where SCC is somatic cell count (cells/mL).

Table 3. Pearson correlation coefficients between milk traits at quarter level.

Trait^1	Milk quality traits
	Fat	Protein	Casein	Lactose	Urea	SCS	DSCC	pH
Milk yield	0.31***	−0.14	−0.13	0.17	−0.005	−0.15	−0.14	0.02
Milk quality traits
Fat		−0.07	−0.08	−0.22*	0.14	0.21*	0.03	0.37***
Protein			1.00***	0.24**	0.50***	0.18	0.20*	0.41***
Casein				0.28**	0.48***	0.14	0.17	0.40***
Lactose					0.17	−0.33***	−0.23*	0.20*
Urea						0.34***	0.30***	0.44***
SCS							0.87***	0.32***
DSCC								0.13

¹ SCS: somatic cell score, calculated as SCS = 3 + log₂(SCC/100,000), where SCC is somatic cell count (cells/mL); DSCC: differential somatic cell count, calculated as the ratio of polymorphonuclear neutrophils plus lymphocytes to total milk SCC.

* p < .05; **p < .01; ***p < .001.

Table 4. F-values and significance of fixed effects included in the model for quarter milk yield and quality traits.

Trait^1	Fixed effects			Random effect
	Quarter	Pathogen	SCS^2	σ^2QC, %^3	RSD^4
Milk yield, kg/ quarter/milking	6.87***	2.28	0.88	90.80	0.30
Milk quality traits
Fat, %	0.63	3.73	5.01**	34.05	0.52
Protein, %	0.28	1.06	2.71	94.41	0.07
Casein, %	0.30	1.20	2.74*	94.60	0.06
Lactose, %	0.55	4.89	4.20**	76.34	0.08
Urea, mg/dL	0.25	0.42	3.18*	72.47	1.94
DSCC, %	2.65	0.02	5.89**	97.47	2.04
pH	0.47	5.09*	4.30**	85.75	0.02

¹ DSCC: differential somatic cell count, calculated as the ratio of polymorphonuclear neutrophils plus lymphocytes to total milk SCC (cells/mL).

² Classes of somatic cell score. Each class was defined based on quartile distribution of SCS: low (SCS ≤ −0.001), medium-low (−0.001 < SCS ≤ 0.94), medium-high (0.94 < SCS ≤ 2.94) and high (SCS > 2.94).

³ Proportion of total variance accounted by the random interaction between quarter position and cow.

⁴ RSD: residual standard deviation.

* p < .05; **p < .01; ***p < .001.

Figure 1. Least squares means (with standard error) of quarter milk yield. ^a,b,cMeans with different superscript letters differ significantly (p < .05).

Table 5. Least squares means (standard error) of quarter milk yield and quality traits.

Trait^1	SCS^2
	Low	Medium-low	Medium-high	High
Milk yield, kg/quarter/milking	3.14 (0.20)	3.22 (0.19)	3.06 (0.19)	2.90 (0.21)
Milk quality traits
Fat, %	2.56 (0.16)^b	2.63 (0.16)^b	3.25 (0.15)^a	3.02 (0.15)^ab
Protein, %	3.28 (0.06)	3.28 (0.06)	3.41 (0.06)	3.36 (0.06)
Casein, %	2.56 (0.05)	2.57 (0.05)	2.68 (0.05)	2.64 (0.05)
Lactose, %	4.82 (0.03)^ab	4.89 (0.03)^a	4.82 (0.03)^ab	4.77 (0.04)^b
Urea, mg/dL	20.22 (0.86)^b	21.40 (0.83)^ab	22.84 (0.80)^ab	23.70 (0.89)^a
DSCC, %	54.47 (2.24)^b	54.81 (2.19)^b	56.80 (2.17)^b	61.53 (2.34)^a
pH	6.57 (0.01)^b	6.59 (0.01)^ab	6.62 (0.01)^a	6.62 (0.01)^a

¹ DSCC: differential somatic cell count, calculated as the ratio of polymorphonuclear neutrophils plus lymphocytes to total milk SCC (cells/mL).

² Classes of somatic cell score. Each class was defined based on quartile distribution of SCS: low (SCS ≤ −0.001), medium-low (−0.001 < SCS ≤ 0.94), medium-high (0.94 < SCS ≤ 2.94) and high (SCS > 2.94).

^{a ,b}Means with different superscript letters within row differ significantly (p < .05).

Data availability statement

None of the data were deposited in an official repository. The data that support the findings presented in this study are available from the first author or corresponding author upon reasonable request.